Analysis Edition: One Question per Week (October 24–30)

Principle of operation of a glass electrode: The main component of a glass electrode is a glass bulb; the lower half of this bulb consists of a glass membrane that is selective in its response to H+. The bulb contains a reference solution of 0.1 mol•L-1 HCl with a constant pH, and an Ag-AgCl electrode is inserted into it as the internal reference electrode. This constitutes the glass electrode. The potential of the internal reference electrode in a glass electrode is constant and independent of the pH of the solution being measured. The glass electrode can measure the pH of a solution because the membrane potential generated by the glass membrane is related to the pH of the solution being tested. The glass electrode must be soaked in an aqueous solution for a certain period of time before use. This results in the formation of a hydrated silica layer on the outer surface of the glass film; due to the effect of the internal reference solution, an inner water and silica layer is also formed on the inner surface of the glass. When the soaked glass electrode is immersed in the solution to be tested, the hydrated layer comes into contact with the solution. Due to the difference in H+ activity between the surface of the silica gel layer and that of the solution, a concentration gradient is created; as a result, H+ ions migrate from the region of higher activity to the region of lower activity. This establishes an equilibrium between the silica gel layer and the H+ ions in the solution, altering the charge distribution at the interface between the gel and the liquid and generating a certain interfacial potential. Similarly, a certain interphase potential also exists at the interface between the hydrated silica layer on the inner side of the glass membrane and the internal solution. Their interphase potential can be expressed by the following equations: Φ_out = k1 + 0.059lg a1/a1ˊ, Φ_in = k2 + 0.059lg a2/a2ˊ. Here, a1 and a2 represent the H+ activities of the external solution and the internal reference solution, respectively ； a 1ˊ and a 2ˊ represent the H+ activity on the surfaces of the outer and inner hydrated silica gel layers of the glass membrane, respectively ； k1 and k2 are constants determined by the properties of the outer and inner surfaces of the glass film, respectively. Since the properties of the inner and outer surfaces of the glass membrane are essentially the same, k1 = k2. Moreover, since all the Na+ ions on the surface of the hydrated silica layer are replaced by H+ ions, a1′ = a2′. Therefore, Φ_membrane = Φ_outside – Φ_inside = 0.059 log(a1/a2). As the H+ activity a2 in the internal reference solution is a constant value, Φ_membrane = K + 0.059 log(a1) = K + 0.059 pH. This shows that, at a constant temperature, the membrane potential of a glass electrode is directly proportional to the pH of the solution being tested.

The main component of a glass electrode is a glass bulb; the lower half of this bulb consists of a glass membrane that is selective in its response to H+. The bulb contains a reference solution of 0.1 mol•L-1 HCl with a constant pH, and an Ag-AgCl electrode is inserted into it as the internal reference electrode. This constitutes the glass electrode. The potential of the internal reference electrode in a glass electrode is constant and independent of the pH of the solution being measured. The glass electrode can measure the pH of a solution because the membrane potential generated by the glass membrane is related to the pH of the solution being tested. The glass electrode must be soaked in an aqueous solution for a certain period of time before use. This results in the formation of a hydrated silica layer on the outer surface of the glass film; due to the effect of the internal reference solution, an inner water and silica layer is also formed on the inner surface of the glass. When the soaked glass electrode is immersed in the solution to be tested, the hydrated layer comes into contact with the solution. Due to the difference in H+ activity between the surface of the silica gel layer and that of the solution, a concentration gradient is created; as a result, H+ ions migrate from the region of higher activity to the region of lower activity. This establishes an equilibrium between the silica gel layer and the H+ ions in the solution, altering the charge distribution at the interface between the gel and the liquid, and thereby generating a certain interfacial potential

The main component of a glass electrode is a glass bulb; the lower half of this bulb consists of a glass membrane that is selective in its response to H+. The bulb contains a reference solution of 0.1 mol•L-1 HCl with a constant pH, and an Ag-AgCl electrode is inserted into it as the internal reference electrode. This constitutes the glass electrode. The potential of the internal reference electrode in a glass electrode is constant and independent of the pH of the solution being measured. The glass electrode is able to measure the pH of a solution because the membrane potential generated by the glass membrane is related to the pH of the solution being tested

There are many methods for measuring pH, mainly chemical analysis, test strip method, and potentiometric method. Here, the pH value measured by the potentiometric method is mainly introduced. The electrode used in potentiometric analysis is called a galvanic cell. A galvanic cell is a system whose function is to convert the energy of chemical reactions into electrical energy. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) is composed of two half-cells. One of these half-cells is called the measuring electrode, and its potential is related to the activity of specific ions, such as ； The other half-cell is the reference half-cell, commonly known as the reference electrode; it is usually in communication with the solution being measured and connected to the measuring instrument.

The membrane potential generated by the glass membrane is related to the pH of the solution being tested

The function of a glass electrode is to generate a potential difference that responds to changes in the hydrogen ion activity of the solution being measured. By placing a pH-sensitive electrode and a reference electrode in the same solution, a galvanic cell is formed, whose potential is the algebraic sum of the potentials of the glass electrode and the reference electrode. The E-cell is an E-reference E-glass cell. If the temperature remains constant, the potential of this cell changes with the pH of the solution being tested. It is difficult to measure the potential generated by the cell in a pH meter, as its electromotive force is very small and the impedance of the circuit is extremely high (1–100 MΩ). Therefore, the signal must be amplified so that it is sufficient to drive a standard millivoltmeter or milliammeter.

Reply 1# *ngguanglueguo Just joining in the conversation~~ A membrane electrode made of a glass film that exhibits a potential response to hydrogen ion activity is a commonly used hydrogen ion indicator electrode. 　　It is usually spherical in shape, with 0.1 mol/L hydrochloric acid and a silver chloride electrode or a calomel electrode built into it. Before use, it is immersed in pure water to form a thin swelling layer on its surface; during use, it is placed together with another reference electrode in the solution to be tested to form a cell, the potential of which is directly related to the pH value of the solution. Due to factors such as asymmetrical potential and junction potential, the pH value cannot be determined directly from the cell potential; instead, a standard buffer solution is used for \"calibration,\" and the pH value is calculated based on the definition of pH. Glass electrodes are not affected by oxidizers, reducers, and other impurities; they have a wide pH measurement range and are widely used.

Glass electrodes have a selective adsorption capacity for hydrogen ions, and the amount of adsorption is proportional to the hydrogen ion concentration； When hydrogen ions adsorb onto the surface of the glass electrode, a potential difference is created between them and the solution inside the glass bulb; the reference electrode can then be used to measure the potential difference across the entire circuit, which is related to the concentration of hydrogen ions. This is how a glass electrode works.

The potential of the internal reference electrode in a glass electrode is constant and independent of the pH of the solution being measured. The glass electrode can measure the pH of a solution because the membrane potential generated by the glass membrane is related to the pH of the solution being tested. At a certain temperature, the membrane potential of the glass electrode is in a linear relationship with the pH of the solution.

Working principle: The pH glass electrode and the reference electrode act as sensors; when inserted into the solution to be measured, they form a galvanic cell. The reference electrode provides a stable potential, while the potential of the pH electrode is related to the pH value of the solution, and these values obey the Nernst equation. The pH value of the solution can be determined by accurately measuring the electromotive force between the two electrodes.