compound. Oxidases require oxygen to react, and so decrease the amount of oxygen in their immediate
environment. The degree of oxygen depletion is hence related to the enzyme activity, which is in turn
determined by the amount of the oxidizable species present. Use of a dissolved oxygen electrode to
measure the degree of oxygen depletion will thus provide a basis for the measurement of the amount of
the oxidizable species which is present.
Specially designed electrodes in which the electrode tip is covered in a gel containing the appropriate
oxidase are commercially available. A semipermeable membrane retaining the gel, and permitting
oxygen diffusion, completes the assembly.
Applications of Enzyme Sensors
The type of enzyme sensor described above is highly selective and can be sensitive in operation. There
are obvious applications for the determination of small amounts of oxidizable organic compounds.
However, it is perhaps too early to give a realistic assessment of the overall importance of enzyme
sensors to analytical chemistry. This is especially so because of parallel developments in other
biochemical sensors which may be based upon a quite different physical principle.
Commercially available kits allow for the determination of compounds such as ethyl alcohol, glucose,
lactic acid and lactose.
Amperometric Titrations
If the limiting current flowing in a polarographic cell is measured during a titration in which any of the
reactants or products are reducible at the micro-electrode, the equivalence point can readily be detected.
The choice of applied potential is not critical provided it corresponds to a point on the limiting current
plateau. Values of id are simply plotted as a function of the volume of titrant added. Three types of
titration curve may be observed, as shown in Figure 6.15(a), (b) and (c). In each case, the equivalence
point is located at the intersection of two straight lines. Curvature in this region is due to partial
dissociation of the products of the titration but as current readings can be taken well away from the
equivalence point, accuracy is not impaired (cf. potentiometric and visual indicator methods). Titrations
which produce curves similar to that shown in Figure 6.15(a) are sometimes referred to as 'dead-stop'
methods because the current falls virtually to zero at the equivalence point. The most accurate
determinations can be made if the curve is V-shaped (Figure 6.15(c)). Unless the titrant is at least
twenty times as concentrated as the analyte solution, current readings must be corrected for volume
changes by multiplying each one by the ratio (V + υ)/V, where V is the solution volume before adding
an increment of titrant of volume υ.
An alternative to the DME is the rotating platinum micro-electrode. Steady currents can be obtained by
rotating the electrode at a constant