The possibilities for a photometric titration of fluorine with lanthanum nitrate are examined. As was to be expected LaF3 is formed. This, however, does not strictly correspond to the formula and thus it is necessary to use an empirical factor which is governed by the conditions of titration, but any deviations from stoichiometry are slight. Lanthanum nitrate, therefore, is clearly superior to some other commonly used standard solutions for fluoride determination.
The method suggested operates at a pH of 6,5 with PAR as an indicator in an aqueous-alcoholic solution. It covers the range from 0.1 to 9 mg F. Titration is made with the recording Metrohm system. For 0.017 M lanthanum nitrate solution the standard deviation is ± 5 μg of F.
Cerium has a similar behaviour. Greater deviations from the stoichiometry are stated for yttrium.