As part of an on-going project with Chemists Without Borders (CWB), students in my group are working on a method to determine the inorganic arsenic concentration in rice that can beimplemented in a rudimentary laboratory at the Asian University for Women, by undergraduate students. Our approach is to adapt the Hach EZ test kit (designed to measure inorganic arsenic in drinking water down to a concentration of 10 µg per L), which has already been selected as the basis of a CWB test for arsenic in groundwater. The test is based on the generation of arsine by reaction of the inorganic species with powdered zinc in an acid solution followed by the reaction of the arsine, purged from the solution by the co-evolution of hydrogen, with mercuric bromide crystals immobilized on a paper support. The resulting depth of the yellow/brown color is compared with colors on a standard chart, either visually or with the help of analysis of a digital image obtained by a cell-phone camera or flat-bed scanner. The initial extraction stage involves extracted from rice by water or dilute mineral acids; however, our preliminary results indicate that the co-extracted matrix components interfere with the reaction at the surface of the zinc, and so we are investigating the use of borohydride as the arsine generation reagent. Although this reagent will also generate volatile methylated arsines from any mono- or di-methyl arsenic species extracted from the rice, these do not react with mercuric bromide. Control of the kinetics of this reaction is crucial. We have tried a number of approaches, the most successful of which involves dissolving the borohydride in an agar/xanthan gum gel stabilized with sodium hydroxide solution. Gels are cast in a 12-well polystyrene tray (Falcon brand, non-tissue culture treated plate, 12 well, flat bottom, Ref 351143). Each well is a cylinder of about 5-mL volume and diameter 23 mm. (2% agar and 0.2% xanthan gum) contains about 400 mg of sodium borohydride in 0.1 M sodium hydroxide. When dropped into the reaction vessels containing 50 mL of rice extract the reaction lasts for over 20 min, producing colors that are identical with those obtained for the same concentration of arsenic when reacted with zinc. Students taking chemistry classes in a nearby high school have helped by evaluating a number of other polysaccharide gel possibilities. We are investigating ways of decreasing the detection limit and of extracting the arsenic compounds from rice.<br/>

Development of a test-kit method for the determination of arsenic in rice in Bangladesh

255th National Meeting (2018)

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