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, °C	Grams Na2Cr2O7 in 100 Grams Solution.	Solid Phase.
0	61.98	Na2Cr2O7.2H2O
18	63.92	Na2Cr2O7.2H2O
34.5	67.36	Na2Cr2O7.2H2O
52	71.76	Na2Cr2O7.2H2O
72	76.9	Na2Cr2O7.2H2O
81	79.8	Na2Cr2O7.2H2O
93	81.19	Na2Cr2O7
98	81.25	Na2Cr2O7

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.