We have modelled for the first time a chemodosimeter for As(III) detection in water. The chemodosimeter modelled is a 1,3-dithiole-2-thione derivative with an anthracene unit which has been previously described as a chemodosimeter for Hg(II) detection. Quantum chemical calculations at the DFT level have been used to describe the binding energies and selectivity of the chemodosimeter. We find that the dosimeter action is intrinsically dependent on the thiophillic affinity and the coordination sphere of the metal ion. Binding studies for a series of metal ions: Pb(II), Cd(II), Hg(II), Ni(II) and As(III) followed by an analysis of the complete reaction pathway explains the high selectivity of the dosimeter towards As(III). The dosimeter efficiency is calculated as 66% for As(III)-ion.