Figure 6.7
Acid base titration curves.Precipitation Titrations
Such titrations involve the progressive removal of a species from solution by precipitation. Ions
frequently used for this purpose include Ag+, Pb2+, Cu+, Zn2+, CNS–, , S^2 – and halides. The
indicator electrode may be of the metallic or membrane type, e.g. Ag for a Ag+–halide titration or F-
membrane for a La3+–fluoride titration. Precipitation titration curves usually show departures from the
theoretical curve due to adsorption of ions present in excess on the surface of the precipitate. Thus, if a
solution of an iodide is titrated with silver nitrate, before the equivalence point the AgI precipitate
adsorbs I– ions thereby changing Ecell. After the equivalence point, the excess of Ag+ ions is adsorbed on
the precipitate producing a similar but opposite effect on Ecell. The net result is to make the inflection in
the titration curve less sharp (Figure 6.8(a)). Mixtures can be titrated where the solubility products of
the individual salts with the titrant ion differ significantly, e.g. mixtures of halides, whose silver salts
have the following solubility products: