Extent of the Titration Reaction

The concept of the sharpness index is quite general and can be applied to all kinds of titrations. 

Remark It is an easy matter to demonstrate that the sharpness index is related to the buffer index of the solution. When we consider the following expressions  

We limit ourselves to the study of the case of the titration of a weak acid with a strong base. The titration reaction is 

According to the working hypothesis, it is not complete. The equilibrium constant is (assuming activities equal to concentrations) 

The link between K and the sharpness index in this case (see Sect. 10.5.2) is 

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Unfortunately, the equivalence point of a titration reaction is the least quantitative one, as we demonstrated in the preceding section. This is why logarithmic titrations are the most sensitive to the extent of the titration reaction. This is not the case for linear titrations. Let s first recall that linear titrations are those in which the dependent variable (the registered one) is directly proportional to the fraction titrated or to the concentration of the independent variable (in no case to the logarithm of its concentration or of its activity). Of course, the independent variable may be the activity rather than the concentration. Examples of linear titrations are spectrophoto-metric, amperometric, thermometric titrations, and so forth. This is the reason why Gran s method is interesting. Indeed, its principle involves the linearization of the logarithmic titration curves. 

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