Determination of the equivalence point in potentiometric titrations. Part II

When the potential - volume curve obtained during potentiometric titrations shows only a small potential change at the end-point, it has been customary to plot a deltaE/deltaV - volume curve and to take the peak of this curve as the equivalent point. In 1950, the author proposed a method of transforming these curves by a numerical manipulation into straight lines intersecting at the equivalence point. In this article another way of transforming titration curves into straight lines has now been developed. A simple theoretical treatment shows that the method can be applied to titrations involving acids and bases, ionic precipitations, formation of complexes, and oxidation - reduction reactions. To facilitate the use of the method a table has been compiled giving quantities to be calculated and plotted against volume of titrant added. These quantities can be evaluated by simple slide rule calculations and, since straight line relationships hold, end-points can be obtained by simple extrapolation. The practice of the method is applicable to potentiometers calibrated either in millivolts or in pH units, even when titrations other than acid - alkali reactions are in use.