Figure 6.17
Circuit diagram for coulometric titrations.The titrant is usually generated directly in the solution whence it can immediately react with the species
to be determined. The electrodes are typically made of platinum coils or sheets. The counter-electrode is
normally isolated from the bulk solution by enclosing it in a tube sealed with a sintered-glass disk to
prevent contamination of the solution with undesirable electrolysis products. An alternative
arrangement, especially useful for acid-base titrations, is to generate the reagent externally before
adding it to the titration vessel. An apparatus for generating and separating hydrogen and hydroxyl ions
using an electrolyte solution such as sodium sulphate is shown in Figure 6.18. To eliminate errors due
to impurities in reagents used in the preparation of the sample, a 'pre-titration' method is sometimes
used. A blank solution is first titrated, the sample is added to the titrated solution and the solution
titrated again. This procedure is particularly advantageous for determinations at very low levels.
Figure 6.18
Device for external generation of H+ and OH–.