Determination of the Equivalent Point in Potentiometric Titrations.

Abstract

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 AE/AV - volume curve and to take the peak of this curve as the equivalence point. In 1950 the author proposed a method of trans- forming these curves by a numerical manipulation into straight lines intersecting at the equivalence point. In this paper another way of trans- forming 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 com- plexes, 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