Question

A titration has the following general reaction: Analyte + Titrant  Product or simply, A + T ->P

A sensor is used to monitor the progress of a titration, providing a signal that relates to the chemical reactions occurring in the titration flask. The sensor responds to the concentration of P only, not A or T, with signal that is linear with the concentration of P.

(a) Sketch a plot of the titration data one would obtain (in general terms), with the Sensor Signal as a function of the Volume of Titrant added. Label both axes, and the equivalence point. Assume for part (a) that Kf is large, eg. 106 or more (recall the  is really an equilibrium).

(b) If the Kf was not large, but much smaller, what qualitative effect would this have on your plot in (a), and in terms of determining the equivalence point? Illustration would be helpful here.

Answer 1

Graphing the signal of the detector against the volume of the titrant, we will obtain in general cases graph A, where the equivalence point is between the interval of the end point of the titration, one of the axes is the volume of the titrant, and the other is the logarithm of the concentration.
If we consider a difference between a large Kf and a small Kf, if we put them on a single graph, it would look like graph B.
Kf must be large enough for the titration to be quantitative, because it is important that the product obtained is stable, that the end point can be distinguished and that the error is small.