The Clark (Rank) oxygen electrode
These instruments measure oxygen in solution using the polarographic
principle, i.e. by monitoring the current flowing between two electrodes when
a voltage is applied. The most widespread electrode is the Clark type (Fig.
34.6), manufactured by Rank Bros, Cambridge, UK, which is suitable for
concentrations in cell, organelle and enzyme suspensions. Pt
and Au electrodes are in contact with a solution of electrolyte (normally
saturated KCl). The electrodes are separated from the medium by a Teflon®
membrane, permeable to O2
. When a potential is applied across the electrodes, this generates a current proportional to the O2
concentration. The reactions
can be summed up as:
||4Ag → 4Ag+ + 4e−
||(at silver anode)
||O2 + 2e− + 2H ↔ H2O2
||(in electrolyte solution; O2 replenished by diffusion from test solution)
||H2O2 + 2e− + 2H+ ↔ 2H2O
||(at platinum cathode)
Clark-type oxygen electrodes are also available in probe form for
immersion in the test solution (Fig. 34.7) e.g, for field studies, allowing
direct measurement of oxygen status in situ, in contrast to chemical assays.
The main point to note is that
|Fig. 34.6 Transverse section through a Clark (Rank) oxygen electrode.
the solution must be stirred during
measurement, to replenish the oxygen consumed by the electrode
('boundary layer' effect).
The glucose electrode
The glucose electrode is a simple type of biosensor, whose basic design is
shown in Fig. 34.8. It consists of a Pt electrode, overlaid by two
membranes. Sandwiched between these membranes is a layer of the
immobilized enzyme glucose oxidase. The outer membrane is glucose
permeable and allows glucose in the sample to diffuse through to the
glucose oxidase layer, where it is converted to gluconic acid and H2
inner membrane is selectively permeable to H2
, which is oxidized to O2
at the surface of the Pt electrode. The current arising from this release of
electrons is proportional to the glucose concentration in the sample within
the range 10−7
Electrochemical detectors used in chromatography operate by voltammetric
principles and currents are produced as the mobile phase flows over
electrodes set at a fixed potential: to achieve maximum sensitivity, this
potential must be set at a level that allows electrochemical reactions to occur
in all analytes of interest.
|Fig. 34.7 A Clark-type oxygen probe.
|Fig. 34.8 Underlying principles of a glucose electrode.