Question

1. You may have already noticed that the shape of your spectrophotometric titration curve is quite different than the shapes of the potentiometric titration curves which you have constructed in earlier experiments. Why? (HINT: Think about the mathematical relationships between pH and concentration and absorbance and concentration . . .).

2. When performing a potentiometric titration it is very important to make a large number of measurements near the endpoint, while for a spectrophotometric titration one actually ignores the measurements obtained near the endpoint. Explain.

3. In order for a photometric method to be used for a titration, what requirement(s) must be met by the titration system?

Answer 1

1.

In any titration we measure the concentration of one of reactants or of one of the products. The chemical reaction show a linear relation between them. If the sensor response is also linear R = f (concentration), then the calibration plot will be linear, in fact a combination of lines with different slopes before and after EP. This is the case for spectrophotometric titration where the system response is A (absorbance) = ecl.

For a potentiometric titration the response is E = const ± constxlogX, where X is the measured species. For a pH controlled titration E = E^o – 0.0596 (pH – const).

2. For a spectrofotometric titration the EP is at the intersection on the plot of the linear variations recorded between and after EP (with different slopes). It is not the case for the potentiometric titrations, when relevant measurements have to be recorded close to EP.

3. The simplest system is a spectrophotometer that have to also allow:

- to introduce a burette above the spectrometer cell

- to stirr the solution after each addition of a new volume of titrant.