Patent ID: 7336362

Claim:
A method for quantitative determination of arsenic concentration in a water sample in the field, wherein the water sample comprises phosphates that have an order of magnitude or more higher concentration than the arsenic concentration, the method comprising: (a) preparing a first and a second water sample aliquot; (b) adding a reducing agent to the first water sample aliquot to reduce arsenic in the aliquot to an arsenite state, whereas the second water sample aliquot is unreduced; (c) adding a color reagent to the first and second sample aliquots, whereby phosphates in the first aliquot and both phosphates and arsenates in the second aliquot are converted into color complexes; (d) using optical probes to measure light absorbance of the color complexes formed in each aliquot; and (e) using the measured light absorbances for the two aliquots to determine the arsenic concentration in the groundwater sample, wherein the optical probes are disposed in a portable battery-powered colorimeter, and wherein the determination of the arsenic concentration has a detection limit of 10 ug/L or lower for groundwater samples having phosphate concentrations in the range of 5-50 μM.