Sodium selenate, Na₂SeO₄

The Sodium selenate, Na₂SeO₄, is prepared from sodium selenite by electrolytic oxidation with platinum electrodes at ordinary temperature. A by-product analogous to the dithionate obtained in the electrolytic oxidation of sodium sulphite is not formed.

Sodium selenate is known in the anhydrous form and as decahydrate. From observations made by Funk, Meyerhoffer gives the solubility of the hydrate in 100 grams of water at 0° C. as 13.25 grams, and at 30° C. as 78.5 grams; and that of the anhydrous selenate at 40° C. as 82.5 grams, and at 100° C. as 72.75 grams. The decrease of solubility of the anhydrous salt with rise of temperature is reminiscent of the behaviour of the corresponding form of sodium sulphate. For the approximate transition- point of the hydrate into the anhydrous compound, Funk gives 32° C. The density of the hydrate is 1.616. For the heat of formation from its elements of the selenate in aqueous solution Thomsen gives 262.3 Cal., and for sodium hydrogen selenate, NaHSeO₄, 203.2 Cal.