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Chromic Sulphate, Cr2(SO4)3

The anhydrous salt is prepared by dehydration of a hydrate in air at 400° C., or in a current of carbon dioxide at temperatures rather above 280° C. It forms bluish-red crystals, of density 3.012, insoluble in water and acids.

Violet Chromium Sulphates. - Chromic sulphate can be obtained in aqueous solution by dissolving chromic hydroxide (dried at 100° C.) in the calculated quantity of warm sulphuric acid, allowing the green solution to stand for a week, when it becomes blue in colour (although reddish by transmitted light) and deposits violet crystals, or by allowing to crystallise a mixture of sulphuric acid and a solution of chrome alum. These regular octahedral crystals, which have density 1.867, are easily soluble in water to a blue solution, but the substance may be reprecipitated by alcohol. Two violet hydrates, viz. Cr2(SO4)3.15H2O and Cr2(SO4)3.17H2O, are definitely known, though the existence of others containing 12H2O, 14H2O, and 16H2O respectively has been suggested. Colson observed that a green solution of chromic sulphate on exposure to sunlight, in becoming turquoise blue in colour, increased in density; it was assumed to contain the compound

Green Chromium Sulphates. - When the solid violet salt is heated above 90° C. a green amorphous mass is obtained. Contrary to the opinion of Wyrouboff, Colson considers that this substance may be regarded as a condensed sulphate; for example,

A green amorphous precipitate is also obtained when a violet aqueous solution is evaporated at temperatures between 70° and 100° C. It has been observed that a violet solution becomes green on the addition of alkalies or alkali carbonates, whereas potassium nitrite and thiocyanate produce the contrary effect. This change from violet to green is accompanied by an increase in acidity, and is apparently due to hydrolysis, together with change of structure - probably as follows:

2Cr2(SO4)3 + H2O ⇔ [Cr4O(SO4)4]SO4 + H2SO4,

a pentasulphate being formed.

This conclusion is based upon considerations of cryoscopic and conductivity measurements, a study of absorption spectra, and upon observations regarding precipitation with barium chloride in aqueous solution. It is found that precipitation often takes place only to a fraction of the theoretical extent; in fact Colson divides normal green chromium sulphates into three classes, which he denotes as trebly, doubly, and singly masked, according as three, two, or one acid radicles are unprecipitated by barium chloride solution in the cold. He suggests that the "masking" is due to the formation of such a group as (OH).Cr.HSO4. Moreover, the barium sulphate precipitated from a solution of the green, but not from one of the violet, salt adsorbs small quantities of a green, strongly basic chromium sulphate.

The green hexahydrate, Cr2(SO4)3.6H2O, or Recoura's sulphate, is obtained by saturating a solution of chromic acid with sulphur dioxide at -4° C. and immediately evaporating. The salt is soluble in water, yielding a green solution which gradually changes to violet.

When heated in dry air at 80° C. a less soluble trihydrate, Cr2(SO4)3.3H2O, results.

It is found that attempts to precipitate the sulphate from Recoura's green sulphate are at first unsuccessful, but that after a time barium sulphate is deposited, the amount increasing at first rapidly, then gradually. The green solutions apparently tend towards a state of equilibrium which is a function of the temperature and concentration, and is the state towards which violet solutions of the same concentration gradually tend to pass. It is probable that the composition of Recoura's green chromic sulphate may be expressed according to Werner's scheme by the formula

The green decahydrate, Cr2(SO4)3.10H2O, separates as an amorphous green hygroscopic mass when the green solution obtained by reducing chromic acid by sulphur dioxide at 0° C. is completely evaporated in vacuo. This compound differs from Recoura's sulphate in that the green colour of its solution is permanent, and one-third of its " sulphate " is precipitated by barium chloride. The constitution may probably be expressed thus:

The solution on boiling is hydrolysed in the same way as the violet solution, and the pentasulphate is formed. The freezing-point of the solution is unaltered by boiling, so that no change in the number of molecules present occurs.

Colson considers that a cold solution of chromic sulphate is an equilibrium mixture of the violet sulphate with three green sulphates, to which he gave the formulae Cr2(SO4)3.6H2O, Cr2(SO4)2.(OH).(HSO4), and Cr2(SO4)(OH)2(HSO4)2. As to the green sulphate obtained by heating the solid violet salt, it has been stated that it may exist in solution at the same temperature in two distinct molecular aggregations, [Cr2(SO4)3]2 and Cr2(SO4)3.

By allowing a 0.5 N solution of chromium sulphate to stand until the state of equilibrium is attained, then evaporating in vacuo to the point of saturation, and adding excess of alcohol, Recoura obtained a lilac-grey precipitate, which, after washing with ether, was found to contain 18H2O. On addition of barium chloride to its solution the whole of the sulphate ion was precipitated. After exposure to air for one day, however, it was found to have lost 2H2O, and the addition of barium chloride to its solution produced no precipitate. After remaining in a desiccator for some time, the compound contained only 12H2O. The constitution of these products has not been determined.

In fresh solutions of the green sulphate, the latter has the power of masking the sulphate ion in other metallic sulphates to the extent of several hundred molecules for each molecule of the green salt present, the actual amount depending on the age of the green solution, its concentration and degree of acidity.

A dihydroxypenta-sulphate, Cr4(SO4)5(OH)2, separates as an amorphous substance when the green solution, obtained by shaking chromium hydroxide with a limited quantity of cold dilute sulphuric acid, is evaporated in vacuo. Only three of the five SO4 groups present are immediately precipitated by barium chloride, but the resistance to precipitation diminishes with rise in temperature or increase of concentration. The following constitution is suggested by Colson:

A number of basic chromium sulphates have been described -
  1. 3Cr2O3.2SO3.14H2O;
  2. Cr2O3.SO3.6H2O;
  3. 2Cr2O3.3SO3.xH2O;
  4. 5Cr2O3.8SO3.xH2O;
  5. Cr2O3.2SO3.5H2O;
  6. 5Cr2O3.12SO3.xH2O;
  7. 7Cr2O3.5SO3.25H2O - as well as the acid sulphates, 2Cr2(SO4)3.H2SO4 and 2Cr2(SO4)3.7H2SO4.
Chromisulphuric Acids, H2(Cr2.4SO4), H4(Cr2.5SO4), and H6(Cr2.6SO4), are obtained by drying at 110° to 120° C. the green residual mass after heating on the water-bath 1 molecule of chromic sulphate with 1, 2, or 3 molecules of sulphuric acid. The acids, which yield metallic salts, are stable amorphous powders when dry; in solution they gradually undergo dissociation. The suggestion has been made that they may be regarded as negative colloids.

A number of double sulphates of trivalent chromium with other metals are known.

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