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Iodometric Determination ofCopper

n ne en ons a e eore an er

XX YY

Pennies are an interesting

. ,an always present means of

currency. The penny is ingrained in

our society for many different

social and economic reasons.


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Pennies made from 1962 to 1982 consist of 95%

copper and 5% zinc. These copper rich pennies are

currently worth more as a metal source than their

face value.

Pennies minted after 1982 to the present day are

only made up of 2.5% copper. This copper is plated

onto a zinc core that makes up 97.5% of the penny.

The determination of copper in both pennies made

before and after 1982 will be determined using the

same method.

Introduction to Iodometric

Determination


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Wet Chemical Analysis

ore accurate an prec se t an nstrumenta

methods.

Systematic errors have been detected in the

past and can be accounted.

More involved process

Help to learn new techniques

Reinforce the techniques learned so far

Process selection - Iodine

se w e y to quant tat ve y measure

chemicals reactions

Many analytical procedures are based on the

release or uptake

An analyte that is an oxidizing agent is

added to excess iodide to produce iodine The iodine produced is determined by

titration with sodium thiosulfate

This method is called "iodometry"


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Iodometry

osen or ts stra g t orwar proce ure

The reaction is rapid and quantitative

The starch used is a clear and determinant

indicator

The reagents involved are all easily obtained

and in good supply.

Volumetric determination with a back titrationis a well documented method of determining

the amount of an ion in a solution.

Volumetric Determination

selected as appropriate to the ion being measured

dissolved into an aqueous solution

used in a titration to standardize a secondary standardsolution of sodium thiosulfate

The analyte is reacted with iodide to oxidize ion toiodine

The iodine is then titrated with thiosulfate with astarch indicator to determine the endpoint

With the known amount of thiosulfate used to titratethe freed iodine, we can determine how much iodidereacted with our analyte and then determine howmuch analyte is present


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Iodometry ,

I3- + 2 e- 3 I-

The iodide ion is a strong reducing agent that many oxidizingagents can react with completely

The amount of iodine liberated in the reaction between iodideion and an oxidizing agent is a measure of the quantity ofoxidizing agent originally present in the solution

The amount of standard sodium thiosulfate solution re uired totitrate the liberated iodine is then equivalent to the amount ofoxidizing agent

Iodometric methods can be used for the quantitativedetermination of strong oxidizing agents such as potassiumdichromate, permanganate, hydrogen peroxide, oxygen, and inmy case, the cupric ion.

Sodium Thiosulfate

a e rom sso v ng so um t osu ate

pentahydrate salt

Solution is standardized against a primary

standard

Iodine oxidizes thiosulfate to the

tetrathionate ion:I2 + 2 S2O3

2- 2 I- + S4O62-


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Primary Standard

o ne s rare y use as a pr mary stan ar

for thiosulfate because it presents a problem

in weighing and maintaining its solution

concentration

Pure copper in the form of copper wire is

thiosulfate when it is to be used for the

determination of copper as is the case with

this experiment

Secondary Standard

copper(II), a precipitate of CuI is formed along withI2. The liberated iodine is then titrated with standardsodium thiosulfate

2 Cu2+ + 5 I- 2 CuI(s) + I3-

I3- + 2 S2O3

2- 3 I- + S4O62-

excess iodide force the equilibrium to the right in arapid quantitative reaction

The net stoichiometry of the reaction is 1:1 since 2moles of copper indirectly requires 2 moles ofthiosulfate


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Indicator

signaled by the color change of the starch indicator.

The active ingredient in starch is a helical polymer of-D-gluocse called -amylose, which forms a deepblue-black complex with molecular iodine

For use in this indirect method, the indicator isadded at a oint when virtuall all of the iodine hasbeen reduced to iodide ion, this helps the thedisappearance of color to be rapid and sudden

The starch indicator solution must be freshlyprepared since it will decompose and its sensitivityis decreased

Preparation of Copper

or ion to two or more ligands by coordinate covalent bonds

A ligand is a negative ion or neutral molecule attached to thecentral metal ion in a complex ion

In this experiment, copper and copper-clad pennies aredissolved in a concentrated aqueous solution of nitric acid,HNO3

In aqueous solution, most of the first-row transition metals form

octahedral complex ions with water as their ligands:Cu(s) + 4 HNO3(aq) + 4 H2O(l)

Cu(H2O)62+(aq) + 2 NO2(g) + 2 NO

3-(aq)

Once the pennies have been dissolved, the copper is free toreact with iodide ions


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Experimental Procedure

Standardizing Thiosulfate Measure out ~1 L of de-ionized water

Heat to boilin boil for at least five minutes

Add 25 g of Na2S2O35H2O to 1-L volumetricflask, and mix with hot boiled water

Add ~0.1 g of Na2CO3 as a preservative

Fill to the mark and mix

Transfer the solution to a 1-L plastic bottle

prevent decomposition of the thiosulfate, whichis catalyzed by light.


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Standardizing Thiosulfate

Three pieces of clean copper wire were weighed using about0.20 to 0.25 g each and placed in seperate 250-mL Erlenmeyerflasks

About 5 mL of 6 M nitric acid is added to each flask

Dissolution is promoted by warming the solution on a hot plate

De-ionized water is added to keep the volume constant.

After the copper has dissolved, ~25 mL of water was addedand the solution boiled for a few more minutes to remove morenitrogen oxides

Add 5 mL of urea solution (1 g in 20 mL of water) withcontinued boiling for another minute.

Boilin removes oxides of nitro en which result from thefollowing reactions:

3 Cu(s) + 8 HNO3 3 Cu(NO3)2 + 2 NO + 4 H2O

2 NO + O2 2 NO2 and the oxides are decomposed by urea:

2 HNO2 + (NH2)2CO CO2 + 2 N2 + 3 H2O

Standardizing Thiosulfate

The solution is removed from the hot

The excess acid is neutralized with 4 Maqueous ammonia added dropwise inthe hood until a blue-green precipitate ofcopper(II) hydroxide barely form

To dissolve the precipitate and bring the

so u on o a p ene c a o e ra onreactions, 5 mL of glacial acetic acid wasadded to each flask.


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Analysis of Pennies

For the pennies, six pennies were selected, three from pre-1982 minting and three from post-1982

, ,weighed on an analytical balance

Each of the pennies are placed into a 250-mL Erlenmeyer flask

To each flask, ~10 mL of 6 M nitric acid is added and the flaskswere placed on a hot plate in the hood to dissolve the pennies

After three hours of heating and additional 6 M nitric acid and15.9 M nitric acid added in small amounts and de-ionized waterto keep the solution level constant, the pennies were dissolved

The flasks are removed from the hot plate and allowed to coolin an ice bath

Then ~10 mL of 9 M sulfuric acid is slowly added to each flaskby pouring down the side of the flask

The solution is reheated until white fumes of sulfur trioxideappeared

Throughout the heating process, red fumes of nitrogen oxidescould been seen over each of the solutions

Analysis of Pennies The flasks are then removed again and cooled in an ice

bath

point where the copper hydroxide precipitate has formed

To dissolve the precipitate and bring the pH to a levelappropriate for the titration system, 5 mL of glacial aceticacid is added

The pre-1982 penny solutions has to be diluted to get aproper amount of copper concentration that will beappropriate for the framework of the experiment

The copper solution is quantitatively transferred to a 250

mL volumetric flask and diluted to the mark After dilution, 25 mL of this solution is transferred with a

transfer pipet quantitatively to another 250 mLErlenmeyer flask which is then ready for titrationpreparation


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Iodometric Titration

At this point, the samples were prepared and titrated one at atime.

The flask is covered with a watch glass and allowed to stand for~2 minutes

The solution is then titrated with thiosulfate solution until thebrownish color of iodine is almost gone indicated by a light tanor creamed coffee color

Iodometric Titration Enough iodine has then been reacted to add the 5mL of

starch solution

And 2 g of sodium thiocyanate is added to displace any

adsorbed iodine from the precipitate


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Iodometric Titration

The flask is swirled for about 15 seconds

Then the titration is completed by adding thiosulfate

At the end point, the bluish-gray color of the solutiondisappears and the precipitate appears whitish, or slightlygray

The second and third samples were treated in the samemanner and titrated with thiosulfate

Data and Analysis


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Standardization of Thiosulfate

SolutionSodium thiosulfate pentahydrateFM

248.18 g/mol

Copper molar mass 63.546 g/mol

Mass of thiosulfate used (g) 25.1074

Volume of solution (L) 1.00

Calculated Molarity of thiosulfate

solution

0.101 M

Standardization of Thiosulfate

SolutionTrial 1 Trial 2 Trial 3

ass o copper w re g . . .

Mass of KI added (g) 3.0024 3.0155 3.0012

Mass of KSCN added (g) 1.9816 2.0383 2.0165

Volume of thiosulfate titrated

mL

33.54 37.72 37.00

Measured Molarity of thiosulfate

solution

0.1013 M 0.1017 M 0.1016 M

The average molarity was determined to be 0.1015 0.0002 M.


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Pre-1982 Pennies

Year of Mint 1977 1978 1979

ass o penny g . . .

Volume of thiosulfate titrated (mL) 45.27 45.85 45.59

Measured mass of copper (g) 2.921 2.958 2.963

Measured % of copper in penny 95.10% 95.82% 94.31%

Calc

. . .

The average %wt of copper in pre-1982 pennies is 95.08 0.10

with a standard deviation of 0.7530 from the three trials.

Post-1982 PenniesYear of Mint 1983 1985 1986

ass o penny g . . .

Volume of thiosulfate titrated (mL) 9.27 9.89 10.40

Measured mass of copper (g) 0.05981 0.06382 0.06711

Measured % of copper in penny 2.379% 2.504% 2.659%

Calc

. . .

The average %wt of copper in post-1982 pennies is 2.514 0.003

with a standard deviation of 0.1405 from the three trials.


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Confidence Interval t-test

The 95% confidence interval range with two degrees offreedom for pre-1982 pennies was found to be 96.95 to 93.20 %wt copper

The true value given at the Department of Treasury website is

(19621982) 95% copper, 5% zinc (brass)

which falls within the CI range

The 95% confidence interval range with two degrees offreedom for post-1982 pennies was found to be . .

The true value given at the Department of Treasury website is

(1982 present) 97.5% zinc core, 2.5% copper plating

which falls within the CI range

Sources of Error Copper(I) iodide forms a weak complex with molecular

iodine which slows down its reaction with thiosulfate. As a consequence, once the starch indicator has turned from

blue to colorless, the blue color returns after a few seconds as I2is slowly released into the solution by the CuI[I2] complex

This "after-bluing" can be avoided by adding potassiumthiocyanate just before the end point is reached

The thiocyanate ion, SCN-, replaces the complexed I2 fromCuI[I2], releasing the I2 to solution where its reaction withthiosulfate is rapid.

This is minimized by having a large excess of iodide in order tokeep the iodine in the tri-iodide ion state

Titrations involving iodine should be made in cold solutions inorder to minimize loss through evaporation.


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Sources of Error

In acidic solutions, the iodide ion can oxidize

Therefore, prompt titration of the liberated iodine isnecessary in order to prevent oxidation.

If the starch solution is not made fresh daily orimproperly prepared, the indicator will notbehave properly at the endpoint resulting in a

.

Sources of Error

The addition of concentrated H2SO4 and

fumes appear will ensure that all the HNO3has been removed

NO3- will oxidize I- and would will seriously

interfere with the procedure. The resence of the concentrated sulfuric acid aids

in the exclusion of the remaining nitrogen oxidesfrom the hot solution.

The red oxides first boil off, then fumes of sulfurtrioxide beginning to escape.


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Sources of Error

If excess ammonia is added, copper(II)

,

copper(II) ammonia complex, Cu(NH3)4

If the excess of ammonia is large, some

ammonium acetate will be produced later which

may keep the reaction between copper(II) and

The excess ammonia can be removed by

boiling, and the precipitate will re-form

Sources of Error Concentrated sulfuric acid has a very strong affinity for water

With nitric acid itself, sulfuric acid acts as both an acid and a

Combined with the heating and evaporation of water in the sample,this resulted in a supersaturated aqueous ionic solution which forcedsome precipitates out at certain stages in the experiment

Before the sulfuric acid was added to the pre-1982 penny solutions,the concentration caused copper nitrate to precipitate out

After the sulfuric acid was added, it caused copper sulfate toprecipitate out in well formed crystals

These two reci itates were easil redissolved b addin more de-

ionized water and slightly mixing

Copper

Sulfate

Crystal


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Sources of Error

Dealing with concentrated acids and evaporation of thelittle amount of water in these acids can cause

ump ng or e exp os ons o s eam n esuperheated liquid

Using tongs will keep hands far enough from the mouthof the flask should the solution suddenly "bump" andcause acid burns from the flung drops of concentratedacid

Bumping can also cause small amounts of the analyte to,

flask occasionally will ensure that none of the analyte islost.

Elimination of Error

This experimental method has built in

mentioned previously that can occur.

These systematic errors have been easilycorrected by the refinement of the process

The replication of the experiment is

ensure y e e m na on o esesystematic errors in a concise andrepeatable manner


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Conclusion

measurement of copper in a sample

The amount of error is low because of the correctionof systematic error, and the lowered possibility ofintroduction of random error through thestraightforward procedure

The indirect titration of the iodine with thiosulfate issim le eas to control accurate and recise

The given value of copper content of a penny fell

within the measured 95% confidence interval withthree trials on each type of penny

With more trials, the confidence interval could behoned down to find an even more precise measuredvalue.

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