Coulometric Titration with Electrogenerated +2 Tin Determination of Iodine, Bromine. and Various Oxidants via Iodometry

Although the roster of coulometric titrations no\\' includes many reactions of classical titrimetry'. the lIse of cl,ectrogeneratecl +2 tin as a coulometric titrant has not previously been reported. We have found that in an acidic bromide medium +4 tin (probably SrlBr6-2) is reduced to the' +2 state (probably SnBr4-2) with 100% current efficiency' at a gold cathode. The formal potential of the stannicstannous couple in bromide medium is +0.27 V vs. N.H.E., a value sufficiently reducing so that (in prind\?le) +2 tin should serve for the titration of divers oxidants. In practice one encounters the inco!1\'enience that the rate of reduction of most substances by +2 tin is relatively small, but this can be circumvented by either of two means. Firstly, an excess of +2 tin can be generated in the test solution. and, after a short waiting period, the polarity of the generating electrode is reversed from cathode to anode and the excess +2 tin is titrated by the bromine electrogenerated at the anode. The other expedient, applicable to the determination of subs~ances capable of oxidizing iodide to iodine, is "coulometric iodometry". An excess of iodide ion is added to the test solution, and the liberated iodine is titrated with the electrogenerated +2 tin. The iodine-stannous reaction is rapid, and equivalence points can be detected accurately either potentiometrically or amperometrically. ROWLEYAND SWIFT2determined iodine (and thus various oxidants via iodometry) by adding a measured excess of standard thiosulfate solution. and back titration of the excess thiosulfate with electrogenerated iodine. This method suffers from the disadvantage of requiring a standard thiosulfate solution, and is only semicoulometric. Derect titration of iodine with electrogenerated +2 tin is simpler and more convenient.