Voltammetric determination of silver at ultratrace levels using a carbon paste electrode with improved surface characteristics

A differential pulse anodic stripping voltammetric method for the determination of silver is described. A new type of electrodeposition for preconcentrating the analyte, Ag+, was investigated, enhancing the electrolytic accumulation efficiency onto a carbon paste electrode by ion pair formation. A carbon paste electrode containing tricresyl phosphate as a pasting liquid was modified in situ with heptylsulfonic acid (sodium salt). The modifier gave rise to an additional accumulative effect towards the target ions based on ion pairing which resulted in an enhanced sensitivity of the electrode. Additional activation of the electrode surface at −1.0 V (vs. SCE) for 60 s improved the detection limit significantly. The detection limit was found to be 2.5 × 10−12 M Ag+ (accumulation time 15 min), and could be even lowered by increasing the period of preelectrolysis. The current response was linearly dependent on the concentration up to 2 × 10−5 M Ag under optimized conditions. The effects of paste composition, voltammetric parameters, composition of the supporting electrolyte as well as the influence of alkylsulfonic acids and of cathodic activation upon the determination of Ag were studied. Interferences from about 20 elements were investigated with particular attention to Cu, Hg, and Au. By masking with ethylenediaminetetraacetate (EDTA), Cu and Hg did not affect the response of Ag up to a 10000-fold and 500-fold molar excess in concentration, respectively. Au did not interfere with the determination of Ag up to a 100-fold molar excess if its absolute concentration was lower than 5 × 10−7 M. For higher concentration levels, [AuCl4]− could be separated by passing the analyte solution through an anion-exchanger column (Dowex-1) prior to stripping analysis. Due to the extremely low detection limit and high selectivity, the method could be applied to determine Ag at its ubiquitous level in tap water. With this method, the corrosion of silver cutlery in tap water could be monitored. The results agreed well with those obtained by using inductively coupled plasma mass spectrometry (ICP-MS).