The 1302 Oxygen Electrode is a precision electrode with a very small diameter. Its rate of oxygen consumption is extremely low so that when used with the relatively low permeability polypropylene membranes, most of the resulting oxygen gradient is confined to the distance between the outside of the membrane and the cathode surface. Consequently there is no requirement for physical movement of the solution to replenish the oxygen at the outer surface of the membrane.
The relaxation of a stirring requirement for the 1302 results in a small stirring artifact (2-3%) when the electrode has been calibrated in stirred solution and is then used in an unstirred environment (and vice versa). In general, electrodes using the fast-responding, highly permeable FEP membranes have a large O2 flux through the membrane. This causes the electrode to behave as a macrocathode making it necessary to stir.
Unfortunately, it is not possible to build a fast electrode with no stirring requirement, and it is important to note that most Clark-type electrodes require stirring. Only in a microcathode electrode such as the 1302, fitted with a low permeability membrane, is stirring not required.
Stirred Clark-type electrodes
The Clark-type oxygen electrode consists of a probe at whose tip is an exposed gold or platinum cathode and a silver or silver/silver chloride anode. When the anode and cathode are polarized so that the cathode is held at a voltage of -0.6 to -0.8 volts relative to the anode, and connected via a solution of electrolyte such as KCl, the following reaction will occur at the anode:
4Ag > 4Ag + + 4e and 4Ag + + 4Cl- > 4AgCl
Simultaneously, at the cathode, any oxygen which is present is reduced:
O2 + 2H2O + 4e > 4OH-
Thus for each oxygen molecule reduced, 4 electrons of current flow in the circuit. Oxygen is therefore continually consumed as it is reduced to OH at the cathode. In practice, the anode and cathode are covered by an oxygen permeable membrane to exclude other species which would interfere. The KCl electrolyte is buffered to remove the OH produced in the cathode reaction. As oxygen is removed at the cathode, a pO2 gradient is set up which extends outwards into the surrounding medium. In unstirred water, oxygen therefore diffuses inwards along the pO2 gradient. Because of the pO2 gradient, the outside of the electrode membrane is effectively sensing a lower pO2 than that in the surrounding water. For this reason, most Clark electrodes require the water to be stirred. The size of the signal generated by the electrode is proportional to the flux of oxygen molecules to the cathode.
Last Revision 6.28.12
This is a Clark-type polarographic sensor with a 22 micron diameter platinum cathode and silver/silver chloride anode connected by a buffered potassium chloride electrolyte solution.
Used in conjunction with both the 782 Dual-Channel Oxygen Meter and 928 Six-Channel Oxygen Meter
May be used with or without medium stirring
Exhibits highly stable signal output
Very low rate of oxygen consumption
Compatible with all Strathkelvin accessories
This is a Clark-type polarographic electrode, with a 22 micron diameter platinum cathode and silver/silver chloride anode, connected by a buffered potassium chloride electrolyte solution.
In the normal configuration the cathode is covered with a relatively low permeability polypropylene membrane, in order for the electrode to be used in unstirred solutions or where minimal stirring is required. With these membranes, there is a relatively slow response time. For fast response, as required for rapidly respiring enzyme preparations, a thin FEP membrane (used with a special electrode jacket) is used. Rapid stirring of the medium is then necessary. The electrodes are not temperature compensated and require to be used at controlled temperatures (within + or -0.05°C). They should always be used in the electrode holder, so that only the tip of the electrode is exposed to the medium.
Response time at 37°C:
18 sec. for 90% change
6 sec. for 90% change
Oxygen consumption (polypropylene membranes)
0.5 to 3x10-10mg O2/min
2% per °C
Resolution of 928 and 782 meter
0.1% with 1302 electrode*
*The display resolution of the 982 and 782 meter is 0.1%. The A/D converter is capable of at least 0.01%, so you can work on an expanded scale. However, the limit is normally set to about 0.1% by the noise level of the 1302 electrode.