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I would like to develop a device that measures pH in aqueous solutions. I'm not a chemist, but I have some basic knowledge about electrochemistry. So, I thought I could use two electrodes quite close together; the first (-) is ideally isolated from the solution under test and within another reference solution with 0V reduction potential. The other electrode (+) is located inside the solution under examination and both are connected by a salt bridge. Then, using a microcontroller I will increase the voltage until I see that current starts to flow. Knowing the applied voltage, I can ideally measure the concentration of H3O+ ions in the solution under test. Do you think it could work or is it a bad idea?

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    $\begingroup$ Measuring pH via dynamic processes involving significant current is a bad idea. Aside of the need of knowledge of the relation pH as function of electrode potential, there are various potential differences/drops due kinetic reasons and there are overpotentials for evolving gaseous hydrogen and mainly oxygen. It would need precise calibration and even then, various solutions with the same pH could have different electrode potential. Aside of the glass electrode, there is also older technique of quinhydrone electrode using a Pt wire. $\endgroup$
    – Poutnik
    Sep 11, 2023 at 9:59
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    $\begingroup$ Correction - the QH electrode seems to use antimony - I falsely remembered Pt from my memory. It has also limitation for pH<=8 and absence of strong oxidation or reduction compounds. $\endgroup$
    – Poutnik
    Sep 11, 2023 at 10:22

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Measuring pH with current flowing is not an appropriate idea. You would like to measure something without chemically disturbing the system. The moment you pass current in a solution, you start to change the local pH near the electrode. There is a reason why the glass electrode is popular despite the fact that its working is not fully understood is that (i) No electrolysis is involved (ii) The thin glass membrane has a very high resistance (iii) It is highly hydrogen ion selective (in a limited pH range). At pH extremes acid or alkaline errors begin to show up.

Now, not to completely disregard your idea, there are contactless conductivity measurments which work pretty well. Think along those lines and see if you can find anything useful for pH measurements.

The key idea for electrode based sensor is that they are selective to the ion being measured. If you just random pick an electrode, and start electrolysis, it will be responding to all ions in solution not just the hydrogen ion.

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