Chemical Ion Test
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Transcript of Chemical Ion Test
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CHEMICAL TESTS FOR SMALL SPECIMENSBy Jesse Crawford
INTRODUCTION
This is a work in progress. The objective is to collect together the
chemical tests that are useful for identifying minerals and that are alsowithin the reach of the typical hobbyist with the typical hobbyists
budget. If anyone would like to suggest a test for the collection,
please email me ([email protected]) with the details. Put Mineral test in
the title so the message can get through my spam filter. Tests should be
easy to perform, and should use materials that are reasonably easy to
obtain.
The tests described here are intended for small samples. For most tests,
a piece that's 1 or 2 millimeters across is adequate for at least 3 or 4
tests. That's about 5 to 20 milligrams.
Scientists have directed a lot of effort toward developing ways to make
chemical tests on tiny samples using a microscope to interpret theresults. Most of these tests have become obsolete in recent years, but
they still offer useful and fairly low cost methods that amateur
scientists can use to test minerals.
What follows is a description of some chemical tests that work reasonably
well when scaled down to a size appropriate for testing tiny samples. The
author has tried most, but not all, of the tests included. Not much
detail is included about what positive or negative test results look like
because it is assumed that the tests will be performed on both the
unknown sample, and a sample that is known to contain the element being
tested for. It's also a good idea to run the test on a blank sample known
to not contain the element being tested for.
No test is perfect. Most of these tests offer a level of confidence
probably no more than the 80 to 90 percent level, which is pretty good in
a world as uncertain as this one. As I see it, it's the uncertainty that
keeps things interesting. Remember, if youre not having fun, then youre
not doing it right.
Chemists (at least old chemists) form the habit early in their careers of
treating all chemicals as if theyre dangerous.
BE SAFE! Respect the fact that chemicals can be hazardous. Scientists
dont know all the ways that chemicals can injure people. You dont want
to be the first to discover a new one. Don't let chemicals stay on your
skin, and don't breathe them. If you can smell them, then you probably
need to improve the ventilation.
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SAMPLE PREPARATION
After we have a sample to test, the next thing to do is dissolve it, or
at least dissolve enough of it to test. Ideally, the objective is to
dissolve as much as possible of the sample so that the result is a drop
of clear liquid about 10 to 50 microliters in volume containing all of
the ions of interest. (in other words, a small drop). Most of the time,it's not necessary to go for complete dissolution. Often there will be a
part of the sample that remains as pulverized fragments or sometimes a
gelatinous mass of silica. If the grinding of the specimen is done with a
mortar and pestle, the acid can be added while the grinding is being
done. Then touching the pestle to a slide makes a drop that's sufficient
for a test.
Almost all samples are prepared by dissolving them in some kind of acid.
The following is a list in the suggested order to use in trying to
dissolve the sample. If nothing in the list attacks the sample, then
that's a lot of information already. The list of minerals that are
impervious to all acids is a comparatively short one. A table of
solubilities of some minerals is included at the end of this paper. Acidsshould be full strength. When one is found that attacks the sample, the
solution can be diluted with a drop of water before beginning the tests.
Some of the tests need to be carried out in a neutral or basic
environment. Ammonia is handy for neutralizing acids.
THESE ACIDS ARE DANGEROUS! Handle them carefully in a well ventilated
environment. Don't breathe the fumes. Be especially careful with fluoride
minerals. Hydrofluoric acid and sometimes elemental fluorine is evolved
when fluorides are treated with some acids. Its very nasty stuff.
Water
Hydrochloric Acid
Nitric AcidSulfuric Acid
Aqua Regia (3 parts Hydrochloric 1 part Nitric) CAUTION! Chlorine
is evolved from aqua regia.
Hydrofluoric acid, if it were less dangerous, would certainly belong on
this list. It neatly solves the problem of dissolving silicate minerals
by converting silicon to a gas, silicon tetrafluoride. With that goal in
mind, there is an alternative to using a strong solution of hydrofluoric
acid. Small samples of silicate minerals can be digested in platinum or
teflon dishes with a mixture of sulfuric acid and calcium fluoride.
Hydrofluoric acid is thereby generated in situ and immediately reacts
with the silica in the mineral. The technique is not without dangers, butwith proper precautions can be used when necessary. The hydrogen fluoride
generated is still dangerous, and must be respected, but the risk is more
manageable.
There is a method that can be employed to dissolve even the minerals that
resist all the above acids. Heating the sample with a flux to a high
temperature until it is thoroughly fused alters the composition of most
minerals so that they can be dissolved in water or one of the above
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acids. The usual flux used is sodium or potassium carbonate, or for some
minerals sodium or potassium bisulfate.
Fusing the mineral sample at red heat with a flux can induce almost any
mineral to dissolve either in water or in one of the acids. These are
extreme measures, and because they involve a lot more handling of the
sample than simply treating it with acid it's usually good to start witha larger piece. 50 to 100 milligrams is good. Carbonate fusions can be
carried out in a platinum crucible or piece of platinum foil, but
bisulfate fusions should not be made on platinum, as the platinum will be
attacked. Fusions with carbonate can be done in a small ceramic crucible,
or on a block of charcoal, or a loop of platinum or nichrome wire using
pretty much any small torch.
To do a carbonate fusion, start by grinding the sample as fine as
possible. Add about twice the volume of dry sodium carbonate, and mix
them. If you have a platinum crucible, then put in the sample mixed with
flux, cover with a little more pure flux and support the crucible for
heating. Begin heating the side of the crucible and as the mass begins to
fuse, regulate the heat so as to avoid any loss of sample. The melt willevolve carbon dioxide and water vapor and possibly other gasses, and it
will probably do a lot of bubbling. After the bubbles stop, raise the
heat to redness and continue heating for 10 or 15 minutes, until its
thoroughly melted. Let everything cool down and add a few drops of nitric
acid and a little water, and let it sit for a while. The melt will loosen
and dissolve. Put the contents into a beaker, rinse the crucible with
water, and add the washings to the beaker. Then set the beaker on a low
source of heat so that the water and nitric acid can evaporate. It should
not boil at any time. A double boiler arrangement is desirable for this
phase of the operation. When the contents of the beaker are dry, add the
minimum amount of water necessary to dissolve the soluble part. There may
be an insoluble residue of silica. If the dried sample doesn't dissolve
in water, one of the acids may be necessary. The fusion can also be doneusing a wire loop. Start with a hot loop, pick up as much sample and flux
mixture as will stick to it, and fuse it. Then, touch the fused bead to
the sample mixture to pick up a little more, and continue. Repeat the
process until enough of the sample is fused.
The procedure for a bisulfate fusion is similar, but should be carried
out in a porcelain crucible. It's messier, and the fumes are more toxic,
so BE CAREFUL. During the bisulfate fusion, there's a lot of bubbling at
first. After the bubbling stops, there comes a point where the melt
solidifies, and a higher heat is needed to get it to fuse again. This is
the point at which the generation of sulfur trioxide and other corrosive
sulfur oxides begins, which is the objective of the procedure. If the
melt is allowed to cool at this point, the process will not be complete.The heat should continue until the mass fuses again, and no further
changes are in evidence. At this point, cool the melt, add a drop of
concentrated sulfuric acid (carefully) and resume heating. This is
repeated two times. Then the melt is cooled and removed from the crucible
as above using a little sulfuric acid and water. Sulfuric acid gives off
dense clouds of white fumes when it is heated to dryness. DON'T BREATHE
ANY OF IT. This procedure is not for the faint hearted. It's noisy and
hot and frightening and suitable only for a well ventilated garage or
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lab. Have a fire extinguisher close by and an escape route cleared in
case of emergency. Other than that, it's kind of fun.
Vycor labware works well for fusions.
Sodium peroxide also makes a good flux. One author asserts that any
mineral can be brought into solution by sodium peroxide fusion. Peroxidefusions are ordinarily carried out in a zirconium crucible.
DECIDING WHAT TESTS TO PERFORM
In deciding what tests to make, it's sometimes handy to remember that it
can be just as valuable to know what isn't present in a sample as what
is.
Once we know what will dissolve the sample, tables of the solubility of
minerals can be consulted to help in selecting which further tests to
undertake. Try to find a test that will split the list of possibilities
in half. This has been called the half-split technique.
Whenever we read about a test, it usually starts out with a list of
needed equipment and reagents, then a description of the procedure, and
somewhere near the end will be a list of ions that interfere with the
test. That's always the catch. There are very few tests that respond only
to one element. Usually there's a list of them.
A lot of the difficulty with interfering ions can be sidestepped by
careful selection of the sample. Picking a well formed crystal of the
mineral of interest improves the chances that there won't be a lot of
interfering ions. Naturally, those are always the prettiest crystals.
MATERIALS for SPOT TESTS
A small mortar and pestle for grinding samples.
A box of microscope slides.
A box of cover slips.
A glass or plastic ring about 15 or 20 millimeters in diameter ( it
must be smaller than the cover slips) and about 2 to 3 millimeters thick
A glass rod 1 to 2 millimeters in diameter.
A small bulb type pipet (eyedropper).
REAGENTS for SPOT TESTS
Acetic Acid (Glacial)
Acetylsalicycilic Acid (Aspirin)
Ethyl AlcoholAluminon 0.1 percent solution
Ammonium Acetate Solution 3N
Ammonium Chloride
Ammonium Hydroxide
Ammonium Molybdate
Ammonium Oxalate
Ammonium Phosphate (Dibasic)
Aniline hydrochloride
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Barium Chloride
Cesium Chloride
Chloroplatinic Acid
Citric Acid
Curcumin
Dimethylglyoxime
Hydrogen PeroxideHydroquinone
Lead Acetate
Oxalic Acid or sodium or potassium oxalate
m-phenylenediamine hydrochloride or sulfate
Potassium Dichromate
Potassium Iodide
Potassium Mercuric Thiocyanate (This reagent is made by combining
mercuric nitrate with potassium thiocyanate in molar proportions of 1
part mercuric nitrate to 4 parts potassium thiocyanate. Tabular and
needle-like crystals separate easily from acidic aqueous solution).
Potassium Nitrite
Potassium Phosphate (Dibasic)
Potassium or Sodium SulfiteRubidium Chloride
Silica sand
Sodium Acetate
Sodium Chloride
Sodium Fluoride
Sodium Phosphate (Dibasic)
Silver Nitrate
Starch
Tartaric Acid
Thiourea
Uranyl Acetate
The following are spot tests that are carried out on microscope slidesand viewed through the microscope. Some authors recommend coating the
microscope slides with wax or some other hydrophobic material to make it
easier to control the drops. Some manufacturers make microscope slides
with small wells that prevent solutions from running off the slide or to
use with the "hanging drop" method (to be described below). They're all
good ideas, yet just a plain microscope slide works fine for most tests.
It's also a good idea to have a piece of black paper and a piece of white
paper handy to put under the slide for contrast when viewing crystalline
precipitates.
TECHNIQUES
The most general method for carrying out tests is to place a drop of thesolution of the sample on a slide and put a drop of a reagent solution
near it. Then a thin glass rod is used to bring the two drops together.
The entire process is observed under the microscope.
Another important technique that's used is the hanging drop method. It's
used to trap gaseous reaction products that are evolved from the sample
as it reacts with a test reagent. For this technique a glass or plastic
ring supports a cover glass with a drop of reagent or water hanging from
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the underside. The hanging drop is positioned over the sample, so it's
close but not touching.
It is occasionally desirable to separate a drop of a solution from solid
material, such as a precipitate or the fragments that remain after
grinding the sample. It's often possible to precipitate an interfering
ion and then move the clear sample solution to another slide for furthertests. To remove the iron, for example, from a drop of solution, the pH
of the sample solution can be raised by adding a drop of ammonia. At high
values of pH, iron forms a dark gelatinous precipitate. To separate the
sample from the iron precipitate a small piece of filter paper, an eighth
of an inch or so in diameter, is placed on the slide near the sample.
Then a dropper tube with an opening a little smaller than the diameter of
the paper is pressed against the paper. The bulb of the dropper should be
squeezed so that a small amount of suction will be supplied when the bulb
is released. The tip of the tube with the filter paper is slid across
into the sample drop, and the pressure on the bulb is released. If the
dropper tube is not pressing too hard on the filter paper, the fluid will
be drawn up into it through the filter paper, and it can be picked up and
moved to another slide. As described, this procedure for separating ironis not selective, and would also leave behind other elements that
precipitate at high values of pH, notably aluminum. Something else is
needed to separate iron and aluminum (See "Aluminon Test" below). It
takes a little practice to get this technique just right, but it opens a
lot of possibilities when mixtures of ions interfere with one another. It
helps to roughen the end of the tip of the dropper tube with fine
sandpaper to prevent it from slipping off the filter paper when sliding
it along the glass.
It is sometimes necessary to protect a glass slide or cover slip from the
action of hydrogen fluoride. Plastic slides can often be used in these
situations, or the glass can be coated with a hydrophobic material.Smearing grease on the glass works, but its difficult to get a uniform
thickness, and the irregularity of the coating can interfere with
visibility. It works well to keep on hand a thin solution of microscope
grease dissolved in xylene for this purpose. A drop is spread easily over
the slide, and the xylene evaporates quickly, leaving a thin film of
grease that prevents the hydrogen fluoride from attacking the glass.
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TESTS for SPECIFIC IONS
In the descriptions of the tests to follow, references in parentheses
following the name of each test are to the sources listed in the
bibliography.
CATIONS
Aluminum
Aluminon Test: (Welcher) (Lange) If the sample is not already
acidic, acidify it with dilute hydrochloric acid (1 drop). Some authors
also recommend adding a drop of ammonium acetate buffer (3N). Place a
drop of 0.1 percent Aluminon nearby and combine the two drops using a
glass rod. A Red precipitate develops in the presence of aluminum and a
number of other ions. If the precipitate persists after adding a drop of
ammonium hydroxide, aluminum is indicated. This is the simplest form of
the test and unless the material being tested is reasonably free of other
ions, it is likely to produce a false positive. Aluminon is a veryversatile reagent that can be used to detect very small amounts of
aluminum, but in order to be confident of the results it must be
recognized that it forms colored precipitates with a number of other
ions. It forms a purple precipitate with iron, and red to brown
precipitates with aluminum, actinium, barium, beryllium, calcium, cerium,
chromium, europium, gadolinium, hafnium, indium, lanthanum, magnesium and
neodymium, and white precipitates with antimony, bismuth, lead, mercury,
and titanium. In order to interpret the result of this test it's
necessary to separate the aluminum from at least some of these other
ions, particularly iron.
Iron may be separated from aluminum by adding tartaric acid or citric
acid to the sample solution. These will bind with the iron in such a wayas to make it soluble in alkaline solution from which aluminum can be
separated as a gelatinous hydrous oxide. Add a little of the tartaric or
citric acid, and stir the drop until it dissolves, then add a small drop
of ammonia to form the aluminum precipitate. It's important to realize
that if the pH will go too high, the precipitate of aluminum will re-
dissolve. This does not ordinarily happen with ammonium hydroxide, but if
it does, warm the slide a little to drive off some of the ammonia. When
the ph of the drop is in the right range, the aluminum will be in the
form of a white translucent gelatinous precipitate and the iron will
still be in solution. The aluminum precipitate will be translucent white.
If it takes on a dark color then it probably means that the iron (or
something else) is also precipitating. Add more tartaric acid and adjust
the amounts until it looks right. Then use the eyedropper to filter offthe liquid phase, or decant it carefully. Add a drop of water to wash the
precipitate and draw that off through the filter too. Repeat the wash a
couple of times. Then dissolve the white precipitate containing the
aluminum and test it with the aluminon reagent as described above. It's
good to practice on some fake "unknowns" until you have a feel for the
amounts needed to make it go right.
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Aluminon works better as a reagent for use with the enhanced spot tests
described later.
Ammonium Molybdate Test: (Chamot and Mason) This test should be
carried out at a fairly neutral pH. To the drop of sample, if it is not
already neutral, add a small drop of saturated sodium acetate buffer.
Then add a small pinch of ammonium molybdate. Watch for the formation oftransparent four sided plates indicating the presence of aluminum. At
first the crystals look square, but on closer examination they prove to
have a more interesting shape. Too much buffer tends to inhibit the
formation of the crystals. These crystals show symmetrical extinction
when viewed between crossed polarizers. The presence of some ions can
inhibit their formation. Warming the slide and/or adding more water to
the drop sometimes helps. These should always be tried before deciding
that the test is negative for aluminum. Nickel and iron both form similar
crystals. Mercury forms six sided slightly elongated crystals under these
conditions.
Barium
Sodium Bicarbonate Test: (Chamot and Mason) See the sodium
bicarbonate test under Calcium below. Barium carbonate crystals form
more slowly than calcium or strontium carbonates, and the crystal are
larger and more well formed.
Beryllium
Aluminon Test: (Welcher) See the test for aluminum. A red
precipitate develops in the presence of aluminum or beryllium. The red
color from the beryllium looks much like the color from aluminum, but
when ammonia is added, the red precipitate dissolves if it's beryllium.
This is not a very good test for beryllium, because several of the otherions that form red precipitates with aluminon behave the same way.
Aluminum is the only one that does not dissolve when the ammonia is
added.
Potassium Oxalate Test:
(Chamot and Mason) This test
produces characteristic crystals
of a double salt of potassium
and beryllium oxalate. A large
drop of potassium oxalate
solution is placed near the
sample and the two drops are
joined using a glass rod. As thewater evaporates, rhombs and
prisms will become evident if
beryllium is present. It is easy
to mistake crystals of potassium
oxalate for the double salt, so
care should be exercised in
interpreting the result of this test. The double salt is strongly
birefringent, and exhibits an extinction angle of 39 degrees. Prisms are
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sometimes formed with difficulty, but if the solution is heated so the
crystals re-dissolve and a tiny drop of a solution containing mercuric
ions is added, the prisms will have more of a tendency to form as the
solution cools.
Both of the photos show the
results of a positive test forberyllium, made on a known
sample of phenakite. The well
developed crystals in the upper
one appeared only after
recrystallization. These are
crystals of the double salt of
potassium and beryllium
oxalate. The extinction angle
is 39 degrees from the
direction of elongation of the
prisms. The colors are due tothe birefringence of the crystals viewed between crossed polarizers. The
lower photo includes crystals of other compounds as well, and is more
difficult to interpret.
Curcumin Test: (Chamot and Mason) (Smith) See Curcumin Test for
borate under Anions below.
Bismuth
Thiourea Test: (Chamot and Mason) (Lange) Thiourea added to a
solution containing bismuth in nitric acid makes a strong yellow colored
solution.
Dimethylglyoxime Test: (Budavari) A sample containing bismuth forms
a bright yellow color and precipitate with this reagent.
Boron
See borate under Anions below.
Cadmium
Potassium Mercuric Thiocyanate Test: (Chamot and Mason) (Schaeffer)
Put a small drop of potassium mercuric thiocyanate solution near the
sample drop. Combine the two drops with a thin glass rod and watch forthe characteristic crystals that indicate cadmium. This test is also used
for other ions. Crystal shapes are distinctive for each type of ion.
Oxalic Acid Test: (Chamot and Mason) Cadmium oxalate crystallizes
as long prisms with oblique ends, or as Xs or radiating groups. From
concentrated solutions it forms octahedrons. Calcium, zinc and strontium
interfere with this test.
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Calcium, Barium, Strontium
These three elements are difficult to separate because they have very
similar chemistries. Getting a good identification is possible by using two
of the following tests in combination. The difference in solubilities of the
sulfates makes a good way to tell the difference between them.
Ammonium Oxalate Test: (Chamot and Mason) Add a small drop of sodium
acetate buffer to bring the pH to neutral. Place a drop of the reagent near
the test sample and join the two drops with a glass rod. A white precipitate
indicates calcium or strontium or barium. The test can also be made using a
crystal of oxalic acid. The crystals typical of calcium are quite small,
squarish tablets. Strontium oxalate looks much the same. The crystals are
larger, and some are elongated but differentiating the two types of crystals
in a mixture of both is not practicable. Barium makes distinctive crystals
with oxalic acid. They assume the form of branching tree-like structures.
The presence of calcium or strontium will suppress the formation of the
barium crystals. Barium oxalate is very soluble in acids. If just a trace of
nitric acid is present, the crystals will not form.
Sodium Bicarbonate Test: (Chamot and Mason) To the sample drop add a
small drop of saturated sodium acetate to adjust the pH to a value near
neutral. Then add a pinch of sodium bicarbonate. If calcium is present,
small crystals of calcium carbonate will begin to separate out, floating on
the surface and adhering to the slide. After some of the water has
evaporated, larger crystals of the double salt of sodium and calcium
carbonate will form beginning at the edge of the test drop. Its not always
obvious which is the calcium carbonate and which is the double salt, however
the double salt is more soluble in water. Calcium carbonate, once it has
precipitated from neutral solution, will not redissolve on the addition of
water. The double salt of calcium and sodium carbonate can be redissolved by
adding more water to the drop. This is a good test for calcium. Strontium
and barium, which also precipitate as insoluble carbonates, do not form a
double salt under these conditions.
Sulfuric Acid Test: (Chamot and Mason) Add a small drop of sodium
acetate buffer to make the pH neutral and join the sample drop with a drop
of dilute sulfuric acid. In the presence of calcium, prisms of calcium
sulfate separate gradually from the solution. The ends of the prisms are
terminated at an angle of 66 degrees, which serves to confirm their
identity. Twinning is common. The precipitation with barium and strontium
is too finely divided to recognize crystal forms. There are several other
elements that react to form insoluble sulfates, so its best to do a
preliminary separation with oxalic acid or sodium bicarbonate, so that the
other elements will not interfere. The oxalates and carbonates of calcium,
barium and strontium tend to adhere to the surface of the microscope slide,
so the precipitate containing these can be carefully washed and redissolved
prior to making the sulfate test. The precipitate containing strontium can
be recrystallized from hydrochloric acid to produce recognizable crystals,
but they look a lot like the calcium oxalate crystals, so its not really
worth the effort. The solubility of the sulfates of calcium, barium, and
strontium differ widely. Calcium sulfate dissolves readily in hydrochloric
acid. Strontium sulfate dissolves slightly, while barium sulfate is
virtually insoluble.
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Cobalt
Potassium Mercuric Thiocyanate
Test: (Chamot and Mason) Adding
potassium mercuric thiocyanate
solution to a sample containing
cobalt results in deep blue crystalslike the ones shown in the photo.
These crystals are somewhat more
soluble than the ones that develop
in the presence of other ions, and
sometimes do not appear until the
drop has been allowed to sit for a
while so that some of the water has
evaporated.
Quinoline - Ammonium Thiocyanate Test: This test responds to Co,
Fe, Mo, Ti, U, V, and Zr. For this test, the sample should be dissolved
in strong hydrochloric acid. A small drop of quinoline mixed with an
equal volume of 6N hydrochloric acid is mixed with the test drop. It isthen connected in the usual way with a drop of saturated ammonium
thiocyanate. An oil phase separates out quickly and over time crystals
develop from the droplets of oil. After a half hour or so, the drop
becomes filled with ammonium chloride crystals from the reaction between
the hydrochloric acid and the ammonium thiocyanate. Antimony and bismuth
must be absent for this test to work, as these cause an immediate
precipitation when the quinoline reagent is added to the sample. In the
case of Mn, Cd, Sn, and Hg (and possibly others), crystals may be formed
immediately before the addition of the ammonium thiocyanate solution.
Cobalt develops light blue dendrites and blue crystalline blades from
blue oil droplets. In the case of titanium, the oil is yellow to orange
and the crystals, if they appear, are small thin yellow discs, scales andelongated hexagons or prisms. Zirconium yields a colorless oil and thin
scales, plates, and rosettes from yellow to orange in color. Vanadium
causes a colorless oil, and crystals are difficult to form. Uranium
causes a yellow oil with rectangular plates and prisms of a light yellow
color. Molybdenum produces a reddish oil but seldom produces crystals.
Nickel and copper both yield dark colored oils but rarely produce
crystals.
Quinaldine Ammonium Thiocyanate Test: Quinaldine is a compound
almost identical to quinoline. Its comprised of the same heterocyclic
ring system with a single methyl substituent. Its chemistry is much the
same as quinoline, and when substuted for quinoline in the above test
protocol, the results are similar. The crystal shapes and colors producedare a little different however, presumably because of steric effects due
to the methyl group. If the test solution is not already in hydrochloric
acid, it should be evaporated to dryness and dissolved in a drop of
hydrochloric acid before adding the quinaldine. If crystals do not
develop readily, sometimes it helps to also add a drop of water to the
sample. In the case of some ions (Co, Cu, Ni, U) crystals can take up to
two or three hours to develop. The slide must be covered in those cases
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with an inverted petri dish or watch glass in order to retard
evaporation.
The presence of cobalt in the test drop is indicated by a blue oil
separating out. Crystals do not form immediately. If conditions are not
perfect, they do not form at all. If the crystals dont make their
appearance before the slide becomes covered with a mixture of ammoniumchloride and unreacted quinaldine hydrochloride crystals, then its too
late. These metastable states are common with a number of ions, making
this test a little frustrating at times. Im still working on improving
it because crystals, when they can be coaxed to appear, can be quite
distinctive.
Copper
Ammonia Test: (Chamot and Mason) A dilute nitric acid solution of
copper ions will turn a strong characteristic blue color with the
addition of a drop of ammonia. This is not a precipitate, tetraamine
copper ions are soluble but strongly colored.
Triple Nitrite Test: (Schaeffer)(Chamot and Mason) Evaporate the
sample to dryness and then just cover the residue with a small drop of 30
percent acetic acid. Add a small crystal of sodium acetate. Wait for the
crystal to dissolve and add a small crystal of lead acetate. When that
has dissolved, add a crystal of potassium nitrite. Characteristic
crystals will form if copper is present. The triple nitrite test has
several variations and is used to test for a number of ions. It requires
some practice, and even then the results can be confusing. Theres a good
discussion in Handbook of Chemical Microscopy by Chamot and Mason.
Quinaldine Ammonium
Thiocyanate Test: See the notes
for this test under Cobaltabove. The coordination compound
made by copper and quinaldine
produces clusters of long
slender crystals arranged in
branching structures, dark
reddish brown in color. These
crystals only appear when
conditions are perfect, and they
take a long time to form.
Ordinarily they dont put in an
appearance before patience runs
out. The appearance of the oil is similar to the oil that separates when
the sample contains nickel.
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Potassium Mercuric Thiocyanate Test: (Chamot and Mason) Copper
causes long green needle shaped crystals to form when combined with
potassium mercuric thiocyanate reagent. These crystals, like the ones
that develop with cobalt, are fairly soluble. Allowing the slide to sit
undisturbed for a period of time while the water evaporates produces
crystals like the ones in the photos above.
Gold
Potassium Mercuric Thiocyanate Test:
(Chamot and Mason) Adding a drop of
potassium mercuric thiocyanate reagent and
joining it to a drop of sample solution
containing gold causes the immediate
separation of a densely branched structure
of finely divided crystals. The crystals
have a reddish hue and are unmistakable,
making this an easy test for the presence
of gold.
Potassium mercuric thiocyanate is a
versatile reagent, but it must be realized that with mixtures of ions,
the results can be variable and confusing. It works best when one ion
dominates the sample mixture. Chamot and Mason give an extensive
discussion of the behavior of this reagent under various conditions in
Handbook of Chemical Microscopy.
Iridium
Thiourea Test: (Chamot and Mason) In a solution of the sample in
concentrated hydrochloric acid, a few crystals of thiourea are added. If
iridium is present, the reddish color of the sample drop will decolorize,
and become water clear. Iridium does not cause the formation of crystals.
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Iron
A quick assessment of iron can be made by adding a little sodium
hydroxide solution, or ammonia to the unknown. Iron will cause a
precipitate immediately of a dirty green color if its ferrous, or brown
if its ferric. This is a quick test for iron, and can easily lead to
false conclusions unless followed up with more specific tests, becausethere are several elements that yield gelatinous precipitates with bases.
Specifically, the following ions all yield gelatinous hydrous oxides
under these conditions: aluminum, chromium, tin, titanium, zirconium,
hafnium, thorium, bismuth, and uranium. There are probably others.
Iron is a common element in the earths crust. In minerals, it assumes
one of two oxidation states, either the +2 or ferrous state, or +3,
ferric. Its sometimes important to be able to determine which of the two
states are present. Often both are, and if both, then its good to get
some idea of the ratio. This ratio is destroyed if the sample is
dissolved in nitric acid, since nitric acid is a strongly oxidizing acid,
all iron in nitric acid solution is in the ferric state, even if it was
originally ferrous. This difficulty does not apply if the solution ismade in hydrochloric acid. Ferric iron can be changed into ferrous iron
by the addition of a reducing agent, such as sodium sulfite. Ferrous iron
can be changed back to the ferric state by adding an oxidizing agent such
as hydrogen peroxide. Because some of the reagents respond only to iron
in one state and not the other, advantage can be taken of these facts to
design a sequence of operations that will give a pretty good idea of the
proportions of each in an unknown sample. Thiocyanate produces a red
color in the presence of ferric iron, but remains colorless if only
ferrous iron is present. If a solution in hydrochloric acid is first
treated with ammonium or potassium thiocyanate (potassium works best)the
red color, if there is any, gives an estimation of the ferric iron
present. If the color increases significantly after adding hydrogen
peroxide, then the change in the color gives an idea of how much ferrousiron was in the sample.
Quinoline Test: (Chamot and Mason) See Quinoline Test under
Cobalt above.
Quinaldine Test: See Quinaldine Test under Cobalt above.Iron
causes the immediate separation of a dark red oily phase and the prompt
separation of dark red, almost black rectangular tabular and skeletal
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crystals, some elongated, as well as elongated blades and rhombohedral
forms. Dendritic clusters predominate. A concentrated solution of iron
turn opaque very quickly. Because of the strong color, this is a very
sensitive test for iron.
Iron (Ferric)
Potassium Ferrocyanide Test: (Chamot and Mason) Potassium
Ferrocyanide and ferric (Fe3+) iron produce Prussian blue.
Ammonium or Potassium Thiocyanate Test: (Chamot and Mason) Either
of these reagents react with a solution of ferric ions to produce a red
color.
The ferrocyanide and thiocyanate tests for iron may fail in the case of
minerals that contain phosphate, fluoride, or borate. Also cobalt,
chromium, nickel and copper interfere.
Iron (Ferrous)
Potassium Ferricyanide Test: (Chamot and Mason) Potassium
Ferricyanide and ferrous (Fe2+) iron produce Turnballs blue.
Orthophenanthroline is an excellent reagent for detecting ferrous
iron.
Lead
Thiourea Test: (Schaeffer)
This test must be carried out on
a solution of the sample innitric acid. Add a small drop of
nitric acid to the sample and a
small lump of thiourea.
Characteristic crystals will
slowly form in the presence of
small amounts of lead. It's
important to observe the form of
the crystals. Other ions may form
other kinds of crystals and if
large quantities of some impurities are present, the crystals may not
form at all. The form of the crystals varies depending on the acidity and
concentration of lead present. Thiourea makes distinctive crystals with
several elements, including gold, platinum, ruthenium, palladium,rhodium, and osmium. Unfortunately, most if not all of these elements can
produce crystals of several different habits, depending on the
concentration, pH, and the nature and amount of interfering ions.
Consequently, interpreting the results of a thiourea test is something of
an art. More so than with most tests, running standards and blanks in
parallel with the test sample is necessary in order to have much
confidence in the results. Its worthwhile to take some extra precautions
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to make sure the thiourea is pure. Thiourea can be purified by
recrystallization from alcohol.
Hydrogen Chloride Ammonia Test: (Chamot and Mason) This test for lead is
also a test for silver and for mercury. In a dilute nitric acid solution
of the sample, add a small drop of hydrochloric acid. A white precipitate
indicates lead, silver, or mercury. Adding a drop of ammonia willdissolve the precipitate if it's silver. If not, then lead or mercury is
indicated. Remove the liquid by touching the edge of the drop with a
piece of filter paper. Then add two drops of water and heat, but not to
boiling. Put a small drop of potassium dichromate solution near the hot
solution and combine the two drops with a glass rod. A yellow precipitate
indicates lead. These tests are sometimes ambiguous because silver and
lead often occur together and sometimes all three can be present. Lead
and mercury chloride are more soluble than silver chloride. This fact can
be exploited by washing the precipitate several times with warm water to
separate the silver from the other two.
Potassium Iodide Test: (Chamot and Mason) Drop a few small crystals
of potassium iodide into the sample solution. Lead will cause a yellowprecipitate.
Magnesium
Sodium Phosphate (dibasic) Test: (Chamot and Mason) To the sample
drop add a few crystals of ammonium chloride, stir and add a few crystals
of citric acid. Warm and stir until dissolved. Add a crystal of disodium
phosphate, warm gently and stir. Put a drop of strong ammonium hydroxide
near the sample and cause the two drops to join using a glass rod.
Ammonium magnesium phosphate slowly develops as dendritic forms, featherystars, and Xs turning into plates and tabular forms. The precipitate can
be recrystallized by decanting, then dissolving the crystals in dilute
hydrochloric acid and precipitating with ammonium hydroxide. This should
be done in order to reduce the chance of false results from interfering
ions. Similar double ammonium phosphates are formed with Fe2+, Mn2+,
Co2+, and Ni2+. Of these only Mn2+ precipitates (partly) in the presence
of citric acid, and then only if the Mn is in high concentration. If in
doubt, decant the crystals, wash with distilled water, and add hydrogen
peroxide. If manganese is present the crystals will turn brown.
In ammoniacal citrate solution, disodium phosphate will completely
precipitate Mg, Ca, Sc, Pb, Au, and the rare earth elements. In addition,
Be, Sr, Ba, Hg, In, U, Zr, and Mn are partially precipitated.
Manganese
Sodium Phosphate (dibasic) Test: (Chamot and Mason) See sodium
phosphate test under magnesium.
Sodium Bismuthate Test: Manganese dissolved in dilute nitric acid
gives a purple color with sodium bismuthate.
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Acetylsalicylic acid test: (Welcher) The reagent must be freshly
prepared by dissolving a 15 grain aspirin tablet in 1 ml of 10 percent
ammonia. Add 0.5 ml of hydrogen peroxide solution. The color developed is
red to reddish brown in the presence of manganese. Iron also produces a
strong color which may mask the results. The color produced with iron is
dark brown at high concentrations and brown to yellow at lowconcentrations.
Ammonium Molybdate Test: (Chamot and Mason) Evaporate the sample
drop to dry without overheating. Place a very small crystal of ammonium
molybdate on the spot and put a drop of water on it. Set aside for a half
hour. The orange crystals produced in the presence of manganese are
markedly dichroic, going from red-orange to a pale yellow color as the
polarization of the light is rotated. Its important not to use too much
ammonium molybdate. If too much is used, the result will be a white
crystalline mass that covers the entire spot, making any red-orange
crystals difficult to see. This is a good test, but not very sensitive.
Manganese concentration needs to be at least two parts per thousand. The
presence of significant amounts of copper, chromium, strontium, titaniumor tungsten can prevent the crystals from developing.
Mercury
Hydrogen Chloride - Ammonia Test: (Chamot and Mason) To a nitric
acid solution of the sample, add a small drop of hydrochloric acid. A
white precipitate forms if silver or mercury or lead are present. Adding
a drop of ammonium hydroxide will cause the precipitate to go back into
solution if it is silver. See notes for the lead test above.
Potassium Iodide Test: (Chamot and Mason) Add a tiny crystal of
copper sulfate to the sample drop. Put a drop of potassium iodide
solution nearby and bring the two drops together in the usual way. Redmercuric iodide indicates the presence of mercury.
Ammonium Molybdate Test: (Chamot and Mason) See the discussion of this
test under aluminum above.
Molybdenum
Dipotassium Phosphate Test: (Chamot and Mason) The test sample must
be strongly acidified with nitric acid, A solution of dipotassium
phosphate is combined in the usual way, and, if no precipitation occurs,
warm the slide gently. Then set the slide aside to cool. Examine at highmagnification. If molybdenum is present, small yellow isotropic
octahedral crystals are formed. If the principal element is tungsten the
crystals will be white. A negative test result does not mean that
molybdenum or tungsten are not present, only that they are not present in
the form of molybdate or tungstate ions. If diammonium phosphate is used
instead of the dipotassium salt, the test is more sensitive, but its
harder to read.
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Nickel
Quinaldine Ammonium Thiocyanate
Test: See the notes for this test above
under Cobalt. Crystals of the nickel
quinaldine coordination compound are
deep garnet red. They are rarely seen
however, because conditions must be
perfect, and even then theyre very slow
to form. Dark, almost black drops of oil
separate out on addition of the reagent.
The initial appearance is similar to the
oil droplets seen when the sample contains copper.
Dimethylglyoxime Test: (Schaeffer) (Lange) This reagent forms a
bright red precipitate in the presence of nickel. Make the sample
alkaline with a drop of ammonium hydroxide. Put a drop of saturated
dimethylglyoxime in water near the sample, and combine the two drops with
a glass rod. A deep pink or magenta precipitate indicates nickel. It
might be necessary to warm the sample.
Ammonium Molybdate Test: (Chamot and Mason) See the discussion of this
test under aluminum above.
Osmium
Thiourea Test: (Chamot and Mason) Add a few crystals of thiourea
to the sample dissolved in concentrated hydrochloric acid. In thepresence of osmium, a red color develops immediately and, over time,
red crystals form.
Palladium
Dimethylglyoxime test: (Smith) Dimethylglyoxime forms a yellow
precipitate with palladium under acid conditions which is soluble in a
solution made basic by ammonia.
Thiourea Test: (Chamot and Mason) Adding a few crystals of thiourea
to a drop of the sample in concentrated hydrochloric acid causes an
orange or yellow region to develop around the crystals, the outer edge of
which is crystalline. In concentrated solutions of palladium, the orange
crystals form closely around the reagent crystals and prevent them from
dissolving.
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Platinum
Potassium Chloride Test: (Schaeffer) Octahedral crystals of
potassium chloroplatinate form when a drop of potassium chloride solution
is joined to a drop of a sample solution containing chloroplatinic acid.
This is the form produced by the action of aqua regia on platinum.Rubidium chloride can also be used in place of the potassium chloride.
The rubidium chloride test is more strongly colored.
Thiourea Test: (Chamot and Mason) Adding a crystal of thiourea to
a solution containing chloroplatinic acid causes a yellow reaction
followed by reddish brown feathery dendrites.
Potassium
Uranyl Acetate Test: (Schaeffer) Characteristic crystals of
potassium uranyl acetate are formed in the presence of potassium.
Tartaric Acid Test: (Schaeffer) Tartaric acid causes crystals
typical of potassium acid tartarate to precipitate if potassium is
present. Ammonia must not be present for this test to work. Add a little
sodium hydroxide solution and warm the slide first to remove it. Then
make the test for potassium.
Chloroplatinic acid Test: (Schaeffer) To use this reagent, ammonium
must not be present. Add a little sodium hydroxide solution and warm the
slide first to remove it. Then place a drop of chloroplatinic acid
solution near the sample, and proceed in the usual way. In the presence
of potassium, characteristic crystals will form. The test can also beused in the same way to test for the presence of ammonium. If a mixture
of ammonium and potassium is suspected, use the hanging drop method to
trap the ammonia in a drop of water. Then test the water drop separately.
Ruthenium
Thiourea Test: (Chamot and Mason) This test works only on a
sample dissolved in concentrated hydrochloric acid. The sample
solution should not be too darkly colored, if it is, dilute it with
concentrated hydrochloric acid. Add several small crystals of
thiourea to the test drop. Warm the slide gently. Over time, a blue
color will develop in the presence of ruthenium.
Silver
Hydrogen Chloride Ammonium Hydroxide Test: (Schaeffer) To a
nitric acid solution of the sample, add a small drop of hydrochloric
acid. A white precipitate forms if silver, lead or mercury are present.
The silver precipitate will dissolve in ammonium hydroxide.
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Sodium
Uranyl Acetate Test: (Schaeffer) This test is conducted in the
usual way. Characteristic crystals of sodium uranyl acetate will form in
the presence of sodium.
Fluosilicic Acid Test: The initial step of this procedure is to create a
drop of water containing fluosilicic acid. Mix powdered silica sand with
powdered calcium fluoride and put it in a small lead dish. Add a drop of
concentrated sulfuric acid. Then put a hanging drop of water over it. It
works well to use a cover slip coated with a very thin film of something
hydrophobic such as stopcock grease. Silicon hexafluoride is evolved from
the acid mixture and is trapped in the drop of water where it breaks down
forming silicic acid which separates out, and fluosilicic acid which
remains dissolved in the water. After a few minutes, lift the cover slip
and touch the drop to a microscope slide. Then put a drop of the sample
solution nearby, and cause the two drops to join using a thin glass rod.
Set the slide aside for several minutes while the water evaporates. If
sodium is present, hexagonal crystals of sodium fluosilicate, some
looking like little flowers, will appear beginning near the edges of the
drop. These crystals have a very low index of refraction so they may be
difficult to see. If necessary, let the slide dry completely and examineit with a high powered objective. Often the crystals appear as small
prisms lying on their sides with irregular terminations.
Strontium
Sodium Bicarbonate Test: (Chamot and Mason) See the sodium
bicarbonate test under Calcium above. Strontium carbonate looks much
the same as calcium carbonate at first, but does not form the double
salt, and after standing for a while it forms small acicular tufts of
crystals attached to the slide near the test reagent. These are easy to
overlook.
Tin
Potassium Iodide Test: (Chamot and Mason) A yellow to reddish
orange precipitate is formed with the addition of a solution of potassium
iodide to a sample containing stannic tin (+4). If the sample contains
stannous ions (+2) the precipitate is a lighter yellowish white which
changes to orange in the presence of an excess of potassium iodide.
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Stannic Tin (Sn+4)
Cesium Chloride or Rubidium Chloride Test: (Schaeffer) (Chamot and
Mason) These tests are conducted in the usual way. Small crystals
characteristic of tin can be recognized. It is difficult to have much
confidence in this test, since the reagents form insoluble crystals witha number of other ions. If the sample is first treated with nitric acid
and evaporated to dryness on a double boiler several times before making
the test, all the tin will be converted to an insoluble hydrous oxide,
which can be washed several times again with dilute nitric acid. This
removes many of the interfering ions. Then the insoluble oxide can be
dissolved in hydrochloric acid and tested for tin as described above.
Stannous Tin (Sn+2)
Oxalic Acid or Alkali Oxalate Test. (Chamot and Mason) The addition
of oxalic acid or a solution of an alkali oxalate causes a precipitate of
irregularly shaped crystals. Prisms, if formed, exhibit either parallelextinction or, if twinned, an extinction angle of approximately 15
degrees to the direction of elongation.
Titanium
Quinoline Test: (Chamot and Mason) See Quinoline Test under
Cobalt above.
Tungsten
Dipotassium Phosphate Test: (Chamot and Mason) See Dipotassium
Phosphate Test under Molybdenum above.
Vanadium
Quinoline Test: (Chamot and Mason) See Quinoline Test under
Cobalt above.
Quinaldine Test: See Quinaldine Test under Cobalt above.
Uranium
Sodium Fluoride Bead Test. This test is incredibly sensitive,
however it is not very specific to uranium. A sodium fluoride bead is
made by heating a loop of platinum wire until it is red hot and
touching it to some sodium fluoride so that a little of it adheres to
the loop. This is re-heated until fused, and the bead is built up in
increments until it reaches the desired size. Then the hot bead is
used to pick up a bit of the pulverized sample and heated thoroughly
until its completely fused. Let the bead cool, and examine it under
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an ultraviolet light source. If uranium is present, the bead will
glow brightly with green fluorescence. Because there are other
elements that can also cause fluorescence, this test should be
followed up with a confirmatory test. There is a lot of old
literature devoted to bead tests. They are still some of the most
useful of field tests. With some practice its possible to glean agreat deal of information from them.
Quinoline Test: (Chamot and Mason) See Quinoline Test under
Cobalt above.
Potassium Oxalate Test: This is not an especially sensitive test
for uranium, but because the crystals produced are dichroic, its fairly
definitive. A large drop of sample containing something on the order of a
half a milligram of uranium in dilute nitric acid is combined with a
similarly sized drop of saturated potassium oxalate solution. Crystals of
oxalic acid are immediately precipitated. Over time these re-dissolve
leaving small pale yellow rectangular prisms and tablets of the uraniumcompound. These are dichroic, going from pale yellow to colorless as the
polarization of the light is turned through 90 degrees. The crystals are
small and require an hour or two to develop. After three or four hours
they will be large enough to easily determine their dichroic character.
The best crystals seem to develop near the edges where the two drops come
together.
Zinc
Potassium Mercuric Thiocyanate
Test: (Schaeffer) (Chamot and Mason) Put
a small drop of potassium mercuric
thiocyanate solution near the sample
drop. Bring the two drops together with
a thin glass rod and watch for the
characteristic crystals that indicate
Zinc.
The photos show crystals of zinc mercuric
thiocyanate. These are typical of the
crystals that form after adding a
solution of potassium mercuric
thiocyanate to a sample containing zinc.
The graceful branching is typical of a
solution with a high concentration of
zinc.
Sodium Bicarbonate Test: (Schaeffer) Expose the test drop to
ammonia fumes long enough to make it alkaline, or add a small drop of
sodium hydroxide solution, then join with a drop of saturated sodium
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bicarbonate solution. Watch for the formation of characteristic crystals
that indicate zinc. The reaction begins as a slightly milky area where
the two drops join, and the crystals grow slowly. Avoid stirring the drop
when adding the baking soda. If it is agitated, the crystals that form
may be too small to be seen even at maximum magnification.
Zirconium
Quinoline Test: (Chamot and
Mason) See Quinoline Test under
Cobalt above.
Quinaldine Test: See
Quinaldine Test under Cobalt
above. Red crystals of the
quinaldine zirconium complex look
like little red footballs. They
tend to be a little slow in
forming and develop from a lightcolored oil that separates on
addition of the reagent. This is
another one thats a little
tempermental. Conditions must be just right for the crystals to develop,
and often they dont.
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ANIONS
Borate
Curcumin Test: (Smith) (Chamot and Mason) Use an alcoholic solution
of curcumin (0.5 percent). This test works well on paper. First put thespot of unknown on the paper and let it dry. Then put a spot of sodium
fluoride solution over the spot and let it dry. Then add a drop of dilute
hydrochloric acid. Let the spot evaporate until almost dry. Then add a
drop of alcoholic solution of curcumin (0.5 percent). A red color
indicates boron or beryllium. Then hold the test strip over a bottle of
ammonia so the fumes can reach it. If the spot turns blue, boron is
indicated. Titanium, columbium, molybdenum, tantalum, and zirconium
interfere.
Bromide
m-phenylenediamine or aniline Test: (Schaeffer) Either of these
reagents can be used to demonstrate the presence of bromide ions. Theprocedure involves the use of the hanging drop technique to trap the
volatile bromine as it is released from the sample. First, place a small
ring about 2 mm in thickness around the sample drop. Add several crystals
of potassium dichromate to the sample and warm it until it's dry. Then
add a drop of concentrated sulfuric acid. If bromide ions are present in
the sample, free elemental bromine will be evolved from the reaction.
This bromine must be trapped in a solution of the reagent by placing a
cover glass with a droplet of a solution of m-phenylenediamine (sulfate
or hydrochloride) in the middle over the reaction mixture, supported on
the ring. After a few minutes, small crystals characteristic of the
tribromo derivative of the test reagent will appear on the underside of
the cover glass. Aniline works the same way.
Chloride (in the absence of fluoride)
Chromyl Chloride Test: (Schaeffer) This is an indirect test for
chloride ions. Add potassium dichromate to the test solution and
evaporate to dryness. Then add a small drop of concentrated sulfuric Acid
to the sample. Set up a hanging drop of water to catch any gas evolved by
the reaction. Allow to stand for a few minutes, and retrieve the hanging
drop. Evaporate it to dryness and add a small drop of water to the dry
residue. Add a small crystal of silver nitrate. A red precipitate of
silver chromate establishes the presence of chloride ions in the sample.
For this test to work, fluoride ions must not be present. Bromide and
iodide ions do not interfere. The reason this test works to identify
chloride is that the mixture of sample containing chloride mixed withpotassium dichromate and treated with sulfuric acid produces chromyl
chloride which is trapped by the hanging drop, where it decomposes to
chromic acid and hydrochloric acid. Evaporation to dryness leaves only
the chromic acid which reacts with the silver nitrate to produce red
silver chromate. if there are no chloride ions in the original sample,
then there will be no chromic acid, and thus no silver chromate.
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Chromate or Dichromate
Silver Nitrate Test: (Chamot and Mason) (Schaeffer) Acidify the
sample with nitric acid. Place a drop of 2 percent silver nitrate
solution close by and combine the two drops. Dark red crystals of silver
chromate form if the sample contains chromate or dichromate ions.
Lead Acetate Test: (Smith) The same procedure using lead acetate
instead of silver nitrate produces a yellow precipitate in the presence
of chromate or dichromate.
Fluoride
Sulfuric acid and silica Test: (Schaeffer) This is another test
that involves the hanging drop technique to catch the reaction product in
a drop of water. The sample is mixed with some pulverized silica sand and
a drop of concentrated sulfuric acid is added. Silicon tetrafluoride gasis evolved if a fluoride is present. This is collected in a hanging drop
of water where the silicon tetrafluoride breaks down into silicic acid
and fluosilicic acid. The silicic acid is insoluble and forms a
precipitate, while the fluosilicic acid remains in solution. It can be
detected by converting it into its insoluble sodium salt by the addition
of a few crystals of sodium chloride. Compare the fluosilicic acid test
for sodium.
Halides (other than Fluoride)
Silver Nitrate Test: (Schaeffer) The presence of a precipitatewhen the sample is combined with a drop of silver nitrate solution
indicates chloride, bromide or iodide.
Iodide
Potassium nitrite and starch Test: (Schaeffer) Potassium (or
sodium) nitrite is an oxidizing agent that releases free iodine from a
mixture containing the iodide ion. Put a few crystals of potassium
nitrite in the test drop together with a few grains of starch. The starch
grains will turn blue if iodine is present. A drop of hydrogen peroxide
to which a little hydrochloric acid has been added can be used instead of
the potassium nitrite. Bleach also works well for this test. As
confirmation, adding potassium or sodium sulfite reduces the iodine back
to iodide, causing the blue color to disappear.
Phosphate or Arsenate
Ammonium Molybdate Test: (Smith) Add a drop of ammonium molybdate
reagent, and a 1 drop of concentrated nitric acid. Warm the slide. A
yellow precipitate indicates phosphate or arsenate.
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Selenium
Hydroquinone Test: (Chamot and Mason) Hydroquinone is a reducing
agent that works well to detect the presence of selenium and tellurium.
The sample, dissolved in nitric acid is placed on a slide and evaporated
to dryness without overheating. The spot is covered with sulfuric acid
and heated until dense fumes of sulfur trioxide begin to come off. Theslide is cooled and another drop of sulfuric acid is added, then separate
the clear solution from any insoluble material. This can be problematic.
Glass fiber microfilters work well, if you have them, otherwise decanting
the drop is about the best that can be done. Let the drop settle for a
long time and then decant very slowly. The clear drop is then heated
again until sulfur trioxide fumes begin to come off. Let the drop cool,
and then combine it in the usual way with a saturated drop of
hydroquinone dissolved in sulfuric acid. Warm the slide gently. Selenium
will separate as a brown or red precipitate. After a few minutes, decant
the clear liquid from the selenium precipitate, and put the drop on a
fresh slide. Heat the drop again until the dense fumes of sulfur trioxide
begin to come off. Tellurium, if present will precipitate as black
bundles and aggregates. Hot sulfuric acid is dangerous. Drops tend tospread out when hot and its a littledifficult to keep things together.
For this reason, this test would probably work better carried out on a
small watch glass, or something that has a shape that helps to keep the
drop in one place.
Silicate
The same reaction that is used to detect fluoride can be used to
test for silicon. The hanging drop setup is used. Concentrated sulfuric
acid is added to a mixture of the unknown material with calcium fluoride.
Any gas that is evolved is trapped in a hanging drop of water. After a
few minutes, if silicon is a major component of the sample, a precipitate
of silicic acid will be visible in the water drop. If the water issubsequently treated with a few crystals of sodium chloride, insoluble
hexagonal crystals of sodium fluosilicate will confirm the presence of
silicon. This test gives a false positive if carried out in the presence
of glass. A small lead dish works well. A glass cover slip can be coated
with a film of stopcock grease to prevent the hydrogen fluoride from
attacking it, or a plastic cover slip can be used.
Sulfate
Barium Chloride Test: (Chamot and Mason) Use the normal procedure.
A white precipitate indicates sulfate.
Tellurium
Hydroquinone Test: (Chamot and Mason) See Hydroquinone Test under
Selenium.
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ENHANCED SPOT TESTS
CHEMICALS
70% Isopropyl Alcohol
8 Hydroxyquinoline
Alizarin
Aluminon
Curcumin
Dimethylaminobenzyledene Rhodanine
Dimethylglyoxime
Sodium Sulfide
Quercetin
Rhodamine B
You don't need all of these, just the Isopropyl alcohol (available at any
drug store) and one or two of the other reagents for making the spotsvisible. 8-hydroxyquinoline and alizarin are the best. They are both
very versatile reagents. The colors that develop are often unique for a
given ion, and some ions produce spots that show fluorescence under
ultraviolet light. Curcumen and quercetin are also pretty good and
they're a lot cheaper, since they're both available at health food
stores. They also show fluorescence with certain ions.
There are a number of reagents that react to a wide variety of ions with
colors that, in many cases, are diagnostic. They can be used as simple
spot test reagents, but the chance for success can be increased by using
the following method to spread the sample over a wider area and separate
the different ions somewhat using a technique borrowed from
chromatography.
Chromatography is a method that has evolved into one of the main
technologies both for detection and for the separation of compounds that
are mixed together. There are many variations on the method, but the one
that seems most useful for the basement scientist uses paper as the
support. There are a lot of possibilities for the mobile phase. The best
solvent system for a given application has been the subject of a lot of
research. Isopropyl alcohol is not optimum, but it has the virtue of
being easily available and seems to work reasonably well. It's also
fairly non-toxic, which is always an important consideration.
It may be stretching the definition a little to call this method
chromatography. It's really just a spot test, with a slight enhancementborrowed from chromatography. Before applying the reagent that develops
the color, the sample spot is caused to spread across a region of the
paper by allowing the isopropyl alcohol to climb the length of the paper
by capillarity. Since different ions in a mixture in the sample have
different solubilities in the isopropyl alcohol (and differing affinities
for the paper), they will move along the paper at different rates. A
particular ion then can be recognized by how far along it moves, and by
the color it develops with a sprayed on reagent. Spreading the test spot
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across a region of the paper overcomes many of the problems of
interfering ions by moving each ion to a different part of the paper
before adding the reagent to detect it. Resolving two ions that may be
eclipsing one another can sometimes be accomplished by using a longer
strip of paper, or by trying a different solvent. Arthur Ritchie's book
"Chromatography in Geology" contains a lot of helpful information.
You'll need a a stock of chromatography paper. It comes in various sizes.
Tests of the method were made with pieces 1x4 inches cut from larger
sheets. Precondition the strips to be used by soaking them in 70 percent
isopropyl alcohol (or whatever solvent youre using) for several hours
and then let them dry before use. Cleanliness is very important. Don't
let fingerprints get on the paper. About a half inch from one end of the
paper, place a spot of the sample solution made by dissolving a small
crystal (5 to 20 milligrams) of your unknown mineral. The spot should be
approximately a quarter inch in diameter. Let the spot dry. Prepare a
jar that is large enough for the paper to stand in without touching the
sides. Fabricate a way to hang the paper in it so that the bottom end of
the paper is about a half inch above the bottom of the jar, and the sides
of the paper do not touch anywhere else. Then put about a half inch of 70percent isopropyl alcohol in the jar and hang the paper that you prepared
with the spot down so that the end just dips into the alcohol. It must
not go deeply enough that the spot is beneath the surface of the alcohol,
or the test will not work. Cover the jar and watch as the alcohol rises
up the paper. The front of the solvent should rise fairly evenly up the
paper over a time of several minutes until it reaches somewhere near the
top. The time required will depend on the kind of paper used. Some papers
are very fast, others may take an hour or more. Don't let the solvent
front reach all the way to the top. When it's ready, remove the paper and
hang it up to dry. After it's dry, spray the paper with a developer made
from one of the reagents described below. Let it dry again. The
positions and colors that will be on the paper will depend on what ions
were present in the spot that you applied, and the type of spray reagentused. The possibilities are many, and this is both the power and the
weakness of the method. The interpretation of the result is strictly
empirical. To determine whether a sample contains, for example, gold,
compare it with a reference strip that was made with a sample known to
contain gold. Ideally, the reference strip should contain about the same
amount of gold that the unknown has, in order for them to look the same.
Even when they don't look exactly the same though, the colors and the
distance moved, expressed as a percentage of the distance moved by the
solvent front, will be the same, or nearly so. This can be an extremely
sensitive test. There is no right or wrong way to do it. The important
thing is to keep the paper clean, and do enough of them that you develop
a system that works for you.
There are a lot of chemicals that can work as developers. Below is a
list of several, and the ions that they are sensitive to. Reagents
can also be applied by dipping the paper in them, but spraying works
better. There are some really cute little chromatography sprayers
available on ebay from time to time.
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8 Hydroxyquinoline 0.5 percent in ethyl alcohol Al, Ag, Au, Ba, Be, Bi,
Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo,
Nb, Ni, Pb, Pd, Pt, Rb, Sb, Sc, Sn, Sr, Ta, Th, Ti, Tl, U, V, W, Zn, Zr
Alizarin: A saturated solution in ethyl alcohol Al, As, Bi, Ce, Cr, Cs,
Cu, Fe, Hg, In, Li, Mg, Mn, Pb, Sb, Ta, Th, Ti, V, W, Y, Zn, Zr
Aluminon: 0.1 percent in 1 percent ammonium acetate in water Ac, Ag, Al,
Ba, Be, Ca, Ce, Cr, Cu, Eu, Ga, Ha, In, La, Li, Mg, Mn, Nd, Ni, Ti
Curcumin: 0.1 percent in ethyl alcohol Ag, Al, Au, B, Be, Cr, Cu, Fe, Li,
Ni, Pt, Ta, Ti, V, W, Zr
Dimethylaminobenzyledene Rhodanine 1 percent in ethyl alcohol Al, Au, Ag,
Co, Cu, Fe, Hg, Li, Mn, Ni, Pb, Pd, Pt, Ta, Ti, V, W, Zn
Dimethylglyoxime 1 percent in ethyl alcohol Al, Cu, Fe, Co, Li, Ni, V, W,
Zn
Sodium Sulfide 0.5 percent in water Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb,
Pd, Pt, Zn
Quercetin 0.2 percent in ethyl alcohol Ag, Al, Bi, Ca, Cd, Co, Cr, Cu,
Fe, Hg, Mg, Mn, Ni, Pb, Sb, Sn, U, Zn
Rhodamine B 0.1 percent in ethyl alcohol Ag, Au, Cu, Fe, Ni, Pt, Sb, V, W
After development, exposing the paper to ammonia fumes will sometimes
enhance the picture and sometimes not. Also, viewing them under
ultraviolet light can reveal features that otherwise are not visible.
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BIBLIOGRAPHY
1. "Identification and Qualitative Chemical Analysis of Minerals" by
Orsino C. Smith 1953
2. "Microscopy for Chemists" by Harold F. Schaeffer 1953
3. "Chromatography in Geology" by Arthur S. Ritchie 1964
4.
Handbook of Chemical Microscopy Chamot and Mason 19405. Handbook of Chemistry (9th Edition) Norbert Adoplph Lange Ph.D.
1956
6. Organic Analytical Reagents (Volume 2) Frank J. Welcher Ph.D.
1947
7. The Merck Index (11thEdition) Susan Budavari Editor 1989
EXPERIMENTAL
SAMPLE: Approx 20 mg sample of heulandite. PROCEDURE: Pulverized sample
in a small mortar. Added 1 drop of concentrated nitric acid. Continued
grinding for a minute or two. Added 1 drop of water. Grind some more andtouch the pestle to a microscope slide, leaving a small drop with quite a
bit of undissolved material suspended in it. Placed a small drop of 0.1
percent aqueous aluminon beside it. Used a glass rod to cause the two
drops to touch. Over a period of several minutes the point where the two
drops meet developed a pink color that spread across to eventually cover
the entire reagent drop. The color persists after adding a drop of
ammonia. CONCLUSION: This test is positive for aluminum. DISCUSSION:
Heulandite is approximately 9 percent aluminum. This experiment
demonstrates the sensitivity of aluminon as a reagent for the detection
of aluminum.
SAMPLE: Approx 25 mg crystal of heulandite. PROCEDURE: Ground the sample
with about twice the volume of calcium fluoride. Placed in a small leaddish with a drop of sulfuric acid. Placed a cover slip with a hanging
drop of water over the sample, supported on a plastic ring about 2 mm
thick. The hanging drop spread out and ran under the plastic ring, but
did not run down into the acid solution because of the hydrophobic nature
of the plastic ring. Over several minutes gas bubbles evolved from the
sulfuric acid and a residue accumulated on the cover slip in a ring
around the inner edge of the plastic ring. It appears to be salicic acid.
CONCLUSION: This test demonstrates a positive test for a silicate
mineral.
SAMPLE: Approximately 50 mg piece of phosphate rock containing mostly
strengite with some rockbridgeite. PROCEDURE: Pulverized the sample with
a drop of concentrated nitric acid. After a minute or so, added a drop ofwater. Grind more, and allow to stand for a few minutes. Touch pestle to
a slide, leaving a small drop with a little solid material. Placed a drop
of 1 percent ammonium molybdate nearby and let the two drops flow
together. There was no reaction immediately. Placed the slide on a low
heat source (a small transformer). After a few minutes the drop, still
wet, shows a border of yellow material visible against a white paper
background. CONCLUSION: Test is positive for phosphate.
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SAMPLE: Approximately 50 mg piece of phosphate rock containing mostly
strengite with some rockbridgeite. PROCEDURE: Pulverized the sample with
a drop of concentrated nitric acid. After a minute or so, added a drop of
water. Grind more, and allow to stand for a few minutes. Sample stands
for 5 to 10 minutes. Touch pestle to a slide, leaving a small drop with a
little solid material. Placed a drop of 9 percent ammonium molybdate
nearby and let the two drops flow together. A yellow color developsimmediately at the interface between the two. Put the slide on a low heat
source (a small transformer). A yellow crystalline mass develops as the
drops proceed to dryness. CONCLUSION: Test is positive for phosphate.
DISCUSSION: The stronger ammonium molybdate reagent develops a stronger
yellow residue, as expected, however, it was not difficult to conclude
that the test was positive, even with the 1 percent reagent.
SAMPLE: Blank. PROCEDURE: Placed about 50 mg of calcium fluoride in a
small lead dish with a drop of concentrated sulfuric acid. A cover slip
with a hanging drop of water was placed over the sample. No bubbles were
observed coming from the sulfuric acid. The hanging drop spread out and
ran under the edges, but did not run down into the acid. Over time the
cover glass appeared to have a white film on the underside where thewater drop was. The cover slip was removed and inverted on a piece of
black paper. There appears to be a residue of salicic acid where the
water drop was. The amount is much less that it was in a similar
experiment in which the sample contained heulandite. CONCLUSION: This
test could be interpreted as a false positive for silicate in a sample.
It is significant that there was no visible evolution of gas bubbles from
the acid. When silicate is present, Silicon tetrafluoride gas bubbles are
distinctly visible, and probably should be part of the criterion for a
positive result. The salicic acid on the cover slip in this case was
probably from the hydrofluoric acid attacking the glass of the cover
slip. A plastic cover slip might work better for this test or a glass one
covered with a thin film of stopcock grease.
SAMPLE: Approximately 25 mg piece of aurorite. Procedure: The sample was
ground together with a small drop of concentrated nitric acid for several
minutes. The sample dissolved almost completely. The reagent solution was
prepared by dissolving an aspirin tablet in a milliliter of ammonia
solution and adding a half milliliter of strong hydrogen peroxide. A
small drop of the sample solution was transferred to a microscope slide
by touching the pestle to the slide. A reagent drop of similar size was
placed nearby using a small glass rod, and the drop was carefully moved
until it just barely touched the sample drop. A dense light gray
precipitate developed immediately at the interface between the two drops
and faint but clearly visible red streamers developed along the reagent
side of the precipitate. The streamers slowly spread and intensified over
time lending a pink cast to the solution. After a few minutes theprecipitate turned a dirty brown. CONCLUSION: Test is positive for
manganese. The red color was weak but unmistakable against a white paper
background at first, but faded to a light brown as it spread.
SAMPLE: About 100 mg piece of aurorite. PROCEDURE: Sample is ground with
a drop of nitric acid, and the solution is diluted with a drop of
distilled water. A small drop is placed on the slide near a drop of a
solution of oxalic acid, made by mixing a few crystals with a drop of
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water on the slide. The oxalic acid solution is then connected to the
sample solution using a glass rod. A white precipitate develops
immediately at the interface between the drops. CONCLUSION: The test is
positive for calcium. DISCUSSION: The results might be due to the
presence of strontium or barium. PROCEDURE: Another drop of sample
solution was placed on a fresh slide, and tested with sodium chloride
solution. The solution remained clear of any precipitate. CONCLUSION:Negative for silver, mercury and lead. DISCUSSION: Silver sometimes
occurs in aurorite, but is absent from this specimen.
PROCEDURE: Another drop of test solution was placed on a slide and caused
to join with a small drop of ammonium thiocyanate solution. No color was
observed. CONCLUSION: Negative for iron. DISCUSSION: At this point, it
has been established that the sample of aurorite probably contains
manganese and calcium, and that it does not contain significant amounts
of silver or iron.
SAMPLE: Approx 50 mg of picotite. PROCEDURE: The steps taken were similar
to the ones described above. The sample was macerated in a drop of nitric
acid and a drop placed on a microscope slide. A similar drop ofacetylsalicylic acid reagent was placed nearby and carefully encouraged
to join. There was a heavy white precipitate at the point where the two
drops joined, and a yellow hue developed in the reagent drop over the
following minute or so. CONCLUSION: Negative for manganese, positive for
iron. DISCUSSION The absence of a red color is consistent with the fact
that picotite does not contain manganese. This yellow color appears to be
due to iron in the sample.
SAMPLE: Approx 50 mg of picotite. PROCEDURE: The sample was ground
together with a drop of nitric acid for one minute. A small drop was
transferred to a microscope slide. Another small drop of aluminon reagent
was placed next to it, and the two drops caused to touch. A red streamer
reached immediately into the reagent drop. Color persists after adding adrop of ammonia. CONCLUSION: Test is positive for aluminum.
SAMPLE: Approx 50 mg of picotite. PROCEDURE: The sample was ground
together with a drop of nitric acid and allowed to stand for some time. A
small drop of the nitric acid solution is placed on a microscope slide
and a small lump of thiourea is placed into it. A reddish brown color
spreads out from the thiourea as it begins to dissolve. Over time, the
red color fades. No crystals are observed to form. CONCLUSION: Test is
negative for lead. DISCUSSION: This test can also be taken to imply that
thallium is absent, since it is also known to form crystals with
thiourea.
SAMPLE: A drop of solution known to contain zinc ions. PROCEDURE: A dropof sample solution was placed on a slide and joined to a nearby drop of
saturated potassium mercuric thiocyanate solution. Within a minute, small
crosses were beginning to become visible. These grew and developed a fine
branching structure, taking on a feathery appearance, eventually becoming
like round fluffy snowflakes. CONCLUSION: Test is positive for zinc.
DISCUSSION: The published description for this test is confirmed in this
experiment. This test is also sensitive to cadmium, and produces crystals
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of a different form in the presence of cadmium, or a cadmium and zinc
mixtures.
SAMPLE: A drop of solution made from dissolving a crystal of bertrandite
in dilute hydrochloric acid. PROCEDURE: A drop of potassium oxalate
solution was made on the slide by the following method. A small quantityof oxalic acid was placed on the slide and several small drops of
potassium hydroxide solution were mixed into it and stirred. After a few
minutes, most of the oxalic acid dissolved. The pH of the drop was
measured to be about 6 to 7. A drop of the solution was decanted from the
solid oxalic acid remaining and this was caused to join the drop of
sample. Then a glass rod was touched to a solution of mercuric nitrate,
and most of the adhering drop of mercury ions was transferred from the
glass rod to an unused part of the slide, so that very little of the
mercury solution remained on the glass rod. Then the glass rod was
lightly touched to the point where the sample drop joined the potassium
oxalate drop, and removed without stirring the drop. Over the next minute
or two, crystals separated from the solution, mostly rhombs, and some
prisms. The slide was placed under a polarizing microscope, and theextinction angle of several crystals was measured. Some of the crystals
measured around 44 degrees, and some measured 38 degrees. CONCLUSION: The
test is positive for beryllium, based on the presence of crystals with a
measured extinction angle of 38 degrees. DISCUSSION: It is apparent that
not all of the crystals are the double oxalate salt of potassium and
beryllium, but some of the observed crystals fit the criterion for this
salt, and therefore the conclusion that beryllium is present is
supported. This test is a little difficult, and the first attempt to
perform it failed. Making the potassium oxalate in situ in the manner
described is probably not the best way to perform the test, but was
necessitated by the fact that no potassium oxalate was available.
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SOLUBILITIES OF SOME COMMON MINERALS
Information for the following table was taken, for the most part,
from Orsino Smiths book Identification and Qualitative Analysis of
Minerals. In Smiths tables, minerals soluble in hydrochloric acid
were not tested for solubility in other acids, so subsequent entries
for that mineral under nitric and sulfuric acids will indicate No,
even though the mineral might in fact be soluble in those acids.
Likewise, minerals not soluble in hydrochloric acid, if soluble in
nitric acid, were not tested in sulfuric acid, and so may show an
erroneous No under sulfuric acid. These errors are regrettable, but
hopefully the table will nevertheless be useful if this caveat is
kept in mind. M