ISO 2124:1972

The principle consists in neutralization of the alkalinity of a test portion, previously dissolved, in presence of methyl red as indicator. Addition of sodium fluoride, which reacts with the silicic acid, forming a euaivalent quantity of sodium hydroxide. Addition of a measured volume of standard volumetric hydrochloric acid solution in excess, followed for diluting by ethanol. Filtration of the silicic compound thus obtained, which is insoluble under these conditions. Titration of the excess of hydrochloric acid in the filtrate with standard volumetric sodium hydroxide solution in presence of methyl red as indicator.