Intrinsic End-Point Errors in Precipitation Titrations with Ion Selective Electrodes

Investigations on the effect of ionic interferences in potentiometric precipitation titrations with ion selective electrodes are reported. When the inflection point of the titration curve is used to locate the end point, serious errors (>1%) can result which are often outside the range of the ultimate analytical precision (0.1%) achievable in such analyses. Results are presented in terms of β, a dimensionless variable related to the solubility product and the initial analyte concentration, and b, a parameter related to the concentration and selectivity ratio of interfering ions. In general, errors result from finite selectivity ratios and from the intrinsic absence of symmetry in the titration curve. The errors are negligible only in the limit of very sparingly soluble materials or in the absence of any interfering ions.