A new lead chlorofluoride method for the determination of fluorine in organic compounds

Abstract

Some years ago, a gravimetric method for the determination of fluorine in organic compounds was described1; this was based on decomposition of the compound by fusion with alkali metal in a nickel bomb followed by precipitation as lead chlorofluoride. The method has given very satisfactory results under routine conditions but, as was indicated previously, it suffers from the limitation that good results can only be obtained if more than 25 mg of fluoride are present. When only small samples are available, we use a thorium nitrate micro-titration ~, which gives results of similar accuracy but is less preferable than the lead chlorofluoride method for routine use because the end-points, as in all thorium nitrate titrations, give trouble to unpractised operators.The solubility losses which occur when less than 25 mg of fluoride are present in the previous lead chlorofluoride method 1 can be reduced by decreasing the volume of lead chloride precipitant added, but the method is not then applicable over a sufficiently wide range. Since the deficiency of the precipitant lies in its comparatively low lead content, it seemed possible that lower amounts of fluoride could be precipitated quantitatively if an equimolecular solution of lead and chloride were used. This would also be more likely to give a precipitate of the theoretical composition. Contrary to expectation, we found that lead chloride dissolves slightly more readily in lead nitrate solutions than in water; there is some evidence s, ~ that a salt, PbCINO~, is formed. Thus the lead concentration can be increased almost indefinitely without fear of precipitating lead chloride. * This paper is dedicated to Professor Dr. Hans Lieb, former ~eacher of R. B., on the occasion of his 70th birthday.

Indirect polarographic microdetermination of fluorine in fluoroorganic compounds after oxygen-flask combustion

“…Several methods exist for the determination of fluorine in organic compounds; the subject has already been reviewed by Macdonald (1). After the sample is decomposed by one of various techniques (2, 3), the F~can be determined gravimetrically (4)(5)(6), titrimetrically (7,8), potentiometrically (9)(10)(11) using fluoride ion-specific electrode, or spectrophotometrically (12)(13)(14). Titration of the F~with thorium nitrate (7), the procedure most commonly used, has limitations; the reaction is reported (15) to be nonstoichiometric over the F~r ange 1 to 50 mg.…”