(a) pH glass electrode
The user requirement for pH glass electrode
1, low impedance glass film
2, 0 ~ 14 pH range of chemical corrosion
3, not easy to damage
4, thermal stability,
5, good reproducibility
6, is not easy to hydrolysis
7, non-alkali error
8, in the pH range of 0 ~ 14 linear potential value
Glass film is mainly composed of SiO2 is tetrahedral network, network framework and network charge points. One final component can make the SiO2 loose network key to make the unit price of cation exchange easily. Early film is generally sodium glass, this film glass within pH10 showed a good linear, and it has over the larger error of alkali. Nowadays, people add lithium in glass compound lithium (glass) makes the film glass has a wide pH range and only a very small base error.
When thin film of glass surface is wetted by water, the surface of the alkali ions will dissolve out, that is to say, the hydrolysis of thin film surface of glass. Depending on the type of the glass, the hydrolysis can make glass surface form 0.3 ~ 0.6 nm thick layer of swelling. For H + ions, the swelling layer is like an ion exchanger. As the change of measured medium pH value, H + ions can spread into the swelling of diffusion layer or from the swelling. For the inside of the thin film glass, the whole process is the same, just because the solution is fixed (such as pH of 7) within the buffer, so just to form a constant H + ion activity. The medial and lateral swelling layer separated by glass organization, because on both inner side and outside of the glass membrane of different surface potential at the same time and set up a potential difference on the glass membrane. A potential difference that can be through the zero current method with a mV meter with a pH scale measure and displayed in the form of pH value.
This potential can follow, equation under the condition of 25 ℃, proton changes in activity level (pH), potential changes in 59.16 mV.
Theoretically, corresponding to all potential of pH curve should be linear, in fact, the characteristic curve of two side have the nonlinear problem.
So-called acid error is mainly due to the irreversible swelling layer storage in H + ions. So about pH in less than 2 within the scope of potential reach can satisfy tesla equation of potential value of 59.16 mV/pH is too high. Advanced glass membrane is almost to eliminate the error.
For alkali error, its performance is to display the pH value is too low. The reason is caused by alkali ions replace the lack of H + ion concentration. This error in the measured medium containing sodium concentration and the occasion of lithium ion. Alkali error through the appropriate formula to reduce.
The concept of a good performance of the electrode should be as follows:
1, Ag/AgCl reference system
2, the stability of the half cell potential and anti pollution reference system with electrolyte salt bridge reference system
3, diaphragm anti-pollution TEFLON circular diaphragm
4, ability of 15 bar pressure maintenance free reference system of KCl and maintenance-free gel filled in
All the above characteristics, reflected in our electrode.
1, the garden of the diaphragm encirclement of pH glass film, forming high ion activity center symmetric area, make it a tapered aperture diaphragm advantages and avoid its disadvantages, such as: due to the eddy current effect, scale and thermal load impact Block and large consumption of KCl diaphragm. Electrode without maintenance, reference system is not easy to jam. In ion of medium by KCl salt reserves can be taken for good measure.
2, do not need to add electrolyte solution hole, so the glass tube is sealed, and the incompressibility of the electrolyte solution, make it can be applied to high pressure of 6 bar (special measures up to 10 bar).
3, different from traditional diaphragm, is not sensitive to the dirt.
Four and a half, Ag/AgCl battery with independent reference tube, that is to say, the stability of the half cell potential through contact with the measured medium salt bridge. The half cell potential remains stable in a long time.
5, a gel contains no silver, so for not measured medium containing sulfur reaction.
6, under thermal shock and cooling in the air, don't like the hole type diaphragm will produce continuous bubbles. These exist in a hole at the back of the diaphragm air bubbles will cause a reference system of electric insulation.
7, compared with holes diaphragm, under the thermal shock, there would be no gel extrusion.