Chromic Hydroxide, Cr₂O₃

Chromic Hydroxide, Cr₂O₃.Aq., is obtained by precipitation of a solution of a chromic salt by means of potassium hydroxide; if excess is used, the precipitate dissolves, forming a green solution from which the hydroxide is again precipitated on keeping or boiling. Such precipitate retains alkali which cannot be removed by repeated washing with hot water; it is usual, therefore, to employ ammonium hydroxide as the precipitating agent. Even in this case, excess of ammonia dissolves chromic hydroxide, yielding a reddish-violet solution: methyl-amine behaves similarly, but di- and tri-methylamine at once precipitate chromic hydroxide completely. When freshly formed, the precipitate appears to be a well-defined chemical compound, the solubility product of which, according to Bjerrum, is 4.2×10^-16 at 0° C. and 54×10^-16 at 17° C. in 0.0001 molar units. According to Weiser, however, the precipitate obtained by the addition of alkali to solutions of chromic salts does not contain any definite hydrate. In the cold, the freshly formed precipitate readily dissolves in acids, but becomes insoluble on keeping or heating; between these two extremes of solubility an indefinite number of hydrous oxides exists. By precipitating at temperatures ranging from 0° C. to 225° C., products have been obtained varying in colour from greyish blue to bright green. Since most of the chromic salts exist in two distinct modifications, the violet and the green, it has long been assumed that there must be two isomeric chromic hydroxides corresponding to these two series of salts. Such isomerides have not, however, been isolated. The properties of chromic hydroxide vary considerably with age, especially as regards solubility in acids and alkalies, and the "ageing" is accelerated if the precipitate is allowed to remain under alkaline solution; the rate appears to increase with hydroxyl-ion concentration and also with increase of temperature, and the change in properties appears to be due to change in the size of the particles.

Chromium hydroxide is not precipitated by ammonia in presence of tartaric acid or glycerol, probably owing to complex formation in the case of the former, but with glycerol a colloidal solution appears to be formed.

An alkaline solution of chromic hydroxide dissolves various metallic oxides. If the hydroxide is precipitated in the presence of other salts, for example, of magnesium, calcium, zinc, or lead, the hydroxides of these metals are also carried down; it has been observed that magnesium hydroxide is thus adsorbed far more readily by violet than by green chromic hydroxide.

Hydrated chromium sesquioxide is an amorphous, green, very hygroscopic powder, which on heating in the air first forms the "dioxide," and finally the green anhydrous sesquioxide. Whereas the hepta- and tetra-hydrates, Cr₂O₃.7H₂O and Cr₂O₃.4H₂O (the former being converted into the latter in vacuo), are soluble in dilute acids, the monohydrate, Cr₂O₃.H₂O, is insoluble.

The hydrosol is obtained as a deep green solution by the peptisation of the hydroxide by means of chromic chloride, or by a solution of copper oxide in ammonia. As already stated, the freshly precipitated hydroxide forms an apparently clear green solution with excess of an alkali hydroxide. That the chromic hydroxide is peptised and not dissolved is shown by the fact that it can be completely filtered out by means of a collodion filter, leaving a colourless filtrate. The colloidal solution is stable while hot, but slowly yields a gel on keeping at ordinary temperatures.

When potassium hydroxide is added to solutions containing ferric chloride and chromium sulphate in varying proportions, the iron is not precipitated so long as the chromium is present in excess. This is due to the protective action of the colloidal chromic hydroxide, the latter being adsorbed by the ferric hydroxide which is thereby prevented from coagulating. If the iron is in excess, it is precipitated, bringing down the adsorbed chromium with it.

Both positively and negatively charged colloids have been prepared. The former results when the hydrated oxide is peptised with chromic chloride, or may be formed by hydrolysis of the chloride or nitrate; the latter is prepared by peptising the hydrous oxide with sodium or potassium hydroxide, or by adding sodium hydroxide to chromium nitrate solution in presence of arsenious acid and then dialysing.

Chromic oxide jellies may be formed by adding sodium or potassium hydroxide or ammonia to a solution of chromic sulphate or chloride containing sodium acetate; or by adding sodium or potassium hydroxide, but not ammonia, to a solution of chrome alum. The jelly is violet if prepared by the addition of ammonia or of a slight excess of the alkali metal hydroxide; if the latter is added in larger quantity the jelly is green. The jellies dissolve in hydrochloric acid, but re-form on neutralising the solution if sufficient sodium acetate is present.

Guignet's Green is an emerald-green hydroxide, having the composition Cr₂O₃.2H₂O, obtained by heating to redness 10 parts of potassium dichromate with 18 parts of crystallised boric acid, and decomposing the double borate of chromium and potassium with boiling water. It is insoluble in alkalies and in nitric acid, and is only slowly dissolved by boiling hydrochloric acid, while by hot sulphuric acid it is converted into an insoluble sulphate. It is used industrially as a pigment.