An improved standard addition potentiometry in water - Organic solvent mixtures

An improved standard addition potentiometry for a sample of water-organic solvent mixtures, in which the adjustments of both the ionic strength and the solvent composition are not required, is proposed. After the sample of the water-organic solvent mixture of volume V, in which an ion-selective electrode and a reference electrode are immersed, is titrated with the aqueous solution of the diluent and the electromotive forces (E-1) which correspond to volumes (v(d)) of the added diluent are measured (the final volume of the added diluent is denoted with v(d0)), the same sample solution of another certain volume (V-0) is added to the titrated sample. Subsequently this mixed sample is titrated again with the aqueous solution of the standard containing the analyte ion of a known concentration (c(s)) and having the same ionic strength as that of the diluent, and the electromotive forces (E-2) which correspond to volumes (v(s)) of the added standard are measured. E-1 and E-2 which correspond to v(d) and v(s) that satisfy a condition of v(s)=[1+(V-0/V)]v(d)-v(d0) are read off from two titration curves. In this condition, the ionic strength and the solvent composition of the sample solution added the diluent are the same as those of the sample solution added the standard. If the degree of association of the analyte ion in the sample solution of E-1 is almost the same as that of E-2 the following equation is held concerning with the total concentration (c(x)) of the analyte: 10(Delta E/S)=1+C-s/CxV(v(d)-v(d0)') where v(d0)'=v(d0)/[1+(V-0/V)], Delta E=E-2-E-1, S is the response slope of an ion-selective electrode. This c(x) is determined from the slope of linear plots of 10(Delta E/S) vs. (v(d)-v(d0)'). The present method can be applied successfully to determinations of the fluoride in the media of the water-ethanol, the water-1,4-dioxane and the water-acetonitrile mixtures with almost the same accuracy and precision as in aqueous solutions.