A Modified Null-Point Potentiometry by the Use of an Ion-selective Electrode

Abstract

A modified null-point potentiometry by the use of aR ion-selective electrode is.described. The procedure is as follows: An indifferent electrolyte is added to a sample solution, standard solution and diluent, in order to bring them to the same ionic strength and thus the activity coefficients of the test ions in all the solutions are identical. After the addition of the standard solution, whose volume and concentration are v, and C, , respectively, to the initial samplel'solution, whose volume is V, in which'the ion-selective electrode and reference electrode are immersed, the diluent, whose volume is v, is added. Prior to the addition of the standard solution, the cell emf of the sample solution is measured to be Ei. The cell emf, E3, after.each., addition of the diluent, is plotted aga.inst log(Vvs), and from this plot the volume, vd, of the added diluent corresponding to the equivalence Xpoint of Ei=Eg is obtained. Thus, the concentration, C, of the sa' mple solution is given by C=Cs vs/(vs vd) Data analysis for the evaluation of the equivalence point is easily accpmplished by the least squares method or by the three points method using three cell emfs (Ei, E3, and th6 cell emf after the addition of the standard solution). The concentrations of fluoride solutions ranging from to 5 10-5 molll are determi:ned wiarelative standard deviation of about o (mean). over the entire range exept the lowest concentration level. The least squares method shows a better precision than the thr.ee pQints