An indirect potentiometric determination method of metal ions not depending on a solvent composition of samples in water-organic solvent mixtures

An indirect potentiometric determination of metal ions using a chelate displacement reaction and a standard addition method, which does not require an adjustment of the ionic strength and the solvent composition of a sample of a water-organic solvent mixture, is proposed. A sample of the water - organic solvent mixture of volume V containing a metal ion M of concentration c. with an immersed N-selective electrode and a reference electrode is titrated with an aqueous solution (titrant-1) containing a metal-chelate (NL) of concentration c(0). The electromotive forces (El) corresponding to the added volumes (v(f)) of titrant-1 are measured, where the final added volume of titrant-1 is denoted by v(f0). Subsequently, the same sample solution of a certain volume (V-0) is added to the titrated sample; this solution is titrated again with an aqueous solution (titrant-2) containing NL of concentration c(0) and M of concentration c(M), and having the same ionic strength as that of titrant-1. The electromotive forces (E-2) corresponding to the added volumes (v(s)) of titrant-2 are measured. If E-1 and E-2 corresponding to v(f) and v(s) that satisfy the condition v(s) = [1 + (V-0/ V)]v(f) - v(f0) are read off from two titration curves, and if a side-reaction coefficient considering the ion association of N in the solution of the E, measurement is almost the same as that in the solution of the E-2 measurement, 10(DeltaE/S) = 1 + [c(M)/(c(x)V)] (v(f) - V-f0') concerning with c(x) is held, where DeltaE = E-2 - E-1, v(f0)'= v(f0)/ [1 + (V-0/V)], and S is the response slope of the N-selective electrode. This c. is determined from the slope of linear plots of 10(DeltaE/S) vs. (v(f) - v(f0)'). The present indirect method can be applied to determinations of the iron (III) and bismuth (111) in media of water- ethanol and water- 1,4-dioxane mixtures with almost the same accuracy and precision as in aqueous solution by using Cu(II)edta and Cu-II-selective electrode, and by measuring at 50degreesC.