An Indirect Potentiometric Determination of Anions (or Cations) Using a Precipitation Reaction and a Standard Addition Method Not Requiring Adjustment of an Ionic Strength of Sample in Water--Organic Solvent Mixtures†

Norihisa ISHIKAWA*, Yasushi TANIGUCHI, 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, Yan-Bian University; 105 Gong-Yuan-Lu, Yan-Ji-Shi 133002 P. R. China

An indirect potentiometric determination of anions (or cations) using a precipitation reaction and a standard addition method which does not require the adjustment of an ionic strength and a solvent composition of the sample of a water--organic solvent mixture is proposed.
A certain volume (Vr) of the aqueous solution (reactant) containing the precipitant cation A of a known concentration cr is added to the sample of the water--organic solvent mixture of volume V containing the analyte anion B of a concentration cx. If the composition of formed precipitate is AmBn, the condition of Vrmcx V/ (ncr) needs to be satisfied. After addition of the reactant, an A-selective electrode and a reference electrode are immersed to the sample and this solution is titrated with the aqueous solution (standard-1) containing A of a known concentration cs1, where added volumes and the final added volume of the standard-1 are denoted with vs1 and vs10 respectively. The electromotive forces (E1) corresponding to added volumes (vs1) of the standard-1 are measured. Subsequently the same sample solution of volume V and the reactant of volume Vr are added to the titrated sample. This solution is titrated again with the aqueous solution (standard-2) containing A of a known concentration cs2 (cs1) and having the same ionic strength as that of the standard-1. The electromotive forces (E2) corresponding to added volumes (vs2) of the standard-2 are measured.
If E1 and E2 corresponding to vs1 and vs2 that satisfy a condition of vs2 = 2vs1|vs10 are read off from two titration curves, and a side reaction coefficient considering an ion association of A in the solution of E1 measurement is almost the same as that in the solution of E2 measurement, the following equation is held concerning with the concentration cx of B:

where y = 10E/S, xvs1{(cs2/cs1)|y}, EE2|E1, and S and gare the response slope of the A-selective electrode and a constant respectively. This cx is determined from the slope of linear plots of y vs. x.
The present indirect method can be applied to determinations of the hexacyanoferrate (II) in the media of water--ethanol and water--1,4-dioxane mixtures with almost the same precision as in aqueous solutions by using a silver ion as the precipitant and a silver ion-selective electrode as an indicator electrode.

† An Indirect Potentiometric Determination Not Requiring Adjustment of an Ionic Strength of Sample. Part 3.


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