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Sodium Chromate, Na2CrO4

Sodium Chromate, Na2CrO4, is prepared from chromite by fusion with lime, sodium sulphate, and calcium chloride in an oxidising atmosphere, when the following reaction occurs:

2FeO.Cr2O3 + 4CaO + 7O = 4CaCrO4 + Fe2O3.

The mass is mixed with water and the calculated quantity of sodium hydrogen sulphate, and heated under pressure; after filtration the solution of sodium chromate is fractionally crystallised. If the dichromate is required, the solution of sodium chromate is slightly acidified with sulphuric acid and evaporated until it crystallises.

Sodium chromate may be obtained anhydrous, and in the form of a deca-, hexa-, and tetra-hydrate, according to the conditions of crystallisation. The anhydrous salt, Na2CrO4, separates from its solution on evaporation at a temperature above 62.8° C. It forms yellow crystals in the rhombic system (bipyramidal):

a:b:c = 0.4643:1:0.7991,

isomorphous with anhydrous sodium sulphate, and of density 2.723. It dissolves in water, forming the above-mentioned hydrates. The following table shows the solubility at various temperatures:

Temperature, °C.Grams Na2CrO4 in 100 Grams H2O.Stable Phase.
031.69Na2CrO4.10H2O
1050.0
18.571.5
21.280.6Na2CrO4.6H2O
23.282.7
23.784.3
30.087.38Na2CrO4.4H2O
3691.8
4095.4
45100.8
54.5109.8
59.5114.4
70122.5Na2CrO4
80124.3
100126.0


The transition points have been determined as follows:



Solubility of sodium chromate
Solubility of sodium chromate
The solubility curves for temperatures between 10° C. and 40° C. are shown in the figure.

The decahydrate, Na2CrO4.10H2O, has a density of 1.483, and crystallises in the monoclinic system:

a:b:c = l.1127:1:1.2133; β = 107° 43'.

The crystals are deliquescent, and are isomorphous with sodium sulphate decahydrate. The tetrahydrate crystallises in the same system and class:

a:b:c = 1.1119:1:1.0624; β = 105° 4'.

A double chromate of sodium and potassium, Na2CrO4.K2CrO4, has been described; while the existence of a double salt, 3K2CrO4. Na2CrO4 ("chromglaserite", corresponding with the sulphate glaserite), is also indicated. This salt, of density 2.767, has been described as crystallising in the trigonal and in the monoclinic systems. The crystallographic elements of LiNa3(CrO4)2.6H2O (ditrigonal pyramidal), and (NH4)NaCrO4.2H2O (rhombic bipyramidal), have also been measured.

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