A Simple Speciation Analysis Using Tristimulus Colorimetry for Arsenic (III) and Arsenic (V) in Environmental Water after Selective Coprecipitation with Barium Sulfate

Simple differential determinations of arsenic(III) and arsenic(V) in environmental water have been developed based on the selective coprecipitation of arsenic(V) with barium sulfate, followed by a tristimulus colorimetric method. Arsenic(V) coprecipitation with barium sulfate occurred in solution, having an excess of barium ion over sulfate ion when barium sulfate was formed. The pH of solution is also a very important factor for arsenic(V) copreciotation. Arsenic(V) was quantitatively coprecipitated with barium sulfate in solutions at pH 3 similar to 9, but arsenic(III) was not coprecipitated even at pH 3 similar to 9. To 20 mL of the sample solution, which was put into a syringe, a sodium sulfate solution and a barium chloride solution were added successively to cause the coprecipitation of arsenic(V) with barium sulfate. After filtration, arsenic(III) in the filtrate was determined with tristimulus colorimetry. Another portion of the sample solution was used for the determination of total-arsenic [As(III) + As(V)]. The arsenic(V) concentration was obtained by calculating the difference between the total-arsenic and arsenic(III) concentrations. The established method was successfully applied to environmental water samples, such as brackish lake water samples to clarify the behavior of arsenic (III) and arsenic(V) in redox-sensitive water regions.