ERDEY
AND RuzAs have described a direct method for the determination of silverl, in which silver ions are reduced to the metal in hot solution with ascorbic acid; the end-point of the titration is detected with variamine blue. However, the simplicity and accuracy of the titration of hexacyanoferrate(II1)
with ascorbic acid suggestecl that this reaction could be utilized also for the indirect cletcrmination of silver. The titration has already been shown to be aclvantagcous for indirect determinations of a number of oxidizing ancl reducing agentsg, where the favourable reclox potential of the hesacyanoferrate(III)-hexacyanoferrate(I1) system can be applied. Hexacyanoferrate(II1) and hexacyanoferrate(I1) are well known as precipitants, this property being widely utilized in qualitative and quantitative analysis. l3oth ions form precipitates with several metals of groups 1-3. It is interesting to note that metal hcsacyanoferrate(I1)
precipitates are gcncrally less soluble than the corresponding hesacyanoferrate(II1)
precipitates, and that metal hexacyanoferrate(II1) precipitates generally contain only one metal ion, whereas the hesacyanoferratc(I1) compounds are usually clouble salts.
We have therefore estenclecl the previous investigations on oxidation-reduction reactions2 to cover reactions involving precipitation. In such methods, the precipitate of metal hexacyanoferrate(II1) is clcterminccl by a titration with ascorbic acid. The determination of silver provicles an excellent example of such a procedure. Silver ions in neutral or nitric acid-containing medium form a reddish brown precipitate with hexacyanoferrate(II1) 3*4. Thi: precipitate is difficult to filter, though the situation can be improvecl by the aclditio; of foreign electrolytes (e.g. ammonium nitrate). The precipitate can be washed with ammonium nitrate solution. If ammonium hydroxide is aclded to the washed precipitite, it partly dissolves quite quickly, and dissolves completely after some time. However, when sodium hesacyanoferrate(I1) solution is added to the precipitate, which has been mixed with ammonium hydroxide, a precipitation exchange reaction takes place and a yellowish-white silver hesacyanoferrate(I1) precipitate is formecl: Ago Fc(CN),, _t ICJ Pc(CN)o = ICAga Pc(CN)n i-IG Fc(CN)a