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Here is my (limited) understanding of my question:

A pH probe consists of two half cells. One is a reference cell, constructed such that the EMF across the cell is stable and well defined. In the case of an Ag/AgCl reference cell the silver wire is connected electrically to the negative terminal of a voltmeter, allowing the movement of electrons through the wire and eventually into the buffer solution in the reference half cell. The reference cell allows free movement of ions (current) without excess diffusion of buffer solution (commonly KCl). Those electrons are part of a chemical reaction which turns AgCl into Ag ions and solute Cl (and vice versa). Therefore, on the other half of the liquid junction (our solution with unknown pH) there is some voltage present. This voltage is equal to the EMF of the reference half cell, which for Ag/AgCl I think is around 220mV.

This reference voltage is added to the EMF across the measurement probe (a measurement glass that is sensitive to pH according to the Nernst equation) and is approximately +/-420mV.

  1. If this is the case, why is the measured output of the pH sensor not something like -200mV to +640mV?

  2. If the pH glass creates it's own EMF why can't we just submerge a copper (or otherwise conductive) wire into the solution as our reference (ground)?

A practical example:

I have a commercial pH probe. I submerge it in some liquid with pH of 7 and I observe that the voltage across the cell is about 0mV as expected. Now I place a copper wire which is electrically connected to the negative terminal of my electrometer and find that an offset voltage appears of approximately 220mV (perhaps by coincidence or as a ground loop - not yet convinced by the second).

  1. Is it still possible to use the pH probe as normal by subtracting or otherwise calibrating out this offset?
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2 Answers 2

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Consider the case where you wish to remember a friend's address with a reference to a famous restaurant, but the restaurant magically keeps shifting its position without warning day by day! This would certainly be a disappointment. Such an unpredictable and constantly shifting "reference" may be useless for addressing purposes. The same holds true for copper electrodes or any electrode that isn't a reference electrode. They produce unpredictable EMF! What will happen when these metallic wires are dipped in solutions without the corresponding ions? As you may recall, Nernst's equation requires the ions to be of the same metal in contact with each other. If you want to use a copper electode, copper ions must be present in solution. For these reasons, you want a stable electrode, whose potential does not change from solution to solution, day to day and minute by minute.

Copper is quite reactive electrochemically and tends to form an oxide layer. Also note that pH measurement belongs the branch of electrochemistry called potentiometry. In potentially, ideally zero current is drawn during measurement.

BTW, nobody knows how the glass electrode works with full certainty. This is a very fancy system, if you think about it carefully. There is a thin glass membrane. The inside contains a known concentration of acid, and the outside is your test solution. In what way does the glass develop its potential difference? This has baffled electrochemists.

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  • $\begingroup$ Yes - this makes things clearer. I am still a little unclear on what potential develops across the reference cell. As I understand, some potential comes from each Galvanic interface (silver wire to silver chloride, silver chloride to reference solution, etc). Why aren't those voltages superimposed - in other words: is the EMF from silver wire to unknown liquid (across liquid junction) zero? $\endgroup$
    – Bobby Carlo
    Sep 19, 2022 at 4:12
  • $\begingroup$ And in a similar vein: my experimental setup shows large DC offsets when a grounded copper wire is placed. Are these offsets electrochemical or electrostatic? $\endgroup$
    – Bobby Carlo
    Sep 19, 2022 at 4:13
  • $\begingroup$ One principle of electrochemistry is that no one knows the potential of a single electrode. So we can never measure individual potentials developing across interfaces. $\endgroup$
    – AChem
    Sep 19, 2022 at 12:42
  • $\begingroup$ @EdV so in this case the copper wire (meant to provide a drain for static charge) should be galvanically isolated from the pH probe? $\endgroup$
    – Bobby Carlo
    Sep 19, 2022 at 14:00
  • $\begingroup$ Yes but in practice (say in an industrial setting) the solution vessel cannot be floating for safety reasons. I understand that copper is galvanically undesirable - but is it a problem if the grounding rod be made from stainless steel for example? $\endgroup$
    – Bobby Carlo
    Sep 19, 2022 at 15:48
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In other words, you propose replacing the reference electrode with its own solution with a logistically simpler "copper vs test solution" electrode.

  1. Your copper electrode potential will depend on copper ions concentration in the solution. You know it, right?

  2. Should there be no much of copper ions, you will contaminate the test solution with copper ions. Well, in some cases this is not of much importance. But the potential will vary a lot depending on the degree of contamination.

  3. Your copper electrode potential will depend on both the pH AND the redox properties of the solution, in a complex interdependent way. Good luck knowing it with an acceptable accuracy.

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