An Indirect Potentiometric Determination of Metal Ions Using
Chelate Displacement Reaction and a Standard Addition Method
Not Requiring Adjustment of an Ionic Strength of Sample

Norihisa ISHIKAWA*, Tetsuya NOMURA, Ming-Jie LI† and Hiroshi MATSUSHITA

Department of Applied Chemistry, College of Engineering, Chubu University; 1200 Matsumoto-cho,
Kasugai-shi 487-8501 Japan
† Department of Chemistry, Yian-Bian University; Gong-Yuan-Lu, Yian-Ji-shi 133002 China

An indirect potentiometric determination of metal ions using chelate displacement reaction and a standard addition method, in which the adjustment of the ionic strength of a sample is not required, is proposed.
When the sample of volume V, containing a metal ion M to be determined, in which N-selective electrode and a reference electrode are immersed, is titrated with the solution (Titrant-1) containing NL-chelate of a concentration c0, where N and L denote a metal ion and a chelate-forming ligand respectively, N is released by displacement reaction. After the electromotive forces (E1) which correspond to volumes (vf) of the added Titrant-1 are measured (the final volume of the added Titrant-1 is denoted with vf0), the same sample solution of another certain volume (V0) is added to the titrated sample. Subsequently this solution is titrated again with the solution (Titrant-2) containing NL-chelate of the concentration c0 and M of a concentration cM and having the same ionic strength as that of Titrant-1. The electromotive forces (E2) which correspond to volumes (vs) of the added Titrant-2 are measured.
E1 and E2 which correspond to vf and vs that satisfy a condition of vs=[1+(V0/V)] vf−vf0 are read off from two titration curves. If side-reaction coefficient of N in the solution of E1 is almost the same as that of E2, the following equation is held concerning with the concentration cx of M:

where vf0'=vf0/[1+(V0/V)], ΔE=E2−E1, S is the response slope of N-selective electrode. This cx is determined from the slope of linear plots of 10ΔE/S vs. (vf−vf0').
By using CuII-edta and CuII-selective electrode, concentrations of bismuth(III) ranging from 2×10−3 to 1×10−5 mol dm−3 in the samples of various ionic strengths were determined with an error of less than ±1 % and a relative standard deviation of less than 0.8%.


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