Question: conductivity sensor electrolysis a concern in moving water?

MadTinker is asking a question about general
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by MadTinker | February 10, 2018 20:01 | #15712

I'm gone through quite a few of the references provided by donblair in conductivity page Many thanks for these! One of the open questions was the issue with electrolysis that might occur between the sensor electrodes when over 1v is applied.

Q: would this really be an issue when the sensor is placed in moving water, e.g. a stream or river?


I know nothing about this topic, but here is the model I concocted from whole cloth to understand it.

Measuring conductivity of water with electrodes is based on measuring the electrical current flowing between the electrodes. If some of that current is being used to split water molecules into OH- and H+, then that current is not reaching the electrode (or else work is being done without conservation of energy). So it seems that it would not matter that the ions and cations are washed away in the river because the measurement of electrical current has already been altered.

However, I would not be surprised to learn that my model of how electricity works is completely wrong.


Yeah, that seems right. I should have thought thru it a bit more.

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There are two types of current that can be generated when two conductive materials are dipped into the water. 1- Faradaic current 2- Non Faradaic current

When is applied an electrical potential greater than the oxidation or reduction potential of the water or dissolved substances or even the material from which the electrodes are made, occur a transfer of electrons between the electrodes and the substances undergoing oxidation or reduction reactions This is the "Faradaic" current.

But when the applied potential is less than the oxy-reduction potentials of water or dissolved substances, or of the material from which the electrodes are made, then occur the migration of the ions present in the solution which moves to the oppositely charged electrodes . See:

With this migration occurs an increase of the concentration of these ions in the vicinity of the electrodes forming what is called "electrical Double Layer".


The increase in the concentration of ions in the vicinity of the electrode is accompanied by the "accumulation" of electrons in the electrode. But these electrons remain on the electrode and are not transferred to the solution. This is the "non faradaic" current (or capacitive current).

In this way there is a high initial current that goes down as the ions accumulate in the vicinity of the electrodes until they reach zero.

"It is a process similar to the charging of a capacitor, in which the electrodes represent the plates of a capacitor and the solution plays the role of a dielectric."

When the polarity of the electrodes is reversed occurs the discharge of the double layer and the recharge with the opposite polarity.


In conventional conductivity measurements a potential variable is used, below the redox potential that alternates the potentials of the electrodes generating non-faradaic currents of loading and unloading of the double layer.

The generated signal is rectified and filtered before the measurement:


Hope this helps. Markos

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