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

Abstract

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 (V_r) of the aqueous solution (reactant) containing the precipitant cation A of a known concentration c_r is added to the sample of the water—organic solvent mixture of volume V containing the analyte anion B of a concentration c_x. If the composition of formed precipitate is A_mB_n, the condition of V_r ≥ mc_xV/(nc_r) 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 c_s1, where added volumes and the final added volume of the standard-1 are denoted with v_s1 and v_s1⁰ respectively. The electromotive forces (E₁) corresponding to added volumes (v_s1) of the standard-1 are measured. Subsequently the same sample solution of volume V and the reactant of volume V_r are added to the titrated sample. This solution is titrated again with the aqueous solution (standard-2) containing A of a known concentration c_s2 ( >c_s1) and having the same ionic strength as that of the standard-1. The electromotive forces (E₂) corresponding to added volumes (v_s2) of the standard-2 are measured.
If E₁ and E₂ corresponding to v_s1 and v_s2 that satisfy a condition of v_s2 = 2v_s1 − v_s1⁰ 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 c_x of B:
_{&nikkashi_2001_83;} (y = [nc_s1/nc_rV_r − mc_xV]x + g)
where y = 10^ΔE/S, x = v_s1{(c_s2/c_s1) − y}, ΔE = E₂ − E₁, and S and g are the response slope of the A-selective electrode and a constant respectively. This c_x 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.