Chemical elements
  Chromium
    Isotopes
    Energy
    Occurrence
    Preparation
    Physical Properties
    Chemical Properties
    Alloys
    Amalgams
    Compounds
      Chromous Fluoride
      Chromic Fluoride
      Chromyl Fluoride
      Chromous Chloride
      Chromic Chloride
      Oxychlorides
      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
    Detection
    Estimation
    PDB 1huo-9icc

Chromic Orthophosphates






Violet chromic phosphate, CrPO4.6H2O, is produced as a lavender amorphous precipitate when cold solutions of equal weights of chrome alum and ordinary sodium phosphate are mixed:

K2SO4.Cr2(SO4)3 + 2Na2HPO4 + 12H2O = 2(CrPO4.6H2O) + K2SO4 + 2Na2SO4 + H2SO4.

If the precipitate is allowed to remain for a day or two in contact with the solution, it becomes dark violet and crystalline; it may then be washed by decantation and dried in the air. The product is slightly soluble in water; readily soluble in sulphuric and hydrochloric acids; soluble in strong alkalies giving chromites. It is converted into a green basic compound by sodium carbonate solution. Concentrated sulphuric and nitric acids and phosphorus trichloride dehydrate it. At 14° C. its density is 2.121. An amorphous green chromic phosphate is precipitated when excess of sodium phosphate is added to a hot solution of chrome alum; the precipitation is more complete in presence of acetic acid. Under the same conditions solutions of the violet and green chlorides, sulphates, and acetates may be used, but not the oxalates. Rammelsberg considered this precipitate to be the trihydrate; Bloxam gave it the formula 2CrPO4.5H2O; Joseph and Rae, after removing all traces of sulphate by repeated washing with boiling water, and then drying in a desiccator, found its composition to be CrPO4.4H2O, which is converted to the dihydrate, CrPO4.2H2O, at 60° C. Carnot, however, states that when dried at 100° C. the precipitate is the trihydrate, CrPO4.3H2O. Owing to the beautiful green colour of the dried phosphate it is sometimes used as a pigment. It is also used in dyeing, as it can easily be precipitated on fibres.

If the precipitated violet hexahydrate remains in contact with its mother-liquid for a week, the green amorphous tetrahydrate, CrPO4. 4H2O, results, a change which will also occur if the crystals are placed in contact with water, sodium phosphate solution, or chrome alum solution, the last being most favourable. The tetrahydrate is soluble in dilute mineral acids and in alkalies; insoluble in acetic acid; slowly soluble in boiling concentrated hydrochloric acid. Fusion with sodium carbonate causes oxidation, chiefly to sodium chromate; on boiling with nitric acid of density 1.4, and adding portions of potassium chlorate, it is completely oxidised and dissolved. A green crystalline variety of the tetrahydrate is obtained by heating the violet hexahydrate at 100° C., or by boiling with water for half an hour. Its density is 2.10.

A green crystalline dihydrate, CrPO4.2H2O, similar in appearance to the tetrahydrate, and of density 2.42, is obtained by boiling the violet hexahydrate with acetic anhydride, or by heating the hexahydrate in dry air. It dissolves with difficulty in hydrochloric acid, but is readily soluble in sulphuric acid and in strong alkalies. Both the di- and tetra-hydrates are stable in moist air, but the hexahydrate on long standing turns green with loss of water.

Anhydrous Chromic Phosphate, CrPO4, is obtained as a fine black or dark brown amorphous powder by heating any of the above hydrates to dull redness. It is insoluble in hydrochloric acid or aqua regia, and is only attacked by sulphuric acid when nearly boiling, when it is converted to an earthy-coloured powder of indefinite composition, containing phosphate and sulphate, which is insoluble in water and acids. To bring it into solution it is necessary to heat strongly with lime. Its density at 32.5° C. is 2.94, but Joseph and Rae found that on strongly heating the powder loses in weight, and there is a gradual increase in the density of the product, probably due to partial conversion into chromium oxide.

Chromic Orthophosphates
Chromic Orthophosphates
The relations between the various chromic orthophosphates may be indicated diagrammatically in fig.

Acid Chromic Orthophosphate, CrH3(PO4)2.8H2O, has been described. It forms asymmetric crystals of the colour of the chrome alums; stable in air. Vauquelin obtained an emerald green uncrystallisable solution by acting upon the hydrated sesquioxide of chromium with aqueous phosphoric acid.

Several double phosphates have been described. Ammonium chromic phosphate, (NH4)2HPO4.2CrPO4.3H2O, is obtained as a green precipitate when diammonium hydrogen phosphate is added in large excess to a solution of chromic chloride containing hydrochloric acid, but not too strongly acid. If the acidity is reduced until there is very little or no hydrochloric acid present, the basic compound, 5(NH4)H2PO4.2CrPO4.4Cr(OH)3, is formed. Sodium chromic phosphate, Na2HPO4.2CrPO4.5H2O, is precipitated under certain conditions when solutions of chromium salts are boiled with sodium phosphate and acetic acid, but on repeatedly washing the precipitate with water a basic salt is formed.


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