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

An indirect potentiometric determination of anions (or cations) using a pre cipitation reaction and a standard addition method which does not require t he 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 pr ecipitant cation A of a known concentration c(r) is added to the sample of the water-organic solvent mixture of volume V con raining the analyte anion B of a concentration c(x). If the composition of formed precipitate is A(m )B(n), the condition of V-r greater than or equal to mc(x)V/(nc(r)) needs t o be satisfied. After addition of the reactant, an A-selective electrode an d a reference electrode are immersed to the sample and this solution is tit rated with the aqueous solution (standard-1) containing A of a known concen tration c(s1), where added volumes and the final added volume of the standa rd-1 are denoted with v(s1) and v(s1)(0) respectively. The electromotive fo rces (E-1) corresponding to added volumes (v(s1)) of the standard-1 are mea sured. Subsequently the same sample solution of volume V and the reactant o f volume V-r are added to the titrated sample. This solution is titrated ag ain with the aqueous solution (standard-2) containing A of a known concentr ation c(s2)(>c(s1)) and having the same ionic strength as that of the stand ard-1. The electromotive forces (E-2) corresponding to added volumes (v(s2) ) of the standard-2 are measured. If E-1 and E-2 corresponding to V-s1 and v(s2) that satisfy a condition of v(s2) = 2v(s1) - v(s1)(0) are read off from two titration curves, and a sid e reaction coefficient considering an ion association of A in the solution of E-1 measurement is almost the same as that in the solution of E2 measure ment, the following equation is held concerning with the concentration c(x) of B: [GRAPHICS] where y = 10(DeltaE/S), x = v(s1)((/c(s2)/c(s1)) -y), DeltaE=E-2 - E-1, 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 hexacya noferrate (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 i on as the precipitant and a silver ion-selective electrode as an indicator electrode.