IUPAC - glass pH-sensitive electrode (09115)

glass pH-sensitive electrode

synonyms: glass electrode, pH electrode

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 $|$ external reference electrolyte $| |$ test solution $|$ glass membrane $|$ internal reference electrolyte $|$ internal reference electrode.
The bulk of the glass membrane is dry (of about $50 μ m$ thickness) with two hydrated layers (of $5 to 100 nm$ thickness) on each side of the membrane that contact the solution.
The hydrated layer acts as an ion-exchanger for the exchange of $Na^{+}$ ions of the membrane with $H_{3} O^{+}$ ions from an aqueous solution. $| Si - O - Na + H^{+} (aq) \overset{- ⇀}{↽ -} | Si - OH + Na^{+} (aq)$ The glass electrode is therefore selective to $Na^{+}$ (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 $H^{+}$ ions on both sides of the membrane. In the absence of interfering ions, the membrane potential can be expressed by the Nernst equation $\begin{aligned} E & = K + (R T / F) \ln (a_{H^{+}} (t e s t) / a_{H^{+}} (i n t e r n a l)) \\ = K^{'} - (R T / F) / (\log_{10} (e)) p H (t e s t) \end{aligned}$ where $K$ and $K^{'}$ are constants, $R$ is the gas constant, $T$ the thermodynamic temperature, $F$ the Faraday constant, and $a$ activity. At $25^{\circ} C$ this becomes $E \approx K^{'} - 0.0592 p H (t e s t)$ .

Source:
PAC, 2020, 92, 641. (Terminology of Electrochemical Methods of Analysis (IUPAC Recommendations 2019)) on page 661 [Terms] [Paper]

Citation: 'glass pH-sensitive electrode' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.09115 RIS BibTex EndNote

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glass pH-sensitive electrode