The measured potential difference of a symmetrical cell with identical solutions and reference electrodes on each side of the glass @MT06878@. There is rarely the need, nor the possibility, of measuring the @A00486@ potential of commercial glass electrodes. Drifts in glass electrode potentials with time and variations from @D01527@ to @D01527@ in the potential measured in a standard buffer may be attributed to changes in @A00486@ potential.