What Difficult Factors Exist in pH Measurement of High-purity Water with pH Meters?
① Pure water has an extremely weak buffering capacity. It is highly susceptible to contamination, which can easily alter its pH value. If 2 ppm impurities are mixed into pure water, the pH will change significantly. For example, adding 2 ppm NaOH shifts pH from 7 to 10; 2 ppm CO₂ shifts pH from 7 to 6; 2 ppm NH₃ shifts pH from 7 to 7.8. In practical pH measurement, the main interferences come from electrolyte leakage into pure water and the dissolution of atmospheric CO₂ in pure water. In both cases, the measured reading is not the true pH value of pure water. Therefore, electrodes filled with potassium chloride (KCl) solution should be avoided as much as possible for pH measurement in pure water.
② High-purity water features poor electrical conductivity, making it vulnerable to external electromagnetic interference. Static electricity and acoustic fields are readily generated during flow, impairing measurement stability and accuracy. Hence, low-impedance sensitive membrane electrodes must be adopted for pH measurement of pure water. Such electrodes can effectively reduce interferences from static electricity, magnetic fields and acoustic fields while delivering fast electrode response.
③ A potential, commonly known as liquid junction potential E6, forms at the interface between different solutions. The stability of liquid junction potential directly affects the stability of pH measurement. Furthermore, a smaller interfacial area leads to a larger liquid junction potential and greater measurement difficulty. Therefore, electrodes with a large interface must be used for pure water pH measurement, and the flow velocity at the interface shall be kept constant and low to maintain a stable interface. Traditional KCl-filled electrodes have a tiny ceramic cross-section, resulting in a large liquid junction potential. If modified to a ground joint or added ceramic core, substantial KCl solution will permeate and contaminate the sample liquid. Such electrodes are not suitable for direct measurement of pure water. Our company Saike Environmental Protection adopts large cross-section annular Teflon diaphragms imported from overseas to solve these problems. The high-molecular polymer filled in the diaphragm maintains a constant low flow rate (10⁻⁸ L/h), while ceramic diaphragm electrodes have a flow rate of 1 drop per 5 minutes. This design prevents pure water contamination caused by KCl permeation and stabilizes the liquid junction potential.
④ Due to the low ion concentration in high-purity water, a diffusion resistance exists between the reference electrode and measuring electrode. The stability of this potential E5 also affects pH measurement stability. In pure water pH testing, excessive spacing between the reference electrode and measuring electrode shall be avoided. A large distance will raise the impedance between the two electrodes and make readings sensitive to flow rate fluctuations. Combination electrodes solve this problem well, while separate electrodes are not suitable.
