DETERMINATION AND APPLICATION OF ION-SELECTIVE ELECTRODE MODEL PARAMETERS USING FLOW-INJECTION AND SIMPLEX OPTIMIZATION

A modified form of the Nikolskii-Eisenman equation is proposed in which the power dependence of the interfering ion activity is removed. Selectivity is determined for poly(vinyl chloride) membrane electrodes (Na+, K+, Ca2+ and NH4+) in terms of constants (which are valid only for the particular experimental design and conditions used) rather than conventional coefficients. These constants and other electrode parameters, such as cell constant and slope, are measured by means of an ion-selective array approach coupled with flow injection. Each electrode in the array is characterized by means of a fractional factorial calibration design and simplex optimization of the electrode parameters. The applicability of the resulting values for these parameters is demonstrated through the determination of unknowns by direct solution of the system of modified Nikolskii-Eisenman equations describing the array response. The approach is suitable for use in real-time monitoring applications where batch calibration techniques cannot easily be implemented.