User requirements for pH glass electrodes

1. Low impedance glass film

2. Chemical corrosion within the pH range of 0-14

3. Not easily damaged

4. Thermal stability

5. Good reproducibility

6. Not easily hydrolyzed

7. No alkali error

8. From 0 to 14pHLinear potential values within the range

Thin film glass is mainly composed of SiO2 tetrahedral network, network framework, and network charge points. The latter component can loosen the SiO2 network bonds to facilitate the exchange of monovalent cations. Early thin film glass was generally sodium glass, and this type of thin film glasspHWithin 10, it exhibits good linearity, while above this, there is a significant alkaline error. Nowadays, adding lithium compounds (lithium glass) to glass liquid allows thin film glass to have a wide pH range and only a small alkali error.

When the surface of thin film glass is wetted by water, the alkali ions on its surface will dissolve, which means that the surface of the thin film glass is hydrolyzed. Depending on the type of glass, this hydrolysis can form a swelling layer on the surface of the glass with a thickness of 0.3-0.6nm. For H+ions, this swelling layer resembles 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 thin film glass, the entire process is the same as described above, except that the internal solution is fixed (such as an internal buffer solution with pH 7), so only a constant H+ion activity is formed. The swelling layers on the inner and outer sides are separated by the glass tissue, and a potential difference is established on the glass film due to the different surface potentials on both sides of the glass film. This potential difference can be measured using the zero current method with a pH calibrated mV meter and displayed in the form of pH value.

This potential follows the Nernst equation, and at 25 ℃, when the proton activity changes by one level (one pH), the potential changes by 59.16mV.

Alkali acid error

In theory, it corresponds to allPH valueThe potential curve should be linear, but in fact, there are nonlinear situations at both ends of the characteristic curve.

The so-called acid error is mainly caused by the irreversible H+ions stored in the swelling layer. Therefore, within the range of approximately pH less than 2, the potential cannot reach the potential value of 59.16mV/pH that satisfies the Nernst equation, which is too high. Advanced glass film almost eliminates this error.

For alkaline errors, their manifestation is that the displayed pH value is too low. The reason is the insufficient concentration of H+ions caused by the substitution of alkali ions. This error particularly occurs in situations where the measured medium contains concentrations of sodium and lithium ions. Alkali errors can be reduced through appropriate blending formulas.

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

1. Reference system Ag/AgCl

2. Stable half cell potential with electrolyte salt bridge reference system and anti pollution reference system

3. Diaphragm Anti pollution TEFLON Circular Diaphragm

4. Maintenance free KCl gel filling capable of withstanding 15bar pressure

Maintenance free reference system

All the above characteristics are reflected in our electrodes.

1. The circular membrane surrounds the pH glass film, forming a central symmetric region with high ion activity, giving it the advantages of a conical gap membrane while avoiding its shortcomings, such as membrane blockage and large KCl consumption caused by eddy currents, scaling, and thermal load impact. The electrode does not require maintenance, and the reference system is not easily clogged. It can also be well measured by using KCl salt reserves in a medium lacking ions.

2. There is no need to fill the holes with electrolyte solution, so the glass tube is sealed. In addition, the incompressibility of electrolyte solution makes it suitable for applications with even higher pressures of 6 bar (special measures can reach 10 bar).

3. Unlike traditional membranes, it is not sensitive to dirt and stains.

4. The Ag/AgCl half cell has an independent reference tube, which means that the stable half cell potential is in contact with the measured medium through a salt bridge. Keep the half cell potential stable for a considerable period of time.

5. There is no silver in the gel, so it will not react to the tested medium containing sulfur.

6. When subjected to thermal shock and cooling in the air, it does not produce continuous bubbles like a perforated diaphragm. These bubbles that exist behind the porous membrane can cause electrical insulation in the reference system.

7. Compared with the hole diaphragm, there is no gel extrusion under thermal shock.