No abstract

The influence of ionic sites on the behavior of charged carrierbased ion-selective liquid membrane electrodes is described by theory and experiments for cationand for anion-selective electrodes. The cation exchanger potassium tetrakis [3,5-bis-(trifluoromethyl)phenyl]borate proved to be a beneficial additive for nitrite-selective electrodes. On the other hand, the anion exchanger tridodecylmethylammonium chloride improved the potentiometric properties of cation-selective electrodes. By the incorporation of a defined amount of these sites, the selectivity was enhanced, the emf functions became theoretical, the electrode resistance was lowered, and the long-term stability improved. The optimal molar ratio of additive to ionophore was in the range of 0.3-0.6 for the ionophores studied. The theory and the experiments show clearly that ionic sites should be used not only with neutral ionophores but also with charged ones. ISE membranes without ionic additives should be avoided, since otherwise inherent ionic impurities could have a decisive influence on the response characteristics.

A simple but powerful numerical simulation for analyzing the electrochemical behavior of ionselective membranes and liquid junctions is presented. The computer modeling makes use of a finiteelement procedure in the space and time domains, which can be easily processed (e. g., with MS Excel software) without the need for complex mathematical evaluations. It leads to convincing results on the dynamic evolution of concentration profiles, potentials, and fluxes in the studied systems. The treatment accounts for influences of convection, flow, or stirring in the sample solution that act on the boundary diffusion layer and it is even capable of including the effects of an electrolyte flow through the whole system. To minimize the number of arbitrary parameters, interfacial reactions are assumed to be near local equilibrium, and space-charge influences are considered via phase-boundary potential differences. The applicability of the computer simulation is demonstrated for different ionselective membranes as well as for liquid junctions. The numerical results are in excellent agreement with experimental data.

The authors thank Joel Harris for his helpful comments regarding the treatment of the data. This paper is dedicated to Cheves Walling on the occasion of his 70th birthday.

No abstract