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Author: Admin | 2025-04-28
Types of Electrodes. (a) The SCE is a reference electrode that consists of a platinum wire inserted into a moist paste of liquid mercury (calomel; Hg2Cl2) and KCl. The interior cell is surrounded by an aqueous KCl solution, which acts as a salt bridge between the interior cell and the exterior solution. (b) In a glass electrode, an internal Ag/AgCl electrode is immersed in a 1 M HCl solution that is separated from the sample solution by a very thin glass membrane. The potential of the electrode depends on the H+ ion concentration of the sample. (c) The potential of an ion-selective electrode depends on the concentration of only a single ionic species in solution.Using Electrochemistry to Measure ConcentrationsOne of the most common uses of electrochemistry is to measure the \(\ce{H^{+}}\) ion concentration of a solution. A glass electrode is generally used for this purpose, in which an internal Ag/AgCl electrode is immersed in a 0.10 M HCl solution that is separated from the solution by a very thin glass membrane (Figure \(\PageIndex{5b}\). The glass membrane absorbs protons, which affects the measured potential. The extent of the adsorption on the inner side is fixed because \(\ce{[H^{+}]}\) is fixed inside the electrode, but the adsorption of protons on the outer surface depends on the pH of the solution. The potential of the glass electrode depends on \(\ce{[H^{+}]}\) as follows (recall that \(pH = −\log[\ce{H^{+}}]\)):\[\begin{align*} E_{glass} &= E′ + (0.0591\; V \times \log[H^+]) \\[4pt] &= E′ − 0.0591\; V \times pH \end{align*}\]The voltage \(E′\) is a constant that depends on the exact construction of the electrode. Although it can be measured, in practice, a glass electrode is calibrated; that is, it is inserted into a solution of known pH, and the display on the pH meter is adjusted to the known value (Figure \(\PageIndex{7}\)). Once the electrode is properly calibrated, it can be placed in a solution and used to determine an unknown pH.Figure \(\PageIndex{7}\): A pH meter measures the hydrogen-ion concentration in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode, and so the pH meter is sometimes referred to as a "potentiometric pH meter". (CC By-SA 2.0 Generic; Sergei Golyshev via Wikipedia)Ion-selective electrodes are used to measure the concentration of a particular species in solution; they are designed so that their potential depends on only the concentration of the desired species (Figure \(\PageIndex{5c}\)). These electrodes usually contain an internal reference electrode that is connected by a solution of an electrolyte to a crystalline inorganic material or a membrane, which acts as the sensor. For example, one type of ion-selective electrode uses a single crystal of Eu-doped \(\ce{LaF3}\) as the inorganic material. When fluoride ions in solution diffuse to the surface of the solid, the potential of the electrode changes, resulting in a so-called fluoride electrode. Similar electrodes are used to measure the concentrations of other species in solution. Some of the species
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