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Chromic Orthophosphates

Violet chromic phosphate, CrPO4.6H2O, is produced as a lavender amorphous precipitate when cold solutions of equal weights of chrome alum and ordinary sodium phosphate are mixed:

K2SO4.Cr2(SO4)3 + 2Na2HPO4 + 12H2O = 2(CrPO4.6H2O) + K2SO4 + 2Na2SO4 + H2SO4.

If the precipitate is allowed to remain for a day or two in contact with the solution, it becomes dark violet and crystalline; it may then be washed by decantation and dried in the air. The product is slightly soluble in water; readily soluble in sulphuric and hydrochloric acids; soluble in strong alkalies giving chromites. It is converted into a green basic compound by sodium carbonate solution. Concentrated sulphuric and nitric acids and phosphorus trichloride dehydrate it. At 14° C. its density is 2.121. An amorphous green chromic phosphate is precipitated when excess of sodium phosphate is added to a hot solution of chrome alum; the precipitation is more complete in presence of acetic acid. Under the same conditions solutions of the violet and green chlorides, sulphates, and acetates may be used, but not the oxalates. Rammelsberg considered this precipitate to be the trihydrate; Bloxam gave it the formula 2CrPO4.5H2O; Joseph and Rae, after removing all traces of sulphate by repeated washing with boiling water, and then drying in a desiccator, found its composition to be CrPO4.4H2O, which is converted to the dihydrate, CrPO4.2H2O, at 60° C. Carnot, however, states that when dried at 100° C. the precipitate is the trihydrate, CrPO4.3H2O. Owing to the beautiful green colour of the dried phosphate it is sometimes used as a pigment. It is also used in dyeing, as it can easily be precipitated on fibres.

If the precipitated violet hexahydrate remains in contact with its mother-liquid for a week, the green amorphous tetrahydrate, CrPO4. 4H2O, results, a change which will also occur if the crystals are placed in contact with water, sodium phosphate solution, or chrome alum solution, the last being most favourable. The tetrahydrate is soluble in dilute mineral acids and in alkalies; insoluble in acetic acid; slowly soluble in boiling concentrated hydrochloric acid. Fusion with sodium carbonate causes oxidation, chiefly to sodium chromate; on boiling with nitric acid of density 1.4, and adding portions of potassium chlorate, it is completely oxidised and dissolved. A green crystalline variety of the tetrahydrate is obtained by heating the violet hexahydrate at 100° C., or by boiling with water for half an hour. Its density is 2.10.

A green crystalline dihydrate, CrPO4.2H2O, similar in appearance to the tetrahydrate, and of density 2.42, is obtained by boiling the violet hexahydrate with acetic anhydride, or by heating the hexahydrate in dry air. It dissolves with difficulty in hydrochloric acid, but is readily soluble in sulphuric acid and in strong alkalies. Both the di- and tetra-hydrates are stable in moist air, but the hexahydrate on long standing turns green with loss of water.

Anhydrous Chromic Phosphate, CrPO4, is obtained as a fine black or dark brown amorphous powder by heating any of the above hydrates to dull redness. It is insoluble in hydrochloric acid or aqua regia, and is only attacked by sulphuric acid when nearly boiling, when it is converted to an earthy-coloured powder of indefinite composition, containing phosphate and sulphate, which is insoluble in water and acids. To bring it into solution it is necessary to heat strongly with lime. Its density at 32.5° C. is 2.94, but Joseph and Rae found that on strongly heating the powder loses in weight, and there is a gradual increase in the density of the product, probably due to partial conversion into chromium oxide.

Chromic Orthophosphates
Chromic Orthophosphates
The relations between the various chromic orthophosphates may be indicated diagrammatically in fig.

Acid Chromic Orthophosphate, CrH3(PO4)2.8H2O, has been described. It forms asymmetric crystals of the colour of the chrome alums; stable in air. Vauquelin obtained an emerald green uncrystallisable solution by acting upon the hydrated sesquioxide of chromium with aqueous phosphoric acid.

Several double phosphates have been described. Ammonium chromic phosphate, (NH4)2HPO4.2CrPO4.3H2O, is obtained as a green precipitate when diammonium hydrogen phosphate is added in large excess to a solution of chromic chloride containing hydrochloric acid, but not too strongly acid. If the acidity is reduced until there is very little or no hydrochloric acid present, the basic compound, 5(NH4)H2PO4.2CrPO4.4Cr(OH)3, is formed. Sodium chromic phosphate, Na2HPO4.2CrPO4.5H2O, is precipitated under certain conditions when solutions of chromium salts are boiled with sodium phosphate and acetic acid, but on repeatedly washing the precipitate with water a basic salt is formed.

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