Studies on thorium nitrate titration of microamounts of fluoride

Abstract

In thorium nitrate titration of fluoride with Alizarin S indicator, a formate buffer has been applied for adjusting pH of the titrand in order to reduce the remarkable interference from traces of oxidizing agents. Detailed investigations have also been made upon the conditions of titration in the presence of large amounts of sodium nitrate or chloride (as in the analysis of fluorine in organic compounds).
The results are summarized as follows:
(1) The end-point can be detected with satis-factory precision (σ=2.54.5μg F) by the aid of a color standard solution.
(2) Interference from carbonate ion can be easily eliminated by using an appropriate indicator for neutralization.
(3) Interference from oxidizing agents is reduced markedly owing to the reducing action of the formate buffer.
(4) Fluoride over the range of 190 μg to 3 mg can be titrated directly with 0.01N thorium nitrate solution with satisfactory accuracy and reproducibility.
(5)Composition of the buffer solution to be used must be altered depending on the amount of co-existing sodium nitrate or chloride.