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
cx 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
c0. The electromotive forces (
E1) corresponding to the added volumes (υ
f) of titrant-1 are measured, where the final added volume of titrant-1 is denoted by υ
f0. Subsequently, the same sample solution of a certain volume (
V0) is added to the titrated sample; this solution is titrated again with an aqueous solution (titrant-2) containing NL of concentration
c0 and M of concentration
cM, and having the same ionic strength as that of titrant-1. The electromotive forces (
E2) corresponding to the added volumes (υ
s) of titrant-2 are measured. If
E1 and
E2 corresponding to υ
f and υ
s that satisfy the condition υ
s=[1+(
V0/
V)]υ
f-υ
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
E1 measurement is almost the same as that in the solution of the
E2 measurement, 10
ΔE/S=1+[
cM/(
cxV)](υ
f-υ
f0') concerning with
cx is held, where Δ
E=
E2-
E1, υ
f0'=υ
f0/[1+(
V0/
V)], and
S is the response slope of the N-selective electrode. This
cx is determined from the slope of linear plots of 10
ΔE/S vs. (υ
f-υ
f0'). The present indirect method can be applied to determinations of the iron (III) and bismuth (III) 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
IIedta and Cu
II-selective electrode, and by measuring at 50°C.
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