Sodium selenides

The monoselenide, Na₂Se, is formed by the action of selenium on a solution of excess of sodium in liquid ammonia, and separates out. It can also be produced by the interaction of selenium and sodium hyposulphite, Na₂S₂O₄. It melts above 875° C., and on exposure to air its solution in water acquires a reddish colour, and deposits selenium. In solution it is extensively hydrolyzed, and under these conditions may be regarded as a mixture of sodium hydrogen selenide, NaSeH, and sodium hydroxide. Four hydrates are known, with 4½, 9, 10, and 16 molecules of water respectively. For the heat of formation of the anhydrous compound Fabre gives 59.7 Cal., and for the heat of solution at 14° C., 18.6 Cal. He has also investigated the heat of solution of the hydrates.

When excess of selenium reacts with a solution of sodium in liquid ammonia, sodium tetraselenide, Na₂Se₄, separates. By fusion of sodium with selenium, Mathewson has isolated between 500° and 250° C. the diselenide Na₂Se₂, triselenide Na₂Se₃, tetraselenide Na₂Se₄, and hexaselenide Na₂Se₆. They are dark-grey substances, unstable in air, and readily soluble in water to red solutions.