Potassium Molybdate, K₂MoO₄

Potassium Molybdate, K₂MoO₄, is obtained by the addition of potassium trimolybdate, in small quantities at a time, to alcoholic caustic potash solution; the oily layer which separates is washed with alcohol and crystallised over sulphuric acid. Another method consists in fusing together suitable proportions of molybdenum trioxide or ammonium molybdate and potassium carbonate, taking up the mass with water and crystallising over sulphuric acid. The solid melts at 919° C., and appears to exist in four different modifications, since it exhibits three transformation points, namely, 327° C., 454° C., and 479° C. The salt is deliquescent and absorbs carbon dioxide from the air. It dissolves readily in water, 100 grams of the saturated solution at 25° C. containing 64.86 grams K₂MoO₄. It is insoluble in alcohol.