A standard addition method with an ion-selective electrode which does not require the adjustment of the ionic strength of a sample is proposed. After the sample, of volume V, is titrated with the first standard solution containing the analyte ion of a known concentration (cat), the sample solution of a certain volume V-o is added. Subsequently it is titrated with the second standard solution of a known concentration (c(s2)) having the same ionic strength as that of the first standard solution. If the electromotive forces, E(1) and E(2), which correspond to the titration volumes, v(S1) and v(S2), by the first and second standard solutions that satisfy a condition of v(S2)=v(S1)(V+V-o)/V-v(S1)(o) are read off respectively on the two titration curves, the concentration, c(x), of the analyte ion is determined from the slope of a linear plot of y vs. x (v(S1)(o) is the final volume of the first standard solution added): y=c(S1)/c(x)V.x+p where y=10(Delta E/S), X=v(S1) (c(S2)/C-S1-Y), Delta E=E(2)-E(1), S and p are a response slope of an ion-selective electrode and a constant respectively. Concentrations of the fluoride ranging from 10(-3) to 5 x 10(-5) mol.dm(-3) in the samples of various ionic strengths were determined with an error of less than 0.4% and a relative standard deviation of less than 0.5%.
