Differential determination of trace amounts of arsenic(III) and arsenic(V) in seawater by solid sampling atomic absorption spectrometry after preconcentration by coprecipitation with a nickel–pyrrolidine dithiocarbamate complex

Abstract

A sensitive and accurate method for the differential determination of trace amounts of arsenic(III) and arsenic(V) in water samples was described. It was found that arsenic(III) was coprecipitated quantitatively with a Ni–ammonium pyrrolidine dithiocarbamate (APDC) complex at the pH range of 2–3, but arsenic(V) was hardly coprecipitated with the Ni–PDC complex in the same pH condition. The coprecipitates obtained were directly measured by electrothermal atomic absorption spectrometry (ETAAS) using the solid sampling technique. In order to determine trace amounts of total arsenic, sodium thiosulfate and potassium iodide were used to reduce arsenic(V) to the trivalent state in the sample solution before coprecipitation. The concentration of arsenic(V) in the sample solution could be calculated by the difference in concentration between arsenic(III) and total arsenic in the sample solution. The coprecipitation conditions for trace amounts of arsenic(III) and arsenic(V) in water samples by the Ni–PDC complex were investigated in detail. The concentration factor by coprecipitation was reached at about 40 000 when 2 mg of nickel as a carrier element was added to 500 ml of the water sample. The proposed method was successfully applied to the determination of trace amounts of arsenic(III) and arsenic(V) in seawater, and the detection limit for arsenic, which was defined as the concentration calculated from three times of the standard deviation of the procedural blanks, was 0.02 ng/ml for 500 ml portions of water sample in the proposed method.