What are the difficult factors in measuring high-purity water with a pH meter?
1. Because it is pure water, its buffering capacity is extremely weak, it is extremely susceptible to contamination, and it is extremely easy to change its pH value. If 2ppm impurities are mixed into pure water, the pH change will be particularly obvious. For example: mix 2ppmNaoH, pH value from 7→10, 2ppmCO2, pH value from 7→6, 2ppmNH3, pH value from 7→7.8. Generally, in actual pH measurement, the influence mainly comes from the influence of electrolyte leakage into pure water on pH value and air. The influence of CO2 dissolved in pure water. In either case, the measured result at this time is not the pH value of pure water. Therefore, when measuring pH value in pure water, the use of electrodes with added potassium chloride (KCL) solution should be avoided as much as possible.
2. The conductivity of high-purity water is very poor and it is easily interfered by external electromagnetic fields. At the same time, during the flow process, it is easy to generate static electricity, sound fields, etc., which affects the stability and accuracy of measurement. Therefore, the measurement of pure water pH must use low-resistance sensitive membrane electrodes, which can effectively reduce interference from static electricity, magnetic fields, and sound fields, while making the electrodes responsive.
3. When different solutions come into contact, their interface will generate an electric potential, commonly known as the junction potential E6. The stability of the junction potential directly affects the stability of pH measurement. Furthermore, the smaller the junction area, the larger the junction potential will be, making measurement more difficult. Therefore, when measuring the pH of pure water, it is necessary to use an electrode with a large interface and keep the flow rate at the interface constant and small, so as to ensure a stable interface! The traditional electrode with KCL solution has a small ceramic core cross-section, so the junction potential is very large. If it is changed to a frosted port or a ceramic core is added, a large amount of KCL solution will penetrate and contaminate the solution. This kind of electrode is not suitable for measuring purity. Water, now Secco Environmental Protection uses the largest cross-section annular Teflon diaphragm in foreign countries to 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 pure water pollution caused by KCL penetration and maintaining the junction potential stability.
4. Since there are very 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 the pH value measurement. Therefore, the reference value should be avoided when measuring the pH value of pure water. The distance between the ratio electrode and the measuring electrode is too far, which causes the impedance between the two electrodes to be too large, making it susceptible to changes in flow rate. The composite electrode solves this problem very 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, making the pH value measurement unstable and inaccurate. Therefore, when measuring the pH of pure water, the flow rate should be kept as constant as possible, so that changes in the flow rate will not cause instability in the relevant potential, 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. This is determined by theoretical characteristics. It is against the theory to claim that its pure water pH electrode is not affected by the flow rate and is impossible.
