The determination of silicic acid is important for both mineral waters (spa waters) and normal water supplies (percolating groundwaters). The content of silicic acid is particularly important for thermal power plants, in which the maximum silica content in boiler feed water must not be exceeded owing to the volatility of this impurity.
The classical gravimetric method of analysis is time-consuming and requires a sample of several liters. The method given in the «Deutsche Einheitsverfahren» requires colloidal silicic acid to be digested by sodium carbonate. Fresenius and Schneider have published a method in which a hydrofluoric/perchloric acid mixture reacts in the cold with quantities of silicic acid which will not react with ammonium molybdate after sodium carbonate treatment.