https://doi.org/10.1351/goldbook.09115
pH-sensitive electrode, usually consisting of a bulb, or other suitable form, of special glass attached to a stem of high-resistance glass, complete with an internal reference electrode and internal filling solution system.
Notes:
- Other geometrical forms may be appropriate for special applications, e.g., a capillary electrode for the measurement of blood pH.
- The potential across the membrane is measured between two reference electrodes (usually silver-silver chloride, chloride) and is related to the ratio of hydronium-ion activities by the Nikolsky–Eisenman equation.
- Line notation for the cell is: external reference electrode \(\vert\) external reference electrolyte \(\vert\vert\) test solution \(\vert\) glass membrane \(\vert\) internal reference electrolyte \(\vert\) internal reference electrode.
- The bulk of the glass membrane is dry (of about \(\pu{50 \upmu\!m}\) thickness) with two hydrated layers (of \(\pu{5 to 100 nm}\) thickness) on each side of the membrane that contact the solution.
- The hydrated layer acts as an ion-exchanger for the exchange of \(\ce{Na+}\) ions of the membrane with \(\ce{H3O+}\) ions from an aqueous solution. \[|\ce{Si{-}O{-}Na + H+(aq)<=>|Si{-}OH + Na+(aq)}\] The glass electrode is therefore selective to \(\ce{Na+}\) (and any other alkali metal ions).
- The membrane potential originates as a Galvani potential difference between the two sides of the glass membrane, which depends on the ratio of the activities of \(\ce{H+}\) ions on both sides of the membrane. In the absence of interfering ions, the membrane potential can be expressed by the Nernst equation \[\begin{align*} E &= K + (RT/F)\ln(a_{\ce{H+}}({\rm{test}})/a_{\ce{H+}}({\rm{internal}})) \\ &= K^{\prime}\!-\!(RT/F)/(\log_{10}({\rm{e}}))\ {\rm{pH(test)}} \end{align*}\] where \(K\) and \(K^{\prime}\) are constants, \(R\) is the gas constant, \(T\) the thermodynamic temperature, \(F\) the Faraday constant, and \(a\) activity. At \(\pu{25^{\circ}C}\) this becomes \(E \approx K^{\prime} - 0.0592\ \rm{pH(test)}\).