Hello! I have a question concerning the functioning of a pH meter. All the sources online that I can find either omit, I think, important information, or describe it in "technical terms" with equations. I would like to know what is happening at the level of the distribution and movement of charge; after all, a pH meter, from my understanding, is simply measuring an electric potential difference. Okay, so there is the reference electrode of a certain metal or compound and the corresponding solution. Generally, I believe this is AgCl electrode in a chloride solution (such as AgCl or HCl). An equilibrium is formed between the solid AgCl and the aqueous Cl- and Ag+ (and AgCl2-?), generating an electrode potential. If another electrode of the same type and with the same solution with the same concentration (i.e. two identical half-cells) are connected. There is zero electric potential difference between the two electrodes. A second electrode of the same type is connected to the first, the reference electrode, via a voltmeter; this second electrode is called the glass electrode. The electic potential difference would therefore be zero between the two electrodes, as the electrodes and the solutions they are bathed in are identical. I hope this is accurate so far. The next step takes into account the nature of the glass electrode half-cell. The glass electrode has a special glass membrane sorrounding the electrolyte solution that can sequester H+ ions on both sides of the membrane. When the glass membrane is therefore submerged in a solution of a pH different to the internal solution bathing the glass electrode, a potential difference is established across the glass membrane. This apparently, can be used to determine the pH of the solution, since the pH inside the membrane (bathing the electrode) is known, and there is a relationship between the H+ concentration inside, H+ concentration outside and the potential difference. So if the inside pH is known, the potential difference across the membrane is known, then the outside pH can be determined. This I can understand; however, some of the sources I have read seem to claim that this is all that is needed, and everything else is constant, but if this was the case, how is a potential difference generated between the glass electrode and the reference electrode? If the two electrodes initially have a potential difference of zero, and it changes (or indeed ANY change is recorded on the voltmeter) then SOMETHING has to be changing between the two electrodes, which means both electrodes are changing in some way. If the voltmeter is accepted as having infinite resistance, then something is at least "attempting" to happen: in the case of a voltmeter connected to a battery, the electrons would be attempting to flow from the negative electrode to the positive electrode, which is where the reading comes from. In the case of the pH meter, if the voltmeter, positioned between the two electrodes, changes, then electrons are at least attempting to move ("attempting" if infinite resistance is accepted for the voltmeter). If electrons are attempting to move between the electrodes, then the electrodes are attempting to change in some fashion, and the electric potential across the glass membrane is affecting the electrodes. I was wondering, therefore, if it has to do with the response on the inside of the glass membrane; say, for example, that the pH was higher outside. This would draw H+ ions inside close to the surface, as they are attracted to the relatively negative charge outside. This sequestration of H+ ions inside (they are "taken out" of solution and held close to the surface of the glass membrane) alters the internal pH local to the glass electrode; this then alters the equilibrium between the ions in solution and the depositon of the atoms on to the electrode, altering the electrode potential. This would then create a difference in electrical potential between the two electrodes: the reference electrode and the glass electrode. Fundamentally, I imagine it as the glass membrane is used to store of release H+ ions into the glass electrode's bathing solution, altering the pH and therefore altering the electrode potential (which is pH-dependent). I suspect this is wrong, but I can't see otherwise how a difference in potential is created between the reference electrode and the glass electrode without the glass electrode's potential being altered in some way. There can't simply be a difference across the glass membrane that is measured, otherwise what it the point of the glass electrode and the reference electrode. Any help is much appreciated. I found this quite difficult to describe, I can upload some pictures drawn with my excellent artistry if that would help.