Potassium Tetramolybdate, K₂Mo₄O₁₃.7H₂O

Potassium Tetramolybdate, Potassium Metamolybdate, K₂Mo₄O₁₃.7H₂O or K₆H₄[H₂(Mo₂O₇)₆].18H₂O, is obtained as a heavy microcrystalline precipitate on warming a solution of sodium octamolybdate with potassium chloride. It forms transparent prismatic crystals, slightly soluble in water. On boiling with water it yields the trimolybdate. The tetramolybdate is also formed in solution when molybdic acid (4 molecules) is treated with potassium carbonate (1 molecule); the reacting mixture must be kept cool or the trimolybdate is obtained.