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

Abstract

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 NLchelate of a concentration co, 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 (ν_f) of the added Titrant-1 are measured (the final volume of the added Titrant-1 is denoted with ν_f0), 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 c_M and having the same ionic strength as that of Titrant-1. The electromotive forces (E₂) which correspond to volumes (v_s) of the added Titrant-2 are measured. E₁ and E₂ which correspond to ν_f and vst hat satisfy a condition of ν_s= [1+ ( V₀/V)] ν_f-ν_f0 foa re 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 c_x of M: _??_where ν_f0'=ν_f/ [1+(V₀/V)], ΔE=₂-E₁, S is the response slope of N-selective electrode. This c_x. is determined from the slope of linear plots of 10^ΔE/S vs. (ν_f-ν_f0').
By using Cu^II-edta and Cu^II- selective electrode, co ncentrations 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%.