DETERMINATION OF SILVER WITH POLYMER-MODIFIED ELECTRODES

Electrodes modified with quaternized polyvinylpyridine into which [Fe(CN)6]4-, [Mo(CN)8]4- or [Ir(Cl)6]3- was incorporated by ion exchange, have been employed in the determination of silver ions from aqueous solutions. Silver ions are preconcentrated at open circuit at the electrode surface by formation of the silver complex of the polyanionic metal species, depicted for ferrocyanide as [Fe(CN)6]4- + 4Ag+ --> Ag4[Fe(CN)6] (1) The electrochemical response associated with the immobilized species is used as the analytical signal. Either the reduction of the silver ions to metallic silver or the stripping of metallic silver, after initial reduction, could be employed as the analytical signal. However, consistently superior results were obtained in the stripping mode. The signal was much sharper and charge propagation appeared much more rapid as compared with that for the reduction process. In the stripping mode, the redox process occurs as a very sharp and easily quantifiable peak at about +0.10 V and is essentially independent of the identity of the polyanionic metal complex employed. Using the surface immobilized ferrocyanide, silver determinations down to 6 x 10-10 M have been carried out with an r.s.d. of +/- 7%. Excellent reproducibility is obtained for electrodes modified prior to each determination.