Semi-micro determination of fluorine in organic compounds by oxygen silica-flask combustion and gran-type potentiometric titration of fluoride with lanthanum nitrate

A sufficient amount of organofluorine compound to yield about 7 mg of fluoride was burned in a 2-l silica flask filled with oxygen. After absorption of the combustion gases in water the solution was adjusted to pH 4.6 and titrated in a polythene beaker with 0.02 M lanthanum nitrate solution using a fluoride ion-selective electrode as an indicator and a calomel electrode as a reference. Use of a calibration graph based on sodium fluoride and Gran-type evaluation of the potentiometric data gave accurate results with a coefficient of variation of 0.17%.