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

Chromic Sulphate, Cr2(SO4)3

The anhydrous salt is prepared by dehydration of a hydrate in air at 400° C., or in a current of carbon dioxide at temperatures rather above 280° C. It forms bluish-red crystals, of density 3.012, insoluble in water and acids.

Violet Chromium Sulphates. - Chromic sulphate can be obtained in aqueous solution by dissolving chromic hydroxide (dried at 100° C.) in the calculated quantity of warm sulphuric acid, allowing the green solution to stand for a week, when it becomes blue in colour (although reddish by transmitted light) and deposits violet crystals, or by allowing to crystallise a mixture of sulphuric acid and a solution of chrome alum. These regular octahedral crystals, which have density 1.867, are easily soluble in water to a blue solution, but the substance may be reprecipitated by alcohol. Two violet hydrates, viz. Cr2(SO4)3.15H2O and Cr2(SO4)3.17H2O, are definitely known, though the existence of others containing 12H2O, 14H2O, and 16H2O respectively has been suggested. Colson observed that a green solution of chromic sulphate on exposure to sunlight, in becoming turquoise blue in colour, increased in density; it was assumed to contain the compound

Green Chromium Sulphates. - When the solid violet salt is heated above 90° C. a green amorphous mass is obtained. Contrary to the opinion of Wyrouboff, Colson considers that this substance may be regarded as a condensed sulphate; for example,

A green amorphous precipitate is also obtained when a violet aqueous solution is evaporated at temperatures between 70° and 100° C. It has been observed that a violet solution becomes green on the addition of alkalies or alkali carbonates, whereas potassium nitrite and thiocyanate produce the contrary effect. This change from violet to green is accompanied by an increase in acidity, and is apparently due to hydrolysis, together with change of structure - probably as follows:

2Cr2(SO4)3 + H2O ⇔ [Cr4O(SO4)4]SO4 + H2SO4,

a pentasulphate being formed.

This conclusion is based upon considerations of cryoscopic and conductivity measurements, a study of absorption spectra, and upon observations regarding precipitation with barium chloride in aqueous solution. It is found that precipitation often takes place only to a fraction of the theoretical extent; in fact Colson divides normal green chromium sulphates into three classes, which he denotes as trebly, doubly, and singly masked, according as three, two, or one acid radicles are unprecipitated by barium chloride solution in the cold. He suggests that the "masking" is due to the formation of such a group as (OH).Cr.HSO4. Moreover, the barium sulphate precipitated from a solution of the green, but not from one of the violet, salt adsorbs small quantities of a green, strongly basic chromium sulphate.

The green hexahydrate, Cr2(SO4)3.6H2O, or Recoura's sulphate, is obtained by saturating a solution of chromic acid with sulphur dioxide at -4° C. and immediately evaporating. The salt is soluble in water, yielding a green solution which gradually changes to violet.

When heated in dry air at 80° C. a less soluble trihydrate, Cr2(SO4)3.3H2O, results.

It is found that attempts to precipitate the sulphate from Recoura's green sulphate are at first unsuccessful, but that after a time barium sulphate is deposited, the amount increasing at first rapidly, then gradually. The green solutions apparently tend towards a state of equilibrium which is a function of the temperature and concentration, and is the state towards which violet solutions of the same concentration gradually tend to pass. It is probable that the composition of Recoura's green chromic sulphate may be expressed according to Werner's scheme by the formula

The green decahydrate, Cr2(SO4)3.10H2O, separates as an amorphous green hygroscopic mass when the green solution obtained by reducing chromic acid by sulphur dioxide at 0° C. is completely evaporated in vacuo. This compound differs from Recoura's sulphate in that the green colour of its solution is permanent, and one-third of its " sulphate " is precipitated by barium chloride. The constitution may probably be expressed thus:

The solution on boiling is hydrolysed in the same way as the violet solution, and the pentasulphate is formed. The freezing-point of the solution is unaltered by boiling, so that no change in the number of molecules present occurs.

Colson considers that a cold solution of chromic sulphate is an equilibrium mixture of the violet sulphate with three green sulphates, to which he gave the formulae Cr2(SO4)3.6H2O, Cr2(SO4)2.(OH).(HSO4), and Cr2(SO4)(OH)2(HSO4)2. As to the green sulphate obtained by heating the solid violet salt, it has been stated that it may exist in solution at the same temperature in two distinct molecular aggregations, [Cr2(SO4)3]2 and Cr2(SO4)3.

By allowing a 0.5 N solution of chromium sulphate to stand until the state of equilibrium is attained, then evaporating in vacuo to the point of saturation, and adding excess of alcohol, Recoura obtained a lilac-grey precipitate, which, after washing with ether, was found to contain 18H2O. On addition of barium chloride to its solution the whole of the sulphate ion was precipitated. After exposure to air for one day, however, it was found to have lost 2H2O, and the addition of barium chloride to its solution produced no precipitate. After remaining in a desiccator for some time, the compound contained only 12H2O. The constitution of these products has not been determined.

In fresh solutions of the green sulphate, the latter has the power of masking the sulphate ion in other metallic sulphates to the extent of several hundred molecules for each molecule of the green salt present, the actual amount depending on the age of the green solution, its concentration and degree of acidity.

A dihydroxypenta-sulphate, Cr4(SO4)5(OH)2, separates as an amorphous substance when the green solution, obtained by shaking chromium hydroxide with a limited quantity of cold dilute sulphuric acid, is evaporated in vacuo. Only three of the five SO4 groups present are immediately precipitated by barium chloride, but the resistance to precipitation diminishes with rise in temperature or increase of concentration. The following constitution is suggested by Colson:

A number of basic chromium sulphates have been described -
  1. 3Cr2O3.2SO3.14H2O;
  2. Cr2O3.SO3.6H2O;
  3. 2Cr2O3.3SO3.xH2O;
  4. 5Cr2O3.8SO3.xH2O;
  5. Cr2O3.2SO3.5H2O;
  6. 5Cr2O3.12SO3.xH2O;
  7. 7Cr2O3.5SO3.25H2O - as well as the acid sulphates, 2Cr2(SO4)3.H2SO4 and 2Cr2(SO4)3.7H2SO4.
Chromisulphuric Acids, H2(Cr2.4SO4), H4(Cr2.5SO4), and H6(Cr2.6SO4), are obtained by drying at 110° to 120° C. the green residual mass after heating on the water-bath 1 molecule of chromic sulphate with 1, 2, or 3 molecules of sulphuric acid. The acids, which yield metallic salts, are stable amorphous powders when dry; in solution they gradually undergo dissociation. The suggestion has been made that they may be regarded as negative colloids.

A number of double sulphates of trivalent chromium with other metals are known.

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