Colorimetric-solid phase extraction method for trace level determination of arsenite in water

This paper introduces a method for the determination of inorganic arsenite [As(III)] in water at low μg L−1 by a sorption-photometric method known as colorimetric-solid phase extraction (C-SPE). The method relies on the selective extraction and concentration of an analyte on a reagent-impregnated SPE membrane, followed by direct detection of the extracted colored complex by a handheld diffuse reflectance spectrophotometer (DRS) operating in the visible spectral region. The well-established chemistry of the classic redox titrimetric method for molecular iodine (I2) standardization by arsenious oxide (As2O3) serves as the basis for this analysis. I2, which is added to the aqueous sample in an excess with respect to the analyte, serves as a colorimetric indicator. The arsenite–iodine reaction is rapid, allowing an exact volume of analyte solution to be immediately passed through an SPE membrane via a syringe after mixing with the indicator. An SPE membrane that is impregnated with the complexing agent poly(vinyl-pyrrolidone) (PVP) serves to complex and concentrate excess I2 not consumed by the As(III) analyte. The amount of complexed I2 is determined by a DRS reading directly on the membrane surface. The spectrophotometric measurement can be made in a few seconds, with a total sample workup and readout time of ∼1–2 min. The limit of detection (LOD) for this determination is below 10 μg L−1. The potential effectiveness of the method for the analysis of spiked tap water and surface water is examined, and results from preliminary interference studies are described. The work herein also shows that by applying the principles of negligible depletion (ND), the analytical procedure could be simplified by eliminating the need to pass an exact volume of a sample through the impregnated membrane as long as it exceeds the predetermined minimum volume.