A study of the titrimetric potassium silicofluoride method for determination of silica in aluminosilicate materials

A critical study of the titrimetric potassium silicofluoride method for determination of silica reveals that quantitative precipitation of the sificofluoride is possible in 3.5-7.5N acid containing 1% sodium fluoride and >/ 12% potassium chloride. An aqueous wash solution (pH approximately 5.3) containing 0.1% sodium fluoride and 12% potassium chloride has been found ideal for washing the precipitate without significant hydrolysis even at 35 degrees C. The interference of large amounts of aluminium (up to 160 mg of Al(2)O(3)) is eliminated by precipitating the silicofluoride in 6-7.5N acid solution, the aluminium then forming practically no fluoro-complex, because of extensive protonation of fluoride. Interference by large amounts of boron (up to 160 mg of B(2)O(3)) is eliminated by precipitating silicofluoride with the minimum permissible potassium chloride concentration (12%) and washing with the aqueous wash solution at room temperature. Nitrazene Yellow has been used as a more sensitive indicator.