Introduction
No polarographic work has apparently yet been reported on the consecutive stability constants of the tin(II)-fluoride system. In what appears to be the only polarographic study of this complex ion system 1, it was reported that the data could not be treated with the theory of DeFord and Hume 2, but values of the stability constant of the SnF 3 ion were obtained using the calculation method of Lingane 3. The closeness to polarographic reversibility of the tin(II) fluoride system, especially in acidic solution, and the large shifts in half-wave potential observed upon addition of fluoride, should in fact make this system ideally suited to a complete polarographic treatment for consecutive stability constants in both neutral media using the DeFord Hume method 2, and in acidic media by the method proposed by Bond 4.
Recently, the possibility of using a.c. polarography and rapid polarographic techniques for studies on complex ions was examined 5'6. These rapid techniques were subsequently shown to be applicable in aqueous hydrofluoric acid, in which media a study of the fluoride complexes of bismuth(III) was carried out 4. A modified form of the DeFord-Hume equation was used which allows for the formation of undissociated hydrofluoric acid which must necessarily be present in acidic fluoride solutions. For bismuth(III) it was not possible to work in neutral media because of the formation of insoluble bismuth oxy or hydroxy hydrolysis products at neutral pH values and consequently the results obtained using the equation for acidic media could not be compared with any other set of results. In this work, tin(II) is stable to hydrolysis at moderate pH values, so results for relatively neutral media can be calculated using the well known DeFord-Hume method. These results can then be used as a basis for comparison with results obtained in acidic media and thus the tin(II)-fluoride system should provide a good check on the usefulness and validity of the equation proposed by Bond for use in acidic media.
EXPERIMENTAL
Reagent grade chemicals were used throughout without further purification. The problem of air oxidation of tin(II) in solution was overcome by preparing standard solutions in argon-filled volumetric flasks, using distilled water deoxygenated by boiling and applying constant argon bubbling. Oxygen free conditions were maintained in all polarographic test solutions by constant passage of argon, except