Selenophosphates and Oxyselenophosphates

By the action of phosphorus pentaselenide on aqueous solutions of the selenides of the respective metals, selenophosphates of the alkali and alkaline earth metals and of magnesium can be prepared. The reaction takes place according to the equation:

3M₂Se + P₂Se₅ = 2M₃PSe₄.

The tetraselenophosphate is not obtained in the solid state, however, being decomposed by water, thus:

M₃PSe₄ + H₂O = M₃PSe₃O + H₂Se.

The decomposition may go still further, the compound M₃PSe₂O₂ or even M₃PSeO₃ being formed. Intermediate compounds such as K₃PSe_2.5O_1.5 are also formed, which may be isomorphous mixtures of various oxyselenophosphates.

The oxyselenophosphates may also be prepared by the action of phosphorus pentaselenide on the hydroxides of the metals. Normal salts are only obtained in the case of the alkali metals; the alkaline earth metals give monohydrogen salts. The following oxyselenophosphates have been prepared and described:

Sodium Trioxysele no phosphate, Na₃PSeO₃.20H₂O, obtained by the action of phosphorus pentaselenide on either sodium hydroxide or sodium hydroselenide, crystallises in needles.

Ammonium Oxytriseleno phosphate, obtained from phosphorus pentaselenide and a saturated solution of hydrogen selenide in strong ammonia, crystallises from the warm mother-liquor in leaflets having the composition (NH₄)₃PSe₃O.10H₂O. The cold mother-liquor afterwards deposits octahedra and four-sided leaflets having the composition (NH₄)₅H(PSe₃O)₂.18H₂O.

Potassium Oxyselenophosphate, K₃PSe_2.5.O_1.5.H₂O, is obtained as greenish-yellow octahedra by using potassium hydroselenide.

Barium Dioxydiseleno phosphate, BaHPSe₂O₂.14H₂O, formed when phosphorus pentaselenide is treated either with a solution of barium hydroxide or with a saturated solution of hydrogen selenide in barium hydroxide, is a white, microcrystalline powder.

Strontium Oxyselenophosphate, obtained as a light yellow precipitate by the action of phosphorus pentasulphide on strontium hydroxide and treatment of the filtrate with alcohol, has the composition SrHPSe_2/3O_10/3.7H₂O. Using a large excess of phosphorus pentaselenide the crystals obtained are colourless and have the composition SrHPSe_4/3O_8/3.8H₂O, which may possibly be 2SrHPSeO₃.SrHPSe₂O₂.24H₂O.

Calcium Oxyselenophosphate has been obtained in the same way, using calcium hydroxide instead of strontium hydroxide. With the smaller proportion of phosphorus pentaselenide the crystals have the composition CaHPSe_⅓O_11/3.5H₂O, whilst with the larger proportion their composition is CaHPSeO₃.8H₂O.