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 (E
1) 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 ( V
0) is added to the titrated sample. Subsequently this solution is titrated again with the solution (Titrant-2) containing NL-chelate of the concentration c
0 and M of a concentration c
M and having the same ionic strength as that of Titrant-1. The electromotive forces (E
2) which correspond to volumes (v
s) of the added Titrant-2 are measured. E
1 and E
2 which correspond to ν
f and vst hat satisfy a condition of ν
s= [1+ ( V
0/V)] ν
f-ν
f0 foa re read off from two titration curves. If side-reaction coefficient of N in the solution of E
1 is almost the same as that of E
2, the following equation is held concerning with the concentration c
x of M: _??_where ν
f0'=ν
f/ [1+(V
0/V)], ΔE=
2-E
1, 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%.
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