The presence of toxic levels of inorganic arsenic in drinking water is a worldwide problem. Measurement of millions of samples in the field for arsenic is an analytical challenge and the first step to solve the problem. Here, we developed a novel electrochemical arsine sensor and system based on the cell: C(s)/I3 −I2, AsH3(g/aq)//I3 −, I2 AgI(s)/Ag(s), where I3 −/I2 mediated AsH3(g/l) oxidation which occurred on the C(s) electrode. The sensor system requires 50μL of 10mM iodine solution and a small piece of filter paper placed inside a novel electrochemical cell. The cell exhibited highly efficient arsine mass transport with a limit of detection of 15μg/L As(III) in water. A mathematical model of mass transfer of AsH3(g/aq) and following electrochemical reaction was developed and applied to explain the experimental data. Groundwater samples containing arsenic were measured to prove its utility in a field deployable instrument.