Use of Online pH Meters for Ultrapure Water Measurement
Ultrapure water: nearly all conductive substances in water are removed, and undissolved colloids, gases and organic matter are reduced to an extremely low level. Its resistivity is greater than 18 MΩ·cm, close to the limit value of 18.3 MΩ·cm.
Due to the unique properties of ultrapure water, it is very difficult to measure its pH value with an online pH meter. In practice, it is hard to obtain repeatable and stable measurement results in samples with low ionic strength. Low-ionic samples are weakly alkaline, with low ionic strength or low conductivity (conductivity generally 0.056–10) and high resistance, such as deionized water. Owing to the liquid junction potential generated at the reference boundary, different pH readings will be obtained when measuring deionized water, even for new, sealed electrodes that have been well calibrated in standard buffer solutions.
① Why is it necessary to measure the pH value of ultrapure water online?During the production of pure water, measuring the pH value can serve as an indicator of process contamination. When air or carbon dioxide intrudes into the pure water production or distribution system, it will cause the pH value to drift.
② How to select an online pH electrode?Extended cable electrodes are refillable and have low resistance blocking. A flow-through reference electrode provides a relatively high flow rate, which reduces the liquid junction potential. Sealed electrodes commonly adopt slow leakage junctions. Their major advantage is long service life because of the very slow leakage rate of the reference electrolyte. However, electrodes with a fast leakage rate are very suitable for ultrapure water, as the pH potential can reach stability rapidly.
