Determination of membranehydration number of NH4+ in conducting polymers

Abstract

Polypyrrole polymer film doped with the large, immobile dodecylbenzene sulfonate anions operating in NH4Cl aqueous solution has been used to determine the hydration numbers of NH4+. Simultaneous Cyclic Voltammetry (CV) and Electrochemical Quartz Crystal Microbalance (EQCM) technique was used to determine the amount of charge inserted and the total mass change during the reduction process in a polypyrrole film. From these values the number of water molecules accompanying NH4+ ion was evaluated. The number of water molecules entering the polymer during the initial part of the first reduction was found to be constant and independent of the concentration of the electrolyte below ~1 M. This well-defined value can be considered as the primary membrane hydration number of the cation involved in the reduction process. The primary (membrane) hydration number obtained for NH4+ in respective chloride aqueous electrolytes having low to moderate concentrations (0.02 M – 1 M) is 2.0-2.2 and the estimated value is very similar to that of K+ estimated by the same method [9 ] and the sizes of the primary hydrated shells of NH4+ and K+ in water are approximately the same.