What are the difficulties in high-purity water measurement with a pH meter?
1. Due to being pure water, its buffering capacity is particularly weak, making it extremely susceptible to contamination and easily altering its pH value. If 2ppm impurities are mixed into pure water, the pH change is particularly significant. For example, when mixing 2ppm NaOH with pH values ranging from 7 to 10, 2ppm CO2 with pH values ranging from 7 to 6, 2ppm NH3 with pH values ranging from 7 to 7.8, the actual pH measurement is mainly affected by the leakage of electrolyte into pure water and the dissolution of CO2 in pure water in the air. In either case, the measured result is not the pH value of pure water. Therefore, the measurement of pH value in pure water should avoid using electrodes with added potassium chloride (KCL) solution as much as possible.
2. High purity water has poor conductivity and is easily affected by external electromagnetic fields. During the flow process, static electricity and sound fields are easily generated, which affects the stability and accuracy of measurements. Therefore, the measurement of pure water pH value must use low resistance sensitive membrane electrodes, which can effectively reduce the interference of static electricity, magnetic field, and sound field, while also making the electrode response sensitive.
3. When different solutions come into contact, a potential is generated at the interface, commonly known as the junction potential E6. The stability of the junction potential directly affects the stability of pH measurement. Furthermore, the smaller the interface area, the higher the interface potential, which can easily lead to measurement difficulties. Therefore, for pure water pH measurement, it is necessary to use electrodes with a large interface, while maintaining a constant and small flow rate at the interface, in order to ensure a stable interface! However, the traditional electrode with KCL solution has a small ceramic core cross-section, resulting in a high junction potential. If it is changed to a frosted mouth or a ceramic core is added, the KCL solution will penetrate a large amount and contaminate the solution. This type of electrode is not suitable for measuring pure water. Now, our company, SECCO Environmental Protection, has adopted a large cross-section circular Teflon diaphragm from overseas, which can effectively solve these problems. The polymer filled in the membrane can ensure a constant and small flow rate (10-8/hour, while the ceramic membrane electrode is 1 drop/5 minutes), which avoids pure water pollution caused by KCL permeation and maintains the stability of the junction potential.
4. Due to the low number of ions in high-purity water, there is still a diffusion resistance between the reference electrode and the measuring electrode. The stability of this potential E5 also affects the stability of pH measurement. Therefore, in pure water pH measurement, it is advisable to avoid the distance between the reference electrode and the measuring electrode being too far, which can cause a large impedance between the two electrodes and be easily affected by changes in flow rate. Composite electrodes solve this problem well, and discrete electrodes are not suitable!
5. The flow rate also has a significant impact on the pH measurement of pure water. If the flow rate is unstable, it can lead to unstable junction potential E6 and diffusion potential E5, resulting in unstable and inaccurate pH measurement. Therefore, in pure water pH measurement, the flow rate should be kept as constant as possible to avoid potential instability caused by changes in flow rate, which can lead to pH fluctuations. This is an unchangeable reality. At present, any pure pH electrode in the world is affected by flow rate, which is a theoretical characteristic. It is impossible to claim that its pure water pH electrode is not affected by flow rate, which violates theory.
