Standard Addition Method Using Ion-selective Electrodes in the Coexistence of Interfering Ions

Abstract

Although the standard addition method using a nitrate-selective electrode brought positive errors in the coexistence of chlorate, it was experimentally shown that the linearity of Y={(V+ vs)/V.10e/s} vs. vs plotting was still kept (V: sample volume, vs: standard solution volume, e: difference between potentials before and after addition of the standard solution, S: response slope of ion-selective electrodes). In an attempt to elucidate the influence of the interfering effect on the standard addition method, investigation was carried out by numerical calculations, to know what factors affect the linearity of Y vs. vs plotting and accuracy. As a result, it was shown that the interfering effect affects most the accuracy and the linearity of Y vs. vs plotting is maintained even under a large interfering effect. When the ratio of the standard solution concentration to the sample concentration is large, the Y vs. vs plotting is figured by a straight line, whereas it is figured by a gentle curve, yielding greater concentration-errors, when the ratio is small. The electric charge of ions does not much affect the linearity as well as the concentration-error. A great concentration error occurs also due to the error of the response slope q. At q>0, positive errors occur, whereas negative ones occur at q<0. Even if errors in the response slope exist, the Y vs. vs plotting is figured by a very gentle curve (by two straight lines, in the approximate sense). Positive errors attributable to the interfering effect are compensated in the range of q<0, and cancelled to be zero in apparently at a certain value of q.