Micro-Determination of Fluorine

The author's 1 modification of the Willard-Winter 2 method has been adapted to the determination of small quantities of fluorine. The distillation of H₂SiF₆ is carried out in the usual manner except that a 50 cc. distilling flask, 0.1 gm. quartz and 5 cc. 60% perchloric acid are employed. The temperature of the flask contents is maintained at 140°, and 150 cc. of distillate are collected. The water employed is freshly boiled after redistillation from alkali. The distillate is made alkaline to phenolphthalein, evaporated to a small volume, rendered just acid to the indicator, and diluted to 10 cc. One cc. aliquots are transferred to 5 cc. vials (such as those in which dentists receive gold foil). The addition of an equal volume of alcohol is omitted in order to permit a direct relationship between the fluorine content of the aliquots and the volume of Th(NO₃)₄ solution used for their titration and to render inoperative the effect of HClO₄ on the determination of more than 5γ (micrograms) F. If the sample contains less than 5γ F, 6 gm. NaClO₄ are added to the charge in the distilling flask, the distillate evaporated to about 5 cc. and the aliquots taken by weight. NaClO₄ reduced the amount of HClO₄ volatilized during the distillation of H₂SiF₆. One drop of 0.05% sodium alizarin sulphonate is added to the aliquots and, after split drops of appx. 0.03 N HCl are added until the indicator has a light orange color, one drop of buffer soln. (1 molar monochloracetic acid and 1 molar sodium monochloracetate) is introduced. The fluorine is titrated with appx. 0.0004 N Th(NO₃)₄ (1 cc. = 7-8γ F) to the appearance of a barely perceptible pink color which matches that of a titrated standard solution containing 5γ F. The Th(NO₃)₄ solution is standardized daily by titration of 1 cc. volumes of solutions containing 10, 5 and 1γ F. The results are substituted in the two-point equation for a straight line and the titration blank calculated.