For conductimetric titrations, the volume V of the titrant is shown on the abissca (x-axis) and the conductance G (in S or mS) or the conductivity σ (in S. cm-1 or S.m-1) is shown on the ordinate (y-axis).
In the case of a titration, the calibration phase of the conductivity meter is not necessary, because the focus is on the development of the conductivity and not necessarily on its absolute value. The equivalence point is identified thanks to a break in the slope and the absence of calibration only shifts the y value: this does not change the value of the equivalent volume. In practice, the linear sections far from the equivalence point are the ones which are extended and used to identify the equivalence points.
Effects of dilution
Dilution influences the shape of the titration curves and makes the determination of the equivalent volume more difficult. This problem can be corrected by correcting the curves obtained:
Predicting the development of conductivity during a titration
In order to predict the development of conductivity during a titration, it is necessary to:
- Identify the ions initially present
- Identify the ions disappearing and appearing before the equivalence point
- Identify the ions disappearing and appearing after the equivalence point
For example, during the titration of a weak acid (uncharged) with a strong base, the titration reaction is:
Effect of acid dissociation
In the case of a weak acid with a low pKA, part of the acid is dissociated at the beginning: the lower the pKA or the more diluted the acid, the more it dissociates.
This dissociation, which is difficult to observe in pH measurement, is much easier to observe in the case of conductimetry.
Indeed, the dissociation of the acid according to the following reaction produces H+ ions which are titrated by the first drops of HO-. A decrease in conductivity is then observed before its increase.