What are the difficulties in measuring high-purity water with a pH meter
1. Due to being pure water, its buffering capacity is particularly weak, making it extremely susceptible to contamination and easily changing its pH value. If 2ppm impurities are mixed in pure water, the pH change is particularly significant. For example, when mixing 2ppmNaoH with a pH value ranging from 7 to 10, 2ppmCO2PH with a pH value ranging from 7 to 6, and 2ppmNH3 with a pH value ranging from 7 to 7.8, the impact in actual pH measurements mainly comes from the leakage of electrolyte into pure water and the dissolution of CO2 in air in pure water. 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 interference. At the same time, during the flow process, it is prone to generate static electricity, sound fields, etc., which affects measurement stability and accuracy. Therefore, the measurement of pure water pH value must use a low resistance sensitive membrane electrode, which can effectively reduce the interference of static electricity, magnetic field, and sound field, while also making the electrode responsive.
3. When different solutions come into contact, an electric potential is generated at their 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 boundary area, the greater the boundary potential, which makes measurement more difficult. Therefore, for measuring the pH of pure water, 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! The traditional electrode with KCL solution has a small cross-section of the ceramic core, which results in a high junction potential. However, if it is changed to a frosted mouth or added with a ceramic core, the KCL solution will permeate a large amount and contaminate the solution. This type of electrode is not suitable for measuring pure water. Currently, our company, SECCO Environmental Protection, uses a circular Teflon diaphragm with a large cross-section from abroad, which effectively solves these problems. The 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 minute), which avoids pure water pollution caused by KCL infiltration and maintains the stability of the junction potential.
4. Due to the scarcity 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 may cause a large impedance between the two electrodes and be susceptible to 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 constant as much as possible, so as not to cause potential instability and pH fluctuations due to changes in flow rate. This is an unchangeable reality. At present, any pure pH electrode in the world is affected by flow rate, which is determined by theoretical characteristics. It is illegal and impossible to claim that its pure water pH electrode is not affected by flow rate.
