Precipitation titrations:
Titrations involving the precipitation reactions are not nearly as numerous in titrimetric analysis for as those involving redox or acid - base reactions. One of the purposes for limited use of such reactions is the lack of suitable indicators. In a few cases, particularly in the titration of dilute solution, the rate of reaction is too slow for convenience of titrations. As the equivalence point is approached and the titrant is added slowly, a high degree of super- saturation does not exist and the precipitation may be very slow. Another difficulty is that the composition of the precipitate is frequently not known because of co-precipitation effects. While the latter could be minimized or partially corrected for by processes such as ageing the precipitate. This is not possible in direct titrations involving the formation of precipitate.
The indicator electrode for a precipitation titration is frequently the metal from which the reacting cation was derived. The Membrane electrodes which are sensitive to one of the ions involved in the titration may also be employed.
Theoretical curves for a potentiometric titration are readily derived. For example, we may describe the potential of a silver electrode during the titration of iodide with silver ion by the expression.
EAg = - 0.151 - 0.0591 log [I¯ ] ... (2.47)
Where - 0.151 is the standard potential for the half reaction
Ag I (s) + e ↔ Ag (s) + I- ... (2.48)
Alternatively, the potential is given by
EAg = 0.799 - 0.0591 log 1/ [Ag+] ... (2.49)
where 0.799 is the standard potential for the reduction of silver ion.