What are the difficult factors in the measurement of high-purity water with a PH meter
1. Because it is pure water, its own buffer capacity is particularly weak, it is extremely easy to be polluted, and it is extremely easy to change its own pH value. If 2ppm impurities are mixed in pure water, the pH change is particularly obvious. For example: mixed with 2ppmNaoH pH value from 7→10, 2ppmCO2 pH value from 7→6, 2ppmNH3, pH value from 7→7.8, in general actual pH measurement, the influence mainly comes from the influence of electrolyte leakage into pure water on pH value and air In any case, the result measured at this time is not the pH value of pure water. Therefore, the measurement of pH value in pure water should try to avoid using electrodes added with potassium chloride (KCL) solution.
2. The conductivity of high-purity water is very poor, and it is easily disturbed by external electromagnetic fields. At the same time, static electricity and sound fields are easily generated during the flow process, which affects the stability and accuracy of measurement. Therefore, the measurement of the pH value of pure water must use a low-resistance sensitive membrane electrode, which can effectively reduce the interference of static electricity, magnetic field, and sound field, and at the same time make the electrode responsive.
3. When different solutions are in contact, a potential will be generated at the interface, commonly known as junction potential E6. Whether the junction potential is stable or not directly affects the stability of pH measurement. Furthermore, the smaller the junction area, the larger the junction potential, which is more likely to cause difficulties in measurement. Therefore, the pH measurement of pure water must use an electrode with a large interface, and at the same time keep the flow rate at the interface constant and small, so as to ensure a stable interface! In the traditional KCL solution electrode, the cross-section of the ceramic core is very small, so the junction potential is very large, and if it is changed to a frosted port or a ceramic core is added, the KCL solution will permeate in a large amount and pollute the solution. This kind of electrode is not suitable for measuring purity. For water, Secco now adopts the annular Teflon diaphragm with the largest cross-section abroad, which can solve these problems well. The high molecular polymer filled in the diaphragm can ensure a constant and small flow rate (10-8 / hour, while the ceramic diaphragm electrode is 1 drop / 5 minutes), thus avoiding the pure water pollution caused by KCL penetration and maintaining the junction Potential stability.
4. Since there are few ions in high-purity water, there is a diffusion resistance between the reference electrode and the measuring electrode. The stability of this potential E5 also affects the stability of pH value measurement. The distance between the specific electrode and the measuring electrode is too far, resulting in too much impedance between the two electrodes, which is easily affected by the change of flow rate, and the compound electrode solves this problem well, and the discrete electrode is not suitable!
5. The flow rate also has a great influence on the pH measurement of pure water. If the flow rate is unstable, the junction potential E6 and the diffusion potential E5 will be unstable, thus making the pH measurement unstable and inaccurate. Therefore, in the pH measurement of pure water, the flow rate should be kept as constant as possible, so that the related potential will not be unstable due to changes in the flow rate, resulting in pH fluctuations. This is an unchangeable reality. At present, any pure pH electrode in the world will be affected by the flow rate, which is determined by the theoretical characteristics. It is against the theory and impossible to claim that the pure water pH electrode is not affected by the flow rate.
