The Clark electrode has a Teflon membrane which is not permeable for methanol. The membrane in your electrode might be damaged or clogged.

Yes, you can get interferenses from diferent gassing. A Teflon membrane is not permeable for liquid until not contaminated is (organic material). CO, CO2, SO2 ... can go trought the membrane and make interferenses especially if you are in lo level of dO2.

make a scanning CV  to detect interferences in solution after membrane difusion 

Most probably your membrane is damaged. There are several types of gas permeable membranes and there are different substances that can affect your membrane. You should first find out what kind of membrane is use and check which substances can affect that type of membrane. As said earlier, there are other gases that can also penetrate your membrane affecting your readings. Furthermore, a clark cell usually has a solution of potassium nitrate of known concentration (often saturated) under your membrane. Make sure that solution or gel is hydrated and that it is still there under the membrane. A clark cell involves a certain electronic circuit to operate so make sure that is providing the correct current levels..

one of advantages of this electrode is Only O2 can pass through membrane; therefore fewer interfering substances. Interfering substances have to be capable of being reduced with the applied voltage.  Methanol not passing through it. your electrode must be checked it may be clogged.

The reliability depends on the design. Stability is the major problem of most Clark sensors.

We counter such problems due to one or more of the following reasons:

lack of cleanliness of the electrode. We generally clean electrode at least ones in a week with simple white toothpaste (which generally has calcium carbonate and some amount of aluminum hydroxide). Kindly go through attached book of Prof. David Walker (a genius) for details. Silver anode often turns black due to oxidation and calcium carbonate can clean it. Similarly aluminum oxide/hydroxide can clean platinum cathode;

membrane might turn brittle due to excessive use or exposure to high concentrations of quinones (such as p-benzoquinone), methyl viologen, DCMU, hydrogen peroxide etc. Yes, theoretically none of them should affect the membrane, but repeated use of same membrane for several assays will cause this brittleness. When we perform several assays, we prefer to change membrane every day.  

Membrane might be damaged during removing the contents and washing after each assay.  Avoid using pointed or hard tips removing the contents.

Kindly go through the book of Prof. Walker.

Hope, it will be useful to look at paper under the link:

Measuring Dissolved Oxygen - Environmental Measurement ...

www.fondriest.com/...measurements/.../dissolved-oxy...

Tłumaczenie strony

Dissolved Oxygen Measurement Methods; Dissolved Oxygen Sensor ... and subject to error due to other redoxing agents that may be present in the water 27.

dear Jan, unfortunately the link is not working

Sorry, the page is available, but could not be saved. The title of the report is FUNDAMENTALS OF ENVIRONMENTAL MEASUREMENTS

The link is as follows:

http://www.fondriest.com/environmental-measurements/equipment/measuring-water-quality/dissolved-oxygen-sensors-and-methods/

Please note particularly  reference 27  available under the address:

http://www.fondriest.com/environmental-measurements/references/#DOERef

Dissolved Oxygen References

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