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

Sodium Dichromate, Na2Cr2O7.2H2O






Sodium Dichromate, Na2Cr2O7.2H2O, may be prepared as described above, or by methods similar to those described for potassium dichromate. It forms hyacinth-red, slender prisms, crystallising in the monoclinic system (prismatic):

a:b:c = 0.5698:1:1.1824; β = 94° 55',

and of density 2.525. It is very soluble in water, the solubility being as follows (see figure and table):

Temperature, °CGrams Na2Cr2O7 in 100 Grams Solution.Solid Phase.
061.98Na2Cr2O7.2H2O
1863.92Na2Cr2O7.2H2O
34.567.36Na2Cr2O7.2H2O
5271.76Na2Cr2O7.2H2O
7276.9Na2Cr2O7.2H2O
8179.8Na2Cr2O7.2H2O
9381.19Na2Cr2O7
9881.25Na2Cr2O7


Solubility of sodium dichromate
Solubility of sodium dichromate.
The density of the saturated solution at 18° C. is 1.745. It melts at 320° C. and decomposes at 400° C.

Sodium dichromate plays an important part in the tanning of leather. There are two methods of chrome tanning in general use - the two-bath process and the one-bath process. The former was invented by Schulze in 1884, and consists in first treating the skins with a weak bath of sodium dichromate acidified with a mineral acid, when chromic acid is absorbed, colouring the skins bright yellow but producing no tanning effect. The acid is then reduced by transferring the skins to a second bath containing an acidified solution of sodium thiosulphate, when basic sulphates are produced which immediately convert the skins into leather, the colour of the latter becoming brown probably owing to the formation of a basic chromium chromate, Cr2CrO4(OH)4, a compound which on heating to about 120° C. yields chromium dioxide. The reactions taking place in the second bath are very complicated; sulphur is deposited on the skins, and sodium tetrathionate is formed in the bath. It has been shown by Stiasny that the tanning is due to basic chromium sulphates even when hydrochloric acid is used in the first bath. The following equations probably show the course of the reactions:

(1) 3CrO3 + 6HCl + 6Na2S2O3 = 3Na2S4O6 + 6NaCl + H2O + Cr2CrO4(OH)4.

On further additions of acid the skins brighten in colour, chromic chloride being formed, and sulphur is deposited:

(2) 2CrO3 + 12HCl + 6Na2S2O3 = 3Na2S4O6 + 6NaCl + 6H2O + 2CrCl3.

(3) 2CrO3 + 6HCl + 3Na2S2O3 = 3Na2SO4 + 3S + 3H2O + 2CrCl3.

Chromic sulphate is produced by the action of the oxidised thiosulphate, and the basic salt results after the free hydrochloric acid has been used up, thus:

(4) Cr2(SO4)3 + Na2S2O3 + H2O = 2Cr(OH)SO4 + SO2 + S + Na2SO4,

sulphur being again deposited, chiefly within the skins. The softness of the leathers produced by this process is largely due to this sulphur, which acts as a filling and fibre-isolating substance, but such leathers made in imitation of chamois cannot be used for polishing silver.

In the one-bath process the skins are immersed directly in a solution of chromic sulphate to which a certain amount of alkali has been added. The action depends upon absorption of colloidal basic sulphate.


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