A sample of pure sodium fluorosilicate (Na₂SiF₆), not contaminated with silicic acid, was prepared and used as a standard to develop a potentiometric technique for its alkali titration and determination of the silicic acid content. A surprising difference in the end point and the equivalence point of titration was observed. Influence of the dissolved colloidal silicic acid on the results of this titration was investigated. It is shown that no hitherto well-known analytical procedure can give the correct result of this titration, and that only precise potentiometric titration could explain the obtained results.