Based on individual degrees of reacting and two-component mixtures of sulfide forming ions the possibility to use potassium dimethyldithiophosphate in potentiometric titration of some sulfide forming ions have been grounded theoretically. The sequence of ion#s reacting was calculated with a help of ionic product of the reagent complexes. The authors forecast selectivity of possible determination of each ion, selectivity of titration of multicomponent mixtures, choosing of indicator electrodes which can be used in potentiometric titration and the opportunity to use potassium diheptyldithiophosphate for ecological object analysis.