196 JOSEPH ANT) RAE CHROMIUM PHOSPHATE.

X XI 11.- Chromium Phosphate. * By ALFRED FRANCIS JOSEPH and WILLIAM NORMAN RAE.

ALTHOUGH chromium phosphate has formed the subject of a number of investigations during the past sixty years, many of its curious properties appear t o have been unrecorded. The more important references to previous work are: Carnot (Compt. rend., 1882, 94, 1313), Bloxam (Chem. News, 1885, 52, 194), Schiff (Zeitsch. anorg. Chem., 1905, 43, 304), and Maddrell (Hem. Chem. SOC., 1845- 1847, 111, 273). Without reviewing the literature a t length, the following statement cont,ains the ordinarily accessible information as to its chemistry: When a hot solution of chrome alum is pre-

* The substance of this paper was communicated to the Chemical Section of the Second Indian Science Congress held in Madras in January, 1915.

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JOSEPH AND RAE : CHROMIUM PHOSPHATE. 197

cipitated with excess of sodium phosphate, an amorphous, green precipitate is obtained, which consists of hydrated chromium phosphate; this, on being heated, leaves an amorphous, brown powder, which is the anhydrous compound. The precipitate obtained in the cold by the action of sodium phosphate on excess of chrome alum is lavender or violet, and, if allowed to remain in contact with the solution, becomes dark violet and crystalline; it then has the formula CrP0,,6H20; the action of heat on the higher hydrated compounds is said to give rise to others contain- ing 5, 4, 3, 2+, 2, and I molecule of water. A pseudomorphous, green, crystalline variety is also said to be formed by heating the violet, crystalline hexahydrate.

The experiments here described are not in complete agreement with all the aboye statements, and they have led t o a number of other observations of sufficient interest to be recorded. It should be borne in mind that the experiments were carried out a t the ordinary temperature of a tropical laboratory, that is, about 2 8 O .

E x P E R I M E N T A L .

Crystalline Chromium Plmphat e .

When cold solutions of equal weights of chrome alum and disodium hydrogen phosphate are mixed, a violet precipitate of amorphous chromium phosphate hexahydrate is produced. This precipitate, when allowed to remain in the solution for a day or two, is converted into a violet, crystalline hexahydrate; it may then be washed by decantation, filtered, and allowed to dry in the air. The water of crystallisation in this substance was determined by heating to low redness; for the determination of the chromium and phosphorus, it was oxidised by means of sodium peroxide, the resulting chromate being estimated with a freshly standardised solution of ferrous ammonium sulphate, and the phosphorus by Woy's method of double precipitation as ammonium phospho- molybdate and weighing as P20,,24M00,. Analysis of a typical preparation gave :

Found: Cr=19*8; P = 1 1 * 9 ; H20=41'2 . CrP04,6H,0 requires Cr = 20.7 ; P = 12.1 ; H,O = 42.4 per cent. It has been said that the first-formed amorphous precipitate

becomes crystalline on remaining in contact with the solution, but a t the room temperature it is essential that it should not be left too long, as after a week it becomes entirely converted into a green, amorphous powder; on analysis, this was found to be chromium phosphate tetrahydrate. The change is readily shown by leaving the crystals in contact with either water, sodium phos-

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198 JOSEPH AND RAE : CHROMIUM PHOSPHATE.

phate solution, or chrome alum solution, the last being the most rapid agent. Thus, in one experiment, 5 grams of the crystals were entirely converted into the green, amorphous substance by being left in contact with 150 C.C. of 10 per cent. chrome alum for five days. The change with sodium phosphate solution was not complete after forty days, whilst with water very little change had taken place after sixty days.

This effect is greatly influenced by temperature; through the kindness of Dr. L. F. Hirst, a mixture of chrome alum and sodium phosphate solution was left in contact with the violet crystals at, about 5O for thirty days before the crystals showed any signs of turning green. I f the violet hexahydrate is heated at looo for some time, or, better, if boiled with water for half an hour, it is completely converted into a green, crystalline hydrate having the composition CrP0,,4H20.

On boiling the violet, crystalline hexahydrate with acetic anhydride, it forms a green, crystalline dihydrate (as stated by Schiff, Zoc. c i t . ) similar in appearance to the tetrahydrate. Pro- longed boiling with acetic anhydride does not appear to cause further dehydration, and both the di- and tetra-hydrate appear stable in the presence of moist air or water. It is very difficult to prepare the tetrahydrate by heating the hexahydrate in dry air, as a continuous loss of water takes place with the formation of the dihydrate. As stated above, however, the chacge from hexa- hydrate to tetrahydrate is easily accomplished by heating with water.

Attempts to make measurements of the vapour pressures of the various hydrates have been unsuccessful up to the present.

As an alternative to direct measurement, use has been made of the time method first described by Hannay and by Ramsay (30zm~. Chem. SOC., 1877, ii, 381, 395; compare also Rae, T., 1916, 109, 1331), and the rates of dehydration have been determined by placing the hydrates in desiccators a t various temperatures and finding the loss of weight from time to time. For experiments above the room temperature, from 60° to 150°, the arrangement of one or two electric lamps placed in the desiccator, surrounded with sawdust in a packing-case, served as a sufficiently good thermostat. The hydrate was stirred during dehydration by means of a platinum wire soldered to a brass rod, passing through a brass tube in the rubber stopper in the centre of the lid of the desiccator, the brass rod being attached to a pulley slowly turned by a motor.

If a curve is plotted showing the loss in weight and time, any clearly marked t'ransition from one hydrate to another should, of course, be shown by a break in the curve. In this way, for example, i t appears that the hexahydrate becomes converted into

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JOSEPH AND RAE : CHRO;IIIU!% PHOSPHATE. 199

a tetrahydrate on boiling with water, because when this product is placed in the desiccator in the moist state a t the room tempera- ture, the first break takes place when its composition is about 4H,O.

The tetrahydrate does not appear to be formed when the hexa- hydrate is heated in the dry state, because then the first well- marked break occurs a t about 2H,O. This would result i f the vapour pressures of the hexahydrate and the tetrahydrate were not very different.

It should be noted that some such method as the above is essential in determining the composition of hydrates other than very stable ones. St'atements frequently used, such as that a hydrate begins to lose water a t a particular temperature, passing into a lower hydrate, are most unsatisfactory, because all hydrates exposed to a dry atmosphere must lose water a t all temperatures; apparent constancy of weight can only be obtained when the lower and higher hydrates have widely different vapour pressures.

The crystalline di- and tetra-hydrates when heated to low red- ness become quickly converted into a fine, black, anhydrous chromium phosphate.

When either the amorphous or the crystalline hexahydrate is left in a glass-stoppered bottle for a long time, the violet, colour changes to green, with loss of water. The change in both cases is slow, but occurs more quickly with the amorphous than with the crystalline variety.

Thus an amorphous violet specimen initially had the composition CrP04,6H,0, containing 42.4 per cent. of water; after two years, the resulting green powder was analysed and found to contain 36.5 per cent. of water. A cryst,aIline specimen of the violet hexahydrate, after the same time, contained 40.7 per cent. of water, and was quite green. The tetrahydrate would contain 32.9 per cent. of water, and i t appears probable that this would ultimately be formed, although the change is incomplete after two years.

The densities of the crystalline hydrates have been determined a t 32*5O, benzene being used as the comparison liquid. The follow- ing is a summary of the twenty-five determinations made:

CrPO, .................................... 2-94

CrP0,,4&O ........................... 2.10 CrPO,,G&O ........................... 2.12

CrP0,,2&O ........................... 2.42

It was found difficult to obtain the same result for the density of different samples of the anhydrous compound, that of a strongly heated product being always greater than that of one obtained at a low red heat. I n order to determine whether the change was accompanied by any decomposition, a weighed quantity was heated for some time, when a loss in weight was observed, and the density

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200 JOSEPH AND RAE : CHROMIUM PHOSPI-I-4TE.

was found to have appreciably increased. At the same time, the black, crystalline powder became converted into a fine green one strongly suggestive of the presence of chromium oxide. The following is a summary of these experiments.

Initial density of sample, 2-94. Specific volume, 0.340. Percentage

loss. Heated 1 hour in Bunsen burner ............ 1.2

Y Y 2 Y Y 7 7 Y Y ............ 2.4 y y 36 y y y y electric furnace at 1100" 4.4 ,, 36 y y y y draught gas furnace ...... 9.8 ), 50 y y ,) y y y y y y ......... 11.7

Chromium oxide ............... -

Density. Specific volume

3-16 0.316 3.29 0.304 3.42 0.292 3.66 0.273 3.78 0.265 6.14 0-195

Since the density of an ordinary sample of ignited chromium oxide was found to be 5.14, it is suggested that these higher values are due to loss of phosphoric oxide, and this view was confirmed by an analysis of the product having a density of 3-66, which was found t o contain P,O, =41.1, CrPO, requiring P,O, =48.3 per cent. If the percentage loss and specific volume are plotted, the relation is seen not to be linear, the specific volume corresponding with a small loss being much less than that calculated for a mixture of chromium phosphate and oxide in the proportion indicated by the loss of phosphoric oxide.

Some of the green powder obtained by prolonged ignition was finely pulverised in an agate mortar, again ignited, and, after cool- ing in a desiccator, was shaken with methylene iodide (D 3*3), but no separation into chromium phosphate and oxide was observed. The change in the density may be due either to the formation of a basic phosphate or to an actual effect produced by heat alone on the density of the phosphate. I n order to preserve a simple additive relation, anhydrous chromium phosphate would require a density of 3-55 to agree with the observed value (3.78) of the density of the product from which 11.7 per cent. of phosphoric oxide has been removed by heat.

It may be noted that the density of chromium phosphate itself, calculated additively from those of chromium oxide and phosphoric oxide, is 3.29, which is greater than that found (2.94); the com- bination of chromium oxide and phosphoric oxide is theref ore attended by a large iricrease in volume.

A mixture of chromium phosphate and chromium oxide heated to low redness has the density required by the additive relation; thus, a mixture of 29.8 per cent. of chromium oxide (D 5.1) with 70.2 per cent. of chromium phosphate (D 2.94) was found to have the density 3.36, the calculated value being 3.37.

I n the experiments dealing with the effect of heat on the anhydrous compound, an extraordinary loss in weight in t,he

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JOSEPH AND RAE: C)HROMIUM PHOSPHATE. 201

platinum crucible was noticed. That platinum volatilises in a current of air is well known, and an account of a number of experiments was given by Hulett and Berger ( J . Amer. Chem. SOC., 1904, 26, 1512), and by Burgess and Waltenberg ( J . I d . Eng. Chem., 1916, 8, 487). The greatest loss in any case observed by these workers was 0-0034 gram per 100 sq. cm. per hour a t 1200O with a platinum crucible containing 2.7 per cent. of iridium and using electric heating The losses here referred to were about fifteen times as great as this, and were undoubtedly due to the gas heating. Thus, in one experiment, a crucible weighing 40 grams and having a surface area of about 100 sq. cm. lost 0.6 gram, or 1.5 per cent., after heating for thirty-six hours in a gas furnace with a good draught at, about 1 1 0 0 O . I n another experiment, in which the empty crucible was heated, it lost 0.42 gram in twelve hours; in these two experiments, more than a gram of platinum was removed. The appearance of a platinum mirror on near objects, and the coarsely crystalline appearance of the platinum described by the above observers, was also noticed. When heated in a small electric furnace a t l l O O o with practically no air current, no loss in weight was observed after thirty-five hours' heating.

Amorphous, green, chromium phosphate was prepared by pre- cipitating a hot chrome alum solution with excess of disodium hydrogen phosphate, and washing repeatedly with boiling water until sulphate could not be detected-a very long operation.

With a view t o determine the hydration of the substance, it was placed in a moist state in a desiccator a t the room temperature, when it rapidly lost weight until the composition was CrP0,,4H20 ; it remained practically steady for some days, and was then heated to 60°, when a further loss of two molecules of water took place.

When heated to dull redness, it was converted to brown, amorphous chromium phosphate ; the density of this substance was found to be 2.991, but it is possible that decomposition had started, as in the case of the crystalline variety; this, although only pro- ceeding to a small extent, may have a marked effect in raising the density.

Action of Reagents.-TEe hydrated forms dissolve in sulphuric and hydrochloric acids, the dihydrate being rather difficult to dis- solve in the latter. The anhydrous compound is very refractory, being insoluble in hydrochloric acid or aqua regia, and only attacked by sulphuric acid when nearly boiling. It is then con- verted into an earthy-coloured powder, insoluble in water and acids, which appears to be a compound of chromium phosphate and sulphate of indefinite composition. The hydrated forms dis- solve in strong alkalis, forming chromite solutions ; the anhydrous

VOL. CXI. K

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202 JOSEPH AND RAE : CHROMIUM PHOSPHATE.

substance requires strong heating with lime to bring it into alkaline Bolution. Sodium carbonate solution immediately converts the violet hexahydrate into a green, basic compound retaining alkali, which cannot be removed by washing with water. Samples of the basic compound, after heating in the air, were found to contain sodium chromate. At the room temperature, chrome alum solu- tion soon turns the violet hexahydrate into a green, amorphous, hydrated phosphate, probably identical with that obtained by hot precipitation with excess of sodium phosphate. A considerable amount of this passes into solution, but is precipitated on dilution.

A t it appeared that anhydrous chromium phosphate lost phos- phoric oxide on heating, it was thought worth while to compare this effect with the action of heat on chromium metaphosphate. This substance was prepared according to Maddrell’s method, by heating a solution of chromium hydroxide in excess of phosphoric acid for some hours a t about 300°, a fine green powder being obtained, which was freed from phosphoric acid by boiling with water. The dry metaphosphate had a density of 2.93-practically the @me as that of the orthophosphate-and, after being heated for some time over a Meker burner, it was found to have nearly the same value, namely, 2.96. On heating, it becomes brown, but regains its green colour on cooling.

The relations between the various chromium phosphates is shown diagrammatically below :

Amorphous green tetrahydrate Amorphous violet hexahydrate (precipitated cold) (precipitated hot )

i

I I

J.

+

I j, 4

Crystalline violet 6K0, (2 days in contact with

Amorphous green? 4-0 + Brown anhydrous (incomplete in air at

years).

Crystalline green Crystalline green Crystalline green Amorphous green 4H,O (boil with 2H,O (boil with ?4l&O(two years at 4l&O (long contact

water). acetic anhydride). room temperature with chrome alum, in air). sodium phosphate,

(low red heat) eolution). room temperature in two

I 4 J. .1 Y

or water). I I I -4

.i. Black anhydrous

(low red heat).

Green basic3 (strong heating).

CEYLON MEDI~AL COLLEGE, COLOMBO. [Received, December 6th, 1916.1

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