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.
If this is the case, why is the measured output of the pH sensor not something like -200mV to +640mV?
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).
- Is it still possible to use the pH probe as normal by subtracting or otherwise calibrating out this offset?