External generation of titrant

The limitations of coulometric titration with internal generation of the titrant include the following. 

No substance may be present which undergoes reaction at the generator electrodes for example, in acidimetric titrations the test solutions must not contain substances which are reduced at the generator cathode. 

When applied on a macro scale — samples of 1 - 5 millimoles — generation rates of 100-500 milliamps are required parasitic currents may be induced in the indicator electrodes at currents in excess of about 10-20 mA consequently precise location of the equivalence point by amperometric methods is not trustworthy. 

A minor disadvantage of external generation of titrant is the dilution of the contents of the titration cell care is therefore necessary in suitably adjusting the rate of flow and the concentration of the generator solution. The procedure is, however, admirably suited for automatic control. 

9 EXPERIMENTAL DETAILS FOR TYPICAL GOULOMETRIC TITRATIONS AT CONSTANT CURRENT 

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Figure 4.5. Apparatus for the external generation of titrant for coulometric titrations.

Figure 5.36 Flow cell for external coulometric generation of a titrant.

As an alternative for the external application of a stepwise change in pH, as described above, titrant can be added by coulometry. Coulometric generation of protons or hydroxyl ions is possible by sending a current between a noble metal actuator electrode, situated closely around the ISFET gate, and a distant counter electrode 115]. 

A major contribution to decreased current efficiency is the presence of electroactive impurities in the sample. This can be a particular problem with acid-base titrations. In such cases, it may be necessary to generate the titrant externally (in the absence of sample), and then add it to the sample in increments. A suitable arrangement is illustrated in Figure 4.5. By the use of stopcocks the titrant can be generated and flushed into the cell in increments. 

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