Pervaporation-flow injection determination of arsenic based on hydride generation and the molybdenum blue reaction

A pervaporation-flow injection system for the determination of As(III) by the molybdenum blue method is described. Samples containing As(V) were pretreated with KI and HCl prior to injection to reduce As(V) to As(III). As(III) samples were injected into a 0.5 M HNO3 stream and then mixed with a stream containing 0.5% sodium borohydride. The arsine generated was transported in the pervaporation unit across a semi-permeable membrane into a 1.97×10-3 M I3- acceptor solution where it was oxidised to As(V). The acceptor stream was merged with a reagent stream containing 0.01% SnCl2, 0.1% hydrazine sulfate and 0.5% ammonium molybdate thermostated at 70°C. The concentration of arsenomolybdenum blue was monitored photometrically at 840 nm. The calibration graph is characterised by two linear ranges between 2.0 and 15 mg l-1 and 25 and 1000 mug l-1, a sampling frequency of 8 h-1 and a detection limit of 15 mug l-1. The presence of phosphate, Ni(II), Cu(II), and Fe(II) which normally interfere with arsenic in the molybdenum blue reaction or with atomic absorption spectrometry does not affect the analytical determination in this new method at concentrations up to 1000 mg l-1. The method was applied successfully to industrial effluents containing As in the concentration range from 15 to 250 mug l-1. Very good agreement with hydride generation atomic absorption spectrometry was obtained.