Optimization studies for the inorganic arsenic [As(III) and As(V)] determination using square wave cathodic stripping voltammetry

Angeline Limok, Mong (2013) Optimization studies for the inorganic arsenic [As(III) and As(V)] determination using square wave cathodic stripping voltammetry. Masters thesis, University Malaysia Sarawak.

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Abstract

The presence of arsenic in the environment has received worldwide attention. The primary exposure of arsenic poisoning is through the ingestion o the contaminated drinking water which seriously effects human’s health such as the cardiovascular system. Thus, a fast, simple and reliable technique with inexpensive equipment such as voltammetry is required for the monitoring of arsenic contamination in the environment. In this study, the determination of inorganic As(III), total inorganic arsenic [As(III) + As(V)] and arsenic total were conducted using square wave cathodic stripping voltammetry at a hanging mercury drop electrode . Parameters that affect the performance of the technique such as Cu(II) concentration, deposition time, deposition potential, pulse amplitude and frequency were studied and optimized. The preconcentration was carried out in 1M HCl in the presence of Cu(II). Due to the limited maximum frequency allowed by the voltammetry instrumentation, the determination of arsenic was conducted under the frequency 149 Hz. In As(III) determination, it was found that 10 ppm Cu(II) under 90 s deposition time gave the highest peak currents. The optimum deposition potential and pulse amplitude was at – 0.35 V and 60 mV respectively. The linearity range up to 450 μg/L offers a great advantage in quantifying the environmental samples without dilution. The detection limit obtained was 0.076 μg/L. In the determination of total inorganic arsenic, the reducing effect of thiosulfate and its efficiency was investigated. The concentration of thiosulfate and Cu(II) and instrumental parameters such as deposition time and deposition potential was also optimized. It was observed that thiosulfate at concentration of 1.5 mM was effective for the reduction of As(V) to As(III). At higher thiosulfate concentrations, split peaks were detected which could be contributed by the presence of colloid of sulfur at the electrode surface. For total inorganic arsenic determination, the optimum condition for the preconcentration of arsenic was in 1 M HCl in the presence of 10 ppm Cu(II) and 1.5 mM thiosulfate at a deposition potential of – 0.45 V and deposition time of 90 s. The detection limit was 0.648 μg/L. The efficiency of the reduction of As(V) by thiosulfate was studied by comparing the results obtained in the analysis of three sample solution with different ratio of As(III)/As(V) concentration. The mean peak current of 8 ppb As(V) and 2 ppb As(III) + 6 ppb As(V) were not significantly different from 8 ppb As(III) which shows that the reduction of As(V) by thiosulfate is quantitative. Significant interferences were observed for ions Fe3+ and Cd2+ that caused more than 50% peak suppression. No significant interference was observed in the presence of Zn2+ ion. The optimized method was applied in the determination of arsenic in environmental samples collected from Tasik Biru, Bau. It was observed that the concentration of arsenic in the environmental samples were more than 10-48 times higher than the maximum permissible value of 10 μg/L arsenic in the drinking water recommended by WHO. For verification purposes, the determination of arsenic total in the digested environmental samples were also conducted using Atomic Absorption Spectometry instrument and no significant differences in the concentration was obtained. Thus, square wave cathodic stripping voltammetry method is suitable for the routine analysis of arsenic in the environmental samples.

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