Potassium Dichromate, K₂Cr₂O₇

Potassium Dichromate, K₂Cr₂O₇, may be prepared from the sodium salt and potassium chloride, or by a direct method. In the latter case, chrome iron ore is calcined in the air with calcium carbonate, and the mass extracted with water containing a little sulphuric acid. Potassium carbonate is then added, and, after filtration and addition of sulphuric acid, the potassium dichromate is separated by fractional crystallisation. An alternative method consists in heating chromite with calcium carbonate and potassium sulphate, and lixiviating the mass with water. Oxidation of chromite by means of fused potassium nitrate is sometimes convenient, especially upon a small scale. Potassium dichromate is formed by the addition of any acid to a solution of potassium chromate.

Dark orange in colour, potassium dichromate exists in two crystalline forms, the stable form belonging, apparently, to the asymmetric class of the triclinic system:

a:b:c = 0.5575:1:0.5511; α = 82° 0'; β = 90° 51'; γ = 83° 47'.

An unstable monoclinic form, isomorphous with rubidium and ammonium dichromates, has been obtained admixed with the triclinic variety, by concentrating, after filtration from aluminium hydroxide, a mixture of potassium chromate and aluminium sulphate, and allowing the solution to crystallise. It may also be obtained in yellowish brown, doubly refracting monoclinic plates by crystallisation of a mixture of solutions of potassium dichromate and potassium thiocyanate.

The stable form of potassium dichromate has a density of 2.67, while the monoclinic variety has a density of 2.10. The specific heat of the former, according to Regnault, is 0.1894. It melts at 397° C., its heat of fusion being 8.77 Calories per gram-molecule.

Potassium dichromate is soluble in water to the extent indicated in the subjoined table:

Temperature, ° C.	0	10	20	30	40	50	60	70	80	90	100
Grams K2Cr2O7 per 100 grams solution	4.43	7.83	11.60	15.40	22.60	27.0	31.30	37.0	42.20	47.0	50.50

A saturated aqueous solution boils at 103° C. The aqueous solution exhibits an acid reaction with litmus paper, has a sharp, metallic taste, and is, of course, poisonous. It absorbs nitric oxide with eventual precipitation of chromium dioxide. Unlike the normal salt, potassium dichromate is strongly adsorbed in solution by animal charcoal.

A concentrated aqueous solution, when mixed with concentrated sulphuric acid and cooled, yields crystals of chromium trioxide; if strongly heated with sulphuric acid, the products are potassium hydrogen sulphate, chromic sulphate, water, and oxygen. At a white heat potassium dichromate itself decomposes, yielding the normal chromate, chromium sesquioxide, and oxygen. It is readily reduced by the usual agents, e.g. by carbon, sulphur, phosphorus, magnesium, hydrogen sulphide, sulphur dioxide (dithionate being formed as well as sulphate), by solid oxalic acid, and by stannous chloride, also electrolytically.

With concentrated hydrochloric acid, or with phosphorus trichloride, potassium chlorochromate is produced. A compound, K₂Cr₂O₇. HgCl₂, has been described. Compounds of dichromates and pyridine have been prepared.

Commercial potassium dichromate is almost chemically pure; it is used in photography, since, in presence of organic substances, it is affected by light.