Complexometric titration

In a complexometric titration, a colored indicator Ind is added to the solution we wish to analyze. The indicator is capable of creating a colored complex E_int -Ind_(aq) with the analyte of interest (1), with a different color from that of the indicator Ind _(aq) alone in solution under the titration conditions.

(1) E _int(aq) + Ind_(aq) = E_int - Ind_(aq)

As with acid-base indicators and redox indicators, care must be taken not to add too much so that most of the analyte of interest remains free (uncomplexed) in solution.

The titrant T is chosen in order to form a complex E_int -T _(aq) with the analyte of interest and so that: Its formation is more likely to occur than the previously formed complex E_int -Ind_(aq); Its color in solution is different from that of E_int -Ind_(aq): ideally colorless.

During titration, E_int -Ind_(aq) complexes are first formed with the free analyte of interest (2) and then reacted with E_int -Ind_(aq) (3). The indicator Ind is released and the solution changes to a characteristic color (different from that of E_int -Ind_(aq) ) specific to Ind _(aq).

(2) E _int(aq) + T_(aq) = E_int -T _(aq)

(3) E_int -Ind_(aq) + T_(aq) = E_int -T _(aq) + Ind _(aq)

An example of colorimetric complexometric titration is the determination of water hardness.