{"term":{"id":"09115","title":"glass pH-sensitive electrode","longtitle":"IUPAC Gold Book - glass pH-sensitive electrode","doi":"10.1351\/goldbook.09115","code":"09115","status":"current","synonym":"<em>synonyms<\/em>: glass electrode, pH electrode","definitions":[{"id":1,"text":"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":{"1":"Other geometrical forms may be appropriate for special applications, e.g., a capillary electrode for the measurement of blood pH.","2":"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.","3":"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.","4":"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.","5":"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).","6":"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\\ \\pu{pH(test)}\\)."},"links":[{"term":"Nernst equation","url":"https:\/\/goldbook.iupac.org\/terms\/view\/09068"},{"term":"Nikolsky–Eisenman equation","url":"https:\/\/goldbook.iupac.org\/terms\/view\/09120"},{"term":"pH-sensitive electrode","url":"https:\/\/goldbook.iupac.org\/terms\/view\/09121"}],"sources":["PAC, 2020, 92, 641. 'Terminology of Electrochemical Methods of Analysis (IUPAC Recommendations 2019)' on page 661 (https:\/\/doi.org\/10.1515\/pac-2018-0109)"]}],"altoutputs":{"html":"https:\/\/goldbook.iupac.org\/terms\/view\/09115\/html","xml":"https:\/\/goldbook.iupac.org\/terms\/view\/09115\/xml","plain":"https:\/\/goldbook.iupac.org\/terms\/view\/09115\/plain"},"citation":"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. 10.1351\/goldbook.09115","license":"The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/) for individual terms.","collection":"If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .","disclaimer":"The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.","accessed":"2026-07-26T15:36:29+00:00"}}