GAS-PHASE ELECTROCHEMICAL DETECTION OF TRACE ARSENIC IN DRINKING WATER

Abstract

The presence of toxic levels of inorganic arsenic in drinking water is a worldwide problem. Measurement of millions of field samples for arsenic is an analytical challenge and the first step to solve the problem. This work is an attempt to develop new sensor chemistry for electrochemical detection of arsine produced from arsenic in water. The sensor based on reaction between AsH3 and Ag(s)/ AgNO3 // Ag+/ Ag(s) redox couple on a filter paper substrate has shown high sensitivity with potentiometry and conductometry as analytical techniques. Amperometry sensor is based on chemical reaction of AsH3 and iodine, followed by electrochemical reaction of iodide generated from the first chemical reaction. In potentiometry, the signal response ranged from 4.3 to 200 mV for 10 µg/L to 400 µg/L As(III), respectively with limitation of detection of 11 µg/L As(III). The conductometric detection based on alternating bipolar square potential pulses exhibited linear calibration curve and a minimum detection limit of 88 µg/L As(III). A novel electrochemical cell: C(s)/ I-/I2, AsH3(g) // I-, AgI /Ag(s) where I-/I2 mediated AsH3 oxidation occurred was developed for chronoamperometric detection with limit of detection of 47 µg/L As(III). Here, Au, Pt, and C exhibited similar anodic current responses but the porous thin carbon fiber electrode yielded highly efficient arsine mass transport. A mathematical model of mass transfer of AsH3(g) and following electrochemical reaction was developed and successfully applied to the simulate experimental data. The sensor developed requires 2.0 mL of water sample, 2.0 mL of NaBH4 as arsine generating agent, and 50 µL of 10 mM iodine. The sensor is robust, compact, easy to fabricate, and field-deployable.