Although the oxygen electrode disc is one of the smallest components of an oxygen measurement system, it is the most crucial component of the system. Therefore, maintaining the disc to a high standard is extremely important.

A significant proportion of technical support requests received by Hansatech Instruments Ltd relate to oxygen electrode disc performance. Of these requests, the vast majority are the result of failure to comply with recommended working practices and maintenance procedures. Some of the more common abuses of electrode discs include:

Failure to maintain electrode disc

If the electrode disc is not maintained, deposits of silver chloride (soft brown deposit) and/or silver oxide (hard black deposit) will build up on the silver ring (anode) of the disc. The formation of silver chloride is normal and desirable in small quantities as it improves the stability of the sensor. It is easily removed by cleaning and will reform when the electrode is next polarised. By contrast, deposits of black silver oxide are to be avoided. Silver oxide is an electrical insulator and its formation will reduce the available surface area of the silver anode and result in a dramatic reduction or complete loss of electrode signal. The electrode disc should be maintained in accordance with our recommended procedure. Please download a copy of the Hansatech Instruments Electrode Care & Maintenance Manual from the Downloads page.

This image shows a typical case of poor maintenance. The disc has not been cleaned after use and was not stored in a sealed container with desiccant away from strong sunlight. As a consequence, the epoxy resin has become prematurely discoloured due to a combination of strong sunlight and absorption of moisture from the environment. The silver anode is almost entirely covered with a deposit of black silver oxide. The platinum stud and dome are both dirty and badly scratched. This will cause an uneven layer of electrolyte beneath the membrane during measurement leading to un-stable and un-responsive signals. This disc is beyond economic repair.

In contrast, this disc has been maintained according to our recommendations and has been stored correctly in an air tight container with desiccant. The epoxy resin is clear, the silver anode is smooth and polished and the platinum cathode is scratch free. The area of the electrode dome surrounding the cathode is also free from abrasions and pits across the entire surface. An electrode disc, when kept in this kind of condition will provide years of high quality measurements.

After use, the electrode disc should be stripped down, cleaned carefully and stored in a dry, air-tight container. If the disc is simply left, electrolyte will crystallise under the membrane and may cause irreversible damage to the electrode. The length of time that an electrode can be left prepared will depend upon local circumstances such as temperature and air flow through the laboratory. In general terms, we would recommend leaving a prepared electrode disc in a liquid-phase measurement system with stirred air-saturated water in the chamber overnight. The electrode disc can be kept polarised or switched off and re-polarised the next day dependant upon local working practices and user experience. In gas-phase measurements, the electrode is likely to dry out more rapidly and we would recommend stripping down and cleaning the electrode after each measurement day.

The KCl electrolyte dries out quickly in air and begins to crystallise. KCl is quite aggressive and will, by capillary attraction, deposit in any minor crevice. If the electrode is left to dry out several times, it is possible that the KCl will finally break down the seal between either of the metal electrodes and the epoxy resin surround and seep into the fissure formed where it will crystallise to further expand the fissure. This is most likely to occur between the platinum cathode and epoxy resin dome and regrettably will cause most damage at this point.

Damage to the electrode can be observed under low power microscopy (x10 magnification) as a broad white collar around the neck of the platinum as shown in the image left. The effect of this will be to cause instability and drift of the electrode signal and a large increase in the level of electrical signal observed in zero oxygen (residual current). This will have the effect of reducing the ability of the electrode to measure small changes in oxygen flux and will eventually render the electrode unserviceable and beyond economic repair. The disc will require replacement.

Hansatech Instruments Ltd offer the S16 cleaning kit for maintenance of the oxygen electrode disc. The kit contains special cleaning pastes which are recommended for optimal electrode performance. In the absence of this kit, users have resorted to cleaning the electrode with various abrasive compounds including commercial metal polishes or smoker's toothpastes etc. We would caution against the use of inappropriate materials as they may cause irreversible damage to the electrode.

Some metal polishes are harmful to the disc as they can contain an ammonia base or solvent additive. These substances can cause irreversible poisoning to the platinum cathode and could also cause significant damage to the epoxy resin. Excessive cleaning with abrasive substances can badly scratch and alter the profile of the electrode dome which may cause an uneven layer of electrolyte across the cathode leading to instability, drift and lack of responsiveness of the oxygen signal.

The S16 electrode cleaning kit can be obtained by contacting us directly or by contacting a relevant International Hansatech Distributor. Details of the cleaning procedure can be found in the Hansatech Instruments Electrode Care & Maintenance Manual available for download from the Downloads page.