Molybdenum Blue Spectrophotometry for Trace Arsenic in Ground Water Using a Soluble Membrane Filter and Calcium Carbonate Column

Abstract

An improved molybdenum blue spectrophotometry using a soluble membrane filter and CaCO₃-column was proposed for determining arsenic in drinking water supplied from ground water in the presence of phosphate. A 100 mL sample solution containing 0.5 - 10 μg arsenic was passed through a CaCO₃-column to remove phosphate, arsenate (As(V)). Arsenite (As(III)) which was not retained on the column was oxidized to As(V). As(V) was converted into a heteropolymolybdenum blue anion. The blue anion was collected on a membrane filter as an ion-associate with n-dodecyltrimethylammonium ion by filtration. The filter was dissolved in 2 mL of 2-methoxyethanol. The absorbance of the solution was measured at 810 nm against a reagent blank. Total inorganic arsenic was determined by reducing As(V) to As(III) before the column treatment. The RSDs for 10 μg L^‒1 of As(III) and As(V) were 2.9%. Phosphate 0.2 mg L^‒1 (as P) and iron 0.1 mg L^‒1 did not interfere with the determination of 10 μg L^‒1 arsenic. The proposed method was successfully applied to ground waters.