Very little work has been done (by any of the experimental techniques for determining stability constants) on the fluoride complexes of Bi(III) existing in aqueous solution. However, polarographic studies by Headridge et al. 1 of Bi(III) in a supporting electrolyte of 0.1 M HF-0.1 M NH4F have at least indicated qualitatively that quite strong complexing of bismuth occurs and it seemed that polarographic techniques could provide a suitable method for studying the nature and magnitude of these fluoride complexes. This report deals with a polarographic study of the Bi(III)-fluoride system. Problems associated with the use ofpolarographic apparatus in the presence of hydrofluoric acid, and the reversibility of the Bi(III) electrode reaction, are discussed, and a method of calculation of the stability constants for the bismuth(Ill) fluoride system designed to overcome difficulties of evaluation of free fluoride concentration in acidic conditions is presented. Results from this study are compared with those inferred from anion exchange studies made by Faris 2 and Nelson et alJ for Bi(III) in hydrofluoric acid media.
EXPERIMENTAL
All chemicals used were of reagent-grade purity. The concentrations of stock solutions from which appropriate aliquots were taken to give the desired concentrations were determined as follows : sodium perchlorate by precipitation of potassium perchlorate ; sodium bromide by potentiometric titration against silver nitrate; and perchloric acid by titration against sodium hydroxide. Fluoride was added as sodium fluoride. Bismuth(Ill) solutions were prepared by dissolving bismuth(Ill) nitrate in perchloric acid.
All measurements were made at (30+0.1)°C and at an ionic strength of 2.0 maintained by sodium perchlorate. Oxygen-free nitrogen was used to de-aerate the solutions. No maxima were encountered and maximum suppressors were not added.
The concentrations of Bi(llI), sodium bromide, and perchloric acid were 3 x 10 -4 M, 1.75 x 10 -4 M and 0.9 M respectively in all solutions measured. The concentrations of fluoride added to each solution are shown in the Tables.
Polarograms were obtained using the Metrohm Polarecord E 261. A.c. polarography was carried out using the Metrohm A. C. Modulator E 393 with an applied a.c. voltage of 10 mV, r.m.s, at 50 Hz. Rapid polarographic techniques with drop times of 0.16 s and scan rate of potential of 0.5 V min-1 were used with both d.c. and a.c.