CHROMATES AND DICHROMATES, inorganic chem ical compounds, or salts, in which the element chromium appears as part of the acid or "negative" component of the salt. Chromium trioxide, commonly but incorrectly termed chromic acid, forms no crystalline hydrates with water, though its strongly acid aqueous solution is supposed to contain chromic acid, The salts of this acid are known as chromates. Only normal salts, for example potassium chromate, are known, which are analogous to and frequently isomorphous with normal sulphates. Acid or hydrogen salts, analogous to potassium bisulphate, are unknown. If an equivalent of acid be added to a solution of a normal chromate, the colour changes from yellow to orange-red, and from the mixture dichromates, for example potassium dichro mate, alone can be isolated. If further equivalents of acid or, better, chromium trioxide, be added, the solutions become darker red and trichromates such as potassium trichromate, and tetrachromates such as potassium tetrachromate, are formed. The solid tri- and tetrachromates are de composed on solution in water to form dichromates and free chromic acid.
Normal chromates are known of nearly all the stronger bases, both organic and inorganic, and are generally yellow. Ammonium, lithium, sodium, potassium, cuprous, cupric, rubidium, caesium, magnesium, calcium, zinc, cobalt and nickel chromates are soluble in water ; strontium and the "rare-earth" chromates are sparingly soluble in water; mercuric chromate is decomposed by water, while silver, thallous, mercurous, cadmium, barium and lead chromates are insoluble in water. With the exception of cuprous, mercurous and magnesium dichromates which are unknown, all the metals forming soluble chromates also yield soluble red dichromates. Barium, lead and silver dichromates are known but are decom posed by water to form normal chromates and chromic acid. Cadmium dichromate is soluble in water, whereas mercuric and thallous dichromates are insoluble. Dark red trichromates are known of ammonium, sodium, potassium, rubidium, caesium, thal lous, zinc, cadmium and nickel. Dark red tetrachromates are known of ammonium, sodium, potassium, rubidium and caesium.
Chromates and polychromates are readily reduced by hydro chloric acid, sulphurous acid, hydrogen sulphide, ferrous salts, alcohol, etc., with formation of chromic salts or hydroxide. The chromates of the strongly basic elements are not readily decom posed by heating, but those of the feebler bases yield chromium sesquioxide and oxygen. Ammonium chromate, on heating or exposure to air, loses ammonia and yields the dichromate, which decomposes with explosive violence on further heating, leaving a residue of chromium sesquioxide, while nitrogen, ammonia, oxides of nitrogen, water and oxygen are also formed.
On treatment of chromates or dichromates with concentrated cold solutions of the halogen acids, or phosphorous halides, salts are obtained, derived from hypothetical half-acid halides ; for ex ample, hydrochloric acid and potassium dichromate yield potas sium chlorochromate, a salt of the unknown chlorochromic acid or the half-acid chloride of chromic acid )2. Fluorochromates, chlorochromates, bromochromates and iodochromates of some of the strong bases are known. If chromates or dichromates are treated with a large excess of hydro fluoric or hydrochloric acid, chromyl fluoride, and chromyl chloride, are formed. These compounds, which are red volatile liquids yielding red vapours, are the acid fluoride and acid chloride of chromic acid. The bromine and iodine analogues are unknown.
Though chromates and dichromates are readily decomposed by warm solutions of hydrogen peroxide to form oxygen and chromic salts or chromic hydroxide, cold solutions yield little or no oxygen and form peroxidized compounds, nearly all of which yield hydro gen peroxide by suitable treatment and all readily evolve oxygen. It is accordingly inferred that all contain sexavalent chromium as in chromates and dichromates, the extra equivalents of oxygen being in the same form as in hydrogen peroxide. The highest stage of peroxidation is found in the red perchromates of the general formula These salts are obtained by the action of 30% hydrogen peroxide solutions on alkaline solutions of the chromates at low temperatures. They are unstable at ordinary temperatures and occasionally evolve oxygen explosively. The acid correspond ing to these salts has not been isolated. When 97% hydrogen peroxide is added at —3o° to chromium trioxide, a dark blue crys talline acid is formed, having the empirical formula or An intense blue coloration is formed on adding hydrogen peroxide to an acidified solution of a chromate, the reac tion being sufficiently delicate to detect traces of either reagent. The blue colour is extracted by ether, and the ethereal solution on neutralization by alkalis yields dark blue salts. These salts have the general formula and are as unstable' as the more oxygenated red salts. If the red or blue salts are treated with excess of an organic base such as pyridine, blue or violet salts are obtained having the general formula If the red per chromates are heated with ammonia to about 40°, or with potas sium cyanide solution, derivatives of a chromium peroxide of the formula Cr04 or are obtained. This peroxide may be regarded as an acid anhydride, formed by the loss of a molecule of hydrogen peroxide from two molecules of the acid or one molecule of the dimeric acid The derivatives of this peroxide yield oxygen and chromates with alkalis, and oxygen, hydrogen peroxide and chromic salts with acids.
Chromates and dichromates are all therapeutically dangerous, being strongly caustic and destructive of the red corpuscles of the blood. They have, however, been used as external caustics and antiseptics for ulcers, etc., with success. (J. D. M. S.)