Determination of Selectivity Coefficients of Ion Electrodes by Continuous Variation Method

Abstract

A new method for the determination of the selectivity coefficient of an ion selective electrode is proposed. In the proposed method, a series of mixed solutions of the primary ion i and interfering ion j is prepared in which the sum of the logarithm of i concentration and logarithm of j concentration is kept constant. The potential of the electrode, E, is measured for the series of the mixed solutions. The result is plotted with the activity of i, a;, as abscissa against E on semi-logarithmic graph paper. The activity at the intersection of two extrapolated linear parts of the log a;-E curve is determined. From the activity at the intersection of the primary ion, the selectivity co-efficient is calculated. The proposed method was applied to the determination of selectivity coefficients of fluoride, chloride, bromide and potassium selective electrodes. The selectivity coefficients determined by the proposed method, named 'continuous variation method'', agreed with those obtained with the conventional mixed solution method