in pH-sensor mechanisms, majority paper shows use of Ag/AgCl reference electrode as reference electrode or some time short with counter electrode. so is it possible to replace it with only one standard electrode like SHE or activated carbon electrode or Pt-electrode?
A potential developed at the interface of two phases is what you have to measure (see a Nernst's equation). At least one phase is liquid. Contacting a liquid phase (for a potential difference measurement) is not a trivial problem: a reference electrode is the appropriate probe for this task.
Case 1: The sensing device uses a metal/metal oxide structure (MO is exposed to solution). For measurement, a REF electrode is used to contact the sample solution. A voltmeter is used to measure the potential difference between metal and sample solution.
Case 2: The sensing device uses a glass membrane, each face of the membrane is in contact with a solution (one solution is the sample). For measurement, two REF electrodes are used to contact the internal solution (inside the pH electrode) and the external one (the sample). A voltmeter is used to measure the potential difference between the two reference electrodes.
A reference electrode must have a constant potential against the solution where it is immersed. SHE is a REF electrode, but it is not practical. An activated carbon electrode or a Pt electrode are not REF electrodes.
The use of a 3 electrodes cell may be used to measure [H+], not pH, in amperometric (not potentiostatic) mode. This situation is also not practical, a titration is simpler to do and more precise.
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