Potassium fluosilicate, K₂SiSiF₆

The Potassium fluosilicate, K₂SiF₆, is precipitated in the form of microscopic crystals of slight solubility by the action of fluosilicic acid on solutions of potassium salts:

H₂SiF₆ + 2KCl = K₂SiF₆ + 2HCl.

It is decomposed by alkali-metal hydroxides with formation of a fluoride and silicic acid, and can be estimated by this method with phenolphthalein as indicator:

4KOH + K₂SiF₆ - 6KF + H₄SiO₄.