Cold. Travers. (Comptes read., 1921, 173,836-838.)-This method, which is the counterpart of that recently given (ANALYST, 1921, 46, 522) for the estimation of silica, consists in precipitating the fluorine in the cold as potassium silicofluoride and in estimating the latter volumetrically. This process gives excellent results with soluble fluorides, calcium fluoride or potassium borofluoride, but has not yet been rendered applicable to fluorine compounds containing aluminium. The fluorine is
... The fluorine is first brought into the condition of soluble alkali fluoride, the solution, usually alkaline, being then treated with about twice as much silica, in the form of potassium silicate, as is required to convert the fluorine completely into potassium silicofluoride. The resulting solution is rendered neutral to methyl orange by means of hydrochloric acid, an excess of about 2 C.C. of the acid being added. Sufficient solid potassium chloride is introduced to give a concentration of about 20 per cent. of this salt, the potassium silicofluoride being collected on a hard filter, washed with 20 per cent. potassium chloride solution until the washings fail to react acid towards methyl orange, and titrated while boiling with 0.2 N potassium hydroxide solution: 1 C.C. of the latter corresponds with 0-0057 grm. of fluorine. The whole of the procedure may be effected in glass vessels, if neutral fluorides in low concentration form the starting material.