Equivalence-point determinations for automated potentiometric titrations with ion-selective electrodes

An improved method is presented for determining the equivalence point in potentiometric titrations. Titration rate and data acquisition are under computer control, and ion-selective electrodes are used as indicators. As the titration proceeds, the potentiometric curve and Gran (antilog) plot are displayed on a CRT. Upon completion of the titration, the operator uses interactive graphic techniques to interrogate the Gran plot and develop an error function. This error function identifies the period during the potentiometric titration when the cell output is most nearly Nernstian. Data obtained during the identified period are used to estimate the equivalence point. Data are presented together with a discussion of the characteristic features of the error function.