Thin-film glass is primarily composed of SiO. ₂ It is composed of a tetrahedral network, a network framework, and network charge points. The last component can be SiO. ₂ The network bonds are loosened to allow the exchange of单价 cations. Early membrane glass was typically soda glass, which exhibited good linearity up to a pH of 10 but showed significant alkaline error at higher pH values. Today, lithium compounds are added to the glass melt (lithium glass), resulting in membrane glass with a broader pH range and only minimal alkaline error.

When the surface of thin-film glass is wetted by water, alkali ions on the surface dissolve out, which means the thin-film glass undergoes hydrolysis. Depending on the type of glass, this hydrolysis can result in the formation of a swelling layer 0.3 to 0.6 nm thick on the glass surface. For H ⁺ From an ionic perspective, this swelling layer closely resembles an ion exchanger. As the pH of the medium being measured changes, H ⁺ Ions diffuse into the swollen layer or out of it. For the inner side of the thin-film glass, the entire process is the same as described above; however, since the internal solution is fixed (e.g., an internal solution with a pH of 7), only a constant concentration of H⁺ is established. ⁺ Ionic activity. The swelling layers on the inner and outer sides are separated by the glass matrix, and the differing surface potentials on the two sides of the glass membrane give rise to a potential difference across the glass. This potential difference can be measured using the zero-current method with a pH-scale mV meter and displayed as a pH value.

This potential obeys the Nernst equation: at 25°C, a one-unit change in proton activity (i.e., a one-unit change in pH) results in a 59.16 mV change in potential.

Alkaline-acid error

In theory, the potential–pH curve should be linear across all pH values; in practice, however, nonlinearity is observed at both ends of the characteristic curve.

The so-called acid error is primarily caused by the irreversible H+ ions stored in the swelling layer. Consequently, within a pH range of approximately less than 2, the potential fails to reach the value of 59.16 mV per pH unit as predicted by the Nernst equation, resulting in an excessively high potential. Advanced glass membranes have virtually eliminated this error.

With regard to alkaline error, it manifests as an abnormally low pH reading. This is caused by the substitution of alkali ions, which leads to a deficiency in H+ ion concentration. Such error is particularly prevalent when the medium being measured contains high concentrations of sodium and lithium ions. Alkaline error can be mitigated through the use of appropriate buffer solutions.

The concept of a high-performance electrode should be as follows:

1. Reference system: (Ag/AgCl)

2. Stable half-cell potential and anti-fouling reference system (reference system with an electrolyte salt bridge).

3. Diaphragm (anti-fouling TEFLON annular diaphragm).

4. Maintenance-free reference system capable of withstanding 15 bar pressure (maintenance-free KCl gel filling).

All of the aforementioned characteristics are reflected in our electrodes.

1. A circular diaphragm encloses the pH glass membrane, creating a centrally symmetric region of high ionic activity. This configuration combines the advantages of conical-slit diaphragms while mitigating their drawbacks, such as diaphragm clogging and excessive KCl consumption caused by flow effects, scaling, and thermal shock. The electrode requires no maintenance, and the reference system is resistant to clogging. Even in media with low ion concentrations, accurate measurements can be obtained by using a KCl backup solution.

2. No electrolyte solution needs to be added to the pores, so the glass tube is sealed. Moreover, due to the incompressibility of the electrolyte solution, this design enables the device to operate under pressures as high as 6 bar (and up to 10 bar with special measures).

3. Unlike traditional diaphragms, it is insensitive to contamination.

4. The Ag/AgCl half-cell is equipped with a separate reference junction, meaning that the stable half-cell potential is maintained by contact with the measured medium via a salt bridge. This ensures that the half-cell potential remains stable over a considerably long period of time.

5. The gel does not contain silver, so it does not react with sulfur-containing test media.

6. When subjected to thermal shock and cooling in air, it does not generate continuous bubbles, unlike perforated diaphragms. Such bubbles trapped behind perforated diaphragms can compromise the insulation of the reference system.

7. Compared with porous separators, gel electrolyte does not leak under thermal shock.

What is pH, and what should you know about pH measurement?

None

Regarding pH

PH3200 Instrument User Manual

PH3000E Instrument User Manual

PH3300E Instrument User Manual

PH Sensor Instruction Manual

Basic Principle of pH Sensor

What is pH, and what should you know about pH measurement?

Regarding Conductivity

Conductivity measurement