METAL DISPLACEMENT AT THE ZINC-DIETHYLDITHIOCARBAMATE-MODIFIED CARBON-PASTE ELECTRODE FOR THE SELECTIVE PRECONCENTRATION AND STRIPPING ANALYSIS OF MERCURY(II)

A carbon paste electrode modified with zinc diethyldithiocarbamate was used for the selective accumulation of mercury(II) based on the metal displacement principle. Selectivity was achieved by exploiting differ ences in conditional extraction constants. After accumulation, medium exchange was performed. The accumulated Hg-II was then reduced to meta llic mercury in 0.10 M KSCN - 0.010 M HClO4 followed by differential p ulse anodic stripping voltammetry in the same medium. Factors affectin g the accumulation, reduction and stripping steps were systematically investigated. An optimized procedure was then developed for the determ ination of Hg-II. Under optimum conditions, a detection limit of 8.0 x 10(-10) M Hg(II) was obtained for a 15 min accumulation. For 6 determ inations each of 1.00 x 10(-7) M and 5.00 x 10(-8) M Hg-II, relative s tandard deviations were 1.5% and 3.3%, respectively. The presence of s elected metal ions and other organic and inorganic substances gave no observable interferences. However, interferences were observed when 1. 00 x 10(-3) M NH4SCN, Na(2)EDTA and NH4Br were present. These interfer ences can be readily overcome by digesting the samples with oxidizing agents. The developed method was applied to the analysis of mercury in a U.S. Environmental Protection Agency water quality control sample a nd also in a human urine sample.