Determination of fluorine in silicates

Abstract

1. It was shown that silicic acid interferes in the colorimetric determination of fluorine by the zirconia-alizarin method by combining with zirconium and reducing the red component of the color. In order to avoid error it is proposed to determine silicic acid colorimetrically in an aliquot of the solution and introduce the amount so found into the standard fluoride solutions. 2. A study Was made of the conditions under which fluorine passes into solution in the process of fusing the material with sodium carbonate and then leaching the melt with water. It was shown that, in absence of silica and alumina in the material to be analyzed (e. g., CaF2), complete extraction of fluorine can be attained by addition of a very small amount of silica — very much less than is usually recommended. 3. The conditions for the separation of fluorine in the form of lead fluorochloride were investigated. It was shown that the separation of the precipitate is retarded by the presence of silicic acid in the solution, and the precipitation of fluorine is incomplete when the silica content is high. 4. A variant of the procedure for the determination of high fluorine contents in silicate materials is proposed: the preparation of the solution is simplified, and fluorine is determined as the sum of the amount found volumetrically in the solution and the amount found colorimetrically in the precipitates.