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The Glass pH Electrode by Petr Vany ́sek Editor’s note: We are pleased to introduce a new magazine column that aims to take the read- er back to the classroom. This series of tutorials will be written by experts for a non-specialist audience. Please let us know what you think of this new feature. A glass electrode is perhaps the most successful and ubiqui- tous electrochemical sensor. It provides information about the activity of hydronium ions, H3O+, in water. Because water, which mildly dissociates to H3O+ and OH- ions, is the most common solvent medium, and chemical reac- tions in water largely depend on H3O+ activity, the ability to measure it is essential. And conversely, because H3O+ activity, or rather, its negative logarithm, the pH, is so easy to measure, pH is the most commonly monitored and recorded parameter of liquid samples. A glass electrode (Fig. 1) is actually a device, not an elec- trode in an electrochemical sense of the word. It consists of a glass bulb membrane, which gives it its name and an elec- trically insulating tubular body, which separates an internal solution and a silver/silver chloride electrode from the stud- ied solution. The Ag/AgCl electrode is connected to a lead cable terminated with some connector that can hook up to a special voltmeter, the pH meter. The pH meter measures the potential difference and its changes across the glass membrane. The potential difference must be obtained between two points; one is the electrode contact- ing the internal solution. A second point is obtained by connecting to a reference electrode, immersed in the studied solution. Often, this refer- ence electrode is built in the glass electrode (a combination electrode), in a concentric double barrel body of the device. Figure 2a shows a dia- gram of such a device. Figure 2a, the combination electrode and Fig. 2b, a glass electrode and separate refer- ence electrode, are functionally iden- tical. It is a common misconception that the combination electrode (Fig. 2a) requires only one lead, fostered because the round coaxial lead to the electrode looks like a single wire. This is not so. In any potentiometric measurement, and pH measurement is an example of one, two inputs, one of which is a reference point, are required. The completed glass electrode with a reference electrode cell is represented by the electrochemical short- hand Ag/AgCl | HCl | glass || probed solution | reference electrode (1) The potential difference relevant to pH measurement builds up across the outside glass/solution interface marked ||. The key functional part, the glass membrane, is manufac- tured by blowing molten glass into a thin-walled bulb with a wall about 0.1 mm thick. The bulb is then sealed to a thicker glass or plastic tube, and filled, for example, with a solution of HCl (0.1 mol/dm3). In this solution is immersed a silver/silver chloride electrode with a lead to the outside through a permanent hermetic seal. The filling solution has constant Cl- concentration, which keeps the Ag/AgCl inner electrode at fixed potential. The pH sensing ability of the glass electrode stems from the ion exchange property of its glass membrane. Glass is mostly amorphous silicon dioxide, with embedded oxides of alkali metals. When the surface of glass is exposed to water, some Si–O- groups become protonated Si-O- + H3O+ ≡ Si-O-H+ + H2O (2) The exchange of hydronium (or written as proton, H+) between the solid membrane and the surrounding solution, and the equilibrium nature of this exchange, is the key prin- ciple of H3O+ sensing. As with any interface separating two phases between which ionic exchange equilibrium is estab- lished, the glass membrane/solution interface becomes the site of a potential difference Eglass wall/solution ~ |RT/2.303F log a(H3O+)| (3) where R is the molar gas constant 8.314 J mol-1 K-1, T is the temperature in kelvins, F is the Faraday constant 96,485.3 C, 2.303 is a conversion between natur- al and common logarithm, and a(H3O+) is the activity of hydroni- um, which can be at lower concen- trations equated with its concentra- tion. At 30°C the value of RT/2.303F is approximately 0.060 V. The glass membrane has two wall/solution interfaces and there is potential buildup on each of them, with opposite polarity. But the pH inside the bulb is constant, because the internal solution is sealed. Therefore, the inner surface poten- tial is constant, adding merely to an offset to the overall potential of the device. Additional contribution to the offset comes from potentials of the inner solution electrode, and the reference electrode, which are also constant. The changes in the device potential are therefore due entirely to the pH changes of the outside solution and the potential of the glass electrode/reference electrode setup is Eglass electrode = E’ + RT/2.303F log a(H3O+) (4) Where E’ represents the sum of the constant offset poten- tials of the inner glass surface/solution and the two Ag/AgCl electrodes. At 30°C the potential of the glass membrane changes by about 60 mV for each one unit of pH (i.e., a ten- fold activity change). The possible range of hydronium activity encountered in aqueous solutions is large, as much as 10 to 10-15 mol/dm3. FIG. 1. Schematics of a glass electrode. The Electrochemical Society Interface • Summer 2004 19PDF Image | Glass pH Electrode
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