Chemical elements
    Physical Properties
    Chemical Properties
      Chromous Fluoride
      Chromic Fluoride
      Chromyl Fluoride
      Chromous Chloride
      Chromic Chloride
      Chromyl Chloride
      Trichromyl Chloride
      Chromium Chlorate
      Chromium Perchlorate
      Chromous Bromide
      Chromic Bromide
      Complex Halogen-halides
      Chromous Iodide
      Chromic Iodide
      Chromium Iodate
      Chromous Oxide
      Chromo-chromic Oxides
      Chromic Oxide
      Chromic Hydroxide
      Barium Chromite
      Cadmium Chromite
      Calcium Chromite
      Cobalt Chromite
      Cupric Chromite
      Cuprous Chromite
      Ferrous Chromite
      Lithium Chromite
      Magnesium Chromite
      Manganese Chromite
      Zinc Chromite
      Chromium Dioxide
      Chromium Trioxide
      Aluminium Chromate
      Ammonium Chromate
      Ammonium Lithium Chromate
      Ammonium Potassium Chromate
      Ammonium Sodium Chromate
      Ammonium Dichromate
      Ammonium Fluochromate
      Barium Chromate
      Barium Dichromate
      Barium Potassium Trichromate
      Beryllium Chromate
      Bismuth Chromate
      Bismuth Potassium Chromates
      Cadmium Chromate
      Cadmium Dichromate
      Cadmium Trichromate
      Caesium Chromate
      Caesium Dichromate
      Calcium Chromate
      Calcium Dichromate
      Cobalt Chromate
      Cobalt Dichromate
      Copper Chromates
      Cupric Dichromate
      Gold Chromates
      Iron Chromates
      Ferric Chromate
      Ferric Ammonium Chromate
      Lead Chromate
      Basic Lead Chromates
      Lead Dichromate
      Lithium Chromate
      Lithium Chlorochromate
      Magnesium Chromates
      Manganese Chromates
      Mercuric Chromate
      Mercuric Dichromate
      Nickel Chromate
      Nickel Dichromate
      Potassium Chromate
      Potassium Dichromate
      Potassium Trichromate
      Potassium Tetrachromate
      Potassium Fluochromate
      Potassium Chlorochromate
      Rubidium Dichromate
      Silver Chromate
      Silver Dichromate
      Sodium Chromate
      Sodium Dichromate
      Sodium Trichromate
      Sodium Chlorochromate
      Stannic Chromate
      Strontium Chromate
      Strontium Dichromate
      Strontium Trichromate
      Thallous Chromate
      Thallic Chromate
      Thallous Dichromate
      Thallous Trichromate
      Thallous Chlorochromate
      Uranyl Chromate
      Zinc Chromate
      Zinc Dichromate
      Zinc Trichromate
      Perchromic Acid
      Chromium Tetroxide Triammine
      Chromous Sulphide
      Chromium Tetrasulphide
      Chromic Sulphide
      Sodium Thiochromite
      Potassium Thiodichromite
      Chromic Sulphite
      Chromous Sulphate
      Chromic Sulphate
      Lithium Chromic Sulphate
      Sodium Chromic Sulphates
      Potassium Chromic Sulphates
      Potassium Chromium Alum
      Ammonium Chromium Alum
      Hydrazine Chromium Alum
      Sulphochromic Acid
      Chromous Selenide
      Chromic Selenide
      Chromic Selenite
      Chromium Nitrides
      Chromium Azide
      Chromic Nitrate
      Chromium Monophosphide
      Chromium Sesquiphosphide
      Chromic Hypophosphite
      Chromous Orthophosphate
      Chromic Orthophosphates
      Chromous Metaphosphate
      Chromic Metaphosphate
      Chromic Pyrophosphate
      Ammonium Chromi-pyrophosphate
      Potassium Chromi-pyrophosphate
      Sodium Chromi-pyrophosphate
      Chromous Thiophosphite
      Chromous Thiopyrophosphite
      Chromous Thiopyrophosphate
      Chromous Arsenide
      Chromium Sesqui-arsenide
      Chromic Arsenite
      Chromic Arsenate
      Chromium Pyroarsenate
      Chromic Thioarsenite
      Chromium Chlorantimonate
      Chromium Orthoantimonichloride
      Tetrachromium Carbide
      Pentachromium Dicarbide
      Tetrachromium Dicarbide
      Chromium Tungsten Carbide
      Chromous Carbonate
      Ammonium Chromous Carbonate
      Potassium Chromous Carbonate
      Sodium Chromous Carbonate
      Chromic Carbonates
      Chromium Thiocarbonate
      Chromous Cyanide
      Chromic Cyanide
      Potassium Chromocyanide
      Hydrogen Chromicyanide
      Ammonium Chromicyanide
      Lithium Chromicyanide
      Sodium Chromicyanide
      Potassium Chromicyanide
      Cobaltous Chromicyanide
      Cupric Chromicyanide
      Lead Chromicyanide
      Manganous Chromicyanide
      Mercury Chromicyanide
      Nickel Chromicyanide
      Silver Chromicyanide
      Zinc Chromicyanide
      Chromous Thiocyanate
      Chromic Thiocyanate
      Chromithiocyanic Acid
      Ammonium Chromithiocyanate
      Sodium Chromithiocyanate
      Potassium Chromithiocyanate
      Barium Chromithiocyanate
      Silver Chromithiocyanate
      Lead Chromithiocyanate
      Chromium Ferrocyanide
      Trichromium Silicide
      Dichromium Silicide
      Trichromium Disilicide
      Chromium Disilicide
      Chromium Aluminium Silicide
      Sodium Chromisilicates
      Chromium Silicofluoride
      Chromium Boride
      Trichromium Diboride
      Chromous Borate
      Chromic Borate
    PDB 1huo-9icc

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.0102030405060708090100
Grams K2Cr2O7 per 100 grams solution4.437.8311.6015.4022.6027.031.3037.042.2047.050.50

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.

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