How to judge the quality of PH glass electrode

User requirements for pH glass electrodes 1. Low impedance glass film 2. Chemical corrosion within the pH range of 0-14. The thin-film glass with linear potential value in the range of 0-14 pH is mainly composed of SiO2 regular tetrahedral network, network frame and network charge points. One of the components can loosen the SiO2 network bonds to allow easy exchange of monovalent cations. The early thin film glass is generally sodium glass, and this kind of thin film glass shows good linearity within pH 10, but there is a large alkali error above this. Today, lithium compounds (lithium glass) are added to the molten glass to make thin-film glasses with a wide pH range and only a small alkali error. When the surface of the film glass is wetted by water, the alkali ions on the surface will dissolve, that is to say, the surface of the film glass is hydrolyzed. Depending on the type of glass, this hydrolysis can form a 0.3-0.6 nm thick swelling layer on the glass surface. For H+ ions, the swelling layer acts much like an ion exchanger. As the pH value of the measured medium changes, H+ ions will diffuse into or out of the swelling layer. For the inner side of the film glass, the whole process is the same as above, except that because the inner solution is fixed (such as the inner buffer solution with a pH of 7), only a constant H+ ion activity is formed. The inner and outer swelling layers are separated by the glass tissue, and a potential difference is established across the glass membrane due to the different surface potentials on the inner and outer sides of the glass membrane. This potential difference can be measured by the zero current method with a mV meter with a pH scale and displayed as a pH value. This potential follows the Nentes equation. Under the condition of 25°C, when the proton activity changes by one level (one pH), the potential changes by 59.16mV. Alkali-acid error Theoretically speaking, the potential curve corresponding to all pH values ​​should be linear. In fact, there are nonlinear situations at both ends of the characteristic curve. The so-called acid error is mainly due to the irreversible H+ ions stored in the swelling layer. Therefore, in the range of about pH less than 2, the potential cannot reach the potential value of 59.16mV/pH that satisfies the Nentes equation, which is too high. Excellent glass film almost eliminates this error. For base errors, the manifestation is that the displayed pH value is too low. The reason is the lack of H+ ion concentration due to the substitution of alkali ions. This error occurs especially when the measured medium contains high concentrations of sodium and lithium ions. Alkali error can be reduced by suitable allocation formula. The concept of an electrode with excellent performance should be as follows: 1. Reference system Ag/AgCl2, stable half-cell potential reference system with electrolyte salt bridge and anti-pollution reference system 3. Diaphragm anti-pollution TEFLON annular diaphragm 4. Maintenance-free KCl gel-filled maintenance-free reference system capable of 15 bar pressure All the above-mentioned features are embodied in our electrodes. 1. The circular diaphragm surrounds the pH glass film, forming a central symmetrical area of ​​high ion activity, which makes it have the advantages of a tapered gap diaphragm and avoids its shortcomings, such as: due to the effect of eddy currents, scaling and thermal load Shock causes diaphragm clogging and large KCl consumption. The electrodes are maintenance-free and the reference system is not prone to clogging. Measurements are also good in ion-deficient media by taking a KCl salt stock. 2. There is no need to fill holes with electrolyte solution, so the glass tube is sealed, and the incompressibility of electrolyte solution makes it applicable to occasions where the maximum pressure is 6 bar (special measures can reach 10 bar). 3. Unlike the traditional diaphragm, it is not sensitive to dirt. 4. The Ag/AgCl half-cell has an independent reference tube, that is, the stable half-cell potential is in contact with the measured medium through the salt bridge. This keeps the half-cell potential stable for a relatively long period of time. 5. The gel does not contain silver, so it does not react to the measured medium containing sulfur. 6. When subjected to thermal shock and cooling in the air, it does not produce continuous air bubbles like the porous diaphragm. These air bubbles present behind the porous diaphragm cause electrical insulation of the reference system. 7. Compared with porous diaphragm, there will be no gel extrusion under thermal shock.