Automatic titration by stepwise addition of equal volumes of titrant. Part VII. Potentiometric precipitation titrations

Abstract

The method used throughout this series to add the titrant stepwise with equal volumes each time is well suited to precipitation titrations. The advantage that one has to wait for equilibrium to be reached at only a few points is of special importance.

Two methods for calculation of the equivalence volume in precipitation titrations have been evaluated. One is based on the solution of a set of linear equations and the other is an extended version of the Gran I method. The methods are characterised by the following facts: (1) side-reactions can be accounted for; (2) the values of the solubility product and the stability constants of complexes may be unknown; (3) the solutions may be so dilute that a complete precipitation is not obtained; and (4) no accurate calibration of the electrode couple is required.

The calculation methods have been tested on the determination of chloride by titration with silver nitrate solution. The two methods agree closely and the errors in the calculations are negligible. The over-all errors in the determinations are 0.1–0.2% at chloride concentrations down to 10^–4M. At lower concentrations the relative errors are greater.

The methods have also been tested on the determination of sodium fluoride with lanthanum nitrate in (a) neutral and unbuffered solution, (b) acidic solution, (c) solution buffered with acetic acid-acetate and (d) solution buffered with formic acid-formate. The calculations were performed on experimental curves according to Lingane. For (a) and (b) good agreement between calculated and expected values were obtained. In the acetate-buffered solution too high a value was obtained in spite of the fact that acetate and mixed acetate-fluoride complexes were considered. For (d) the buffer capacity was too small to give satisfactory results.