| Chromic hydroxide | Cr(OH)3. |
| Chromic chloride | CrCl3·6H2O. |
| Chromic sulphate | Cr2(SO4)3. |
| Chrome alums |
Chromic hydroxide (Cr(OH)3). This substance, being insoluble, can be obtained by precipitating a solution of the chloride or sulphate with a soluble hydroxide. It is a greenish substance which, like aluminium hydroxide, dissolves in alkalis, forming soluble salts.
Dehydration of chromium hydroxide. When heated gently chromic hydroxide loses a part of its oxygen and hydrogen, forming the substance CrO·OH, which, like the corresponding aluminium compound, has more pronounced acid properties than the hydroxide. It forms a series of salts very similar to the spinels; chromite is the ferrous salt of this acid, having the formula Fe(CrO2)2. When heated to a higher temperature chromic hydroxide is completely dehydrated, forming the trioxide Cr2O3. This resembles the corresponding oxides of aluminium and iron in many respects. It is a bright green powder, and when ignited strongly becomes almost insoluble in acids, as is also the case with aluminium oxide.
Chromic sulphate (Cr2(SO4)3). This compound is a violet-colored solid which dissolves in water, forming a solution of the same color. This solution, however, turns green on heating, owing to the formation of basic salts. Chromic sulphate, like ferric and aluminium sulphates, unites with the sulphates of the alkali metals to form alums, of which the best known are potassium chrome alum (KCr(SO4)2·12H2O) and ammonium chrome alum (NH4Cr(SO4)2·12H2O).
These form beautiful dark purple crystals and have some practical uses in the tanning industry and in photography. A number of the salts of chromium are also used in the dyeing industry, for they hydrolyze like aluminium salts and the hydroxide forms a good mordant.
Hydrolysis of chromium salts. When ammonium sulphide is added to a solution of a chromium salt, such as the sulphate, chromium hydroxide precipitates instead of the sulphide. This is due to the fact that chromic sulphide, like aluminium sulphide, hydrolyzes in the presence of water, forming chromic hydroxide and hydrosulphuric acid. Similarly, a soluble carbonate precipitates a basic carbonate of chromium.
Compounds containing chromium as an acid-forming element. Like manganese, chromium forms two unstable acids, namely, chromic acid and dichromic acid. Their salts, the chromates and dichromates, are important compounds.
Chromates. When a chromium compound is fused with an alkali and an oxidizing agent a chromate is produced. When potassium hydroxide is used as the alkali the equation is
2Cr(OH)3 + 4KOH + 3O = 2K2CrO4 + 5H2O.