Automatic titration by stepwise addition of equal volumes of titrant. Part VI. Further extension of the Gran I method for calculation of the equivalence volume in acid-base titrations

Abstract

In Part V of this series a very simple method for the calculation of the equivalence volume (V_e) from titration data was presented. It was based on the method of stepwise addition of equal volumes of titrant combined with an extended version of the Gran I method. The method presented had some limitations, however, as it could not be used for acids with log K between 2 and 4.2 or for very weak acids (log K > 7).

In this paper equations of a more complicated nature are given, which can be used for monoprotic acids with log K⩽ 10. The equations contain, besides titrant volumes and hydrogen ion concentration, values for K and K_w, which, however, need not be known but are calculated from the titration data at the same time as V_e. The same type of calculation can be used in the calculation of V_e for most di- and triprotic acids.

A discussion is given of possible causes for errors in the equivalence volume, calculated using the equations presented. When accurate automatic pipettes are used for adding the titrant the uncertainty in the determination of the hydrogen ion concentration is the main source of error.