Key Considerations for pH Measurement Using pH Meters
pH Measurement of Colloids and Turbid Liquids
Glass electrodes generally respond normally for such samples. If the glass electrode becomes contaminated, clean it following the cleaning and maintenance procedures for glass electrodes. Problems usually occur at the liquid junction surface of the reference electrode. Charged colloidal micelles or particles tend to cause unstable liquid junction potential. Two methods are commonly adopted to solve this problem: increase the outflow rate of the salt bridge solution to reduce the impact of colloidal micelles and particles on the liquid junction.
pH Measurement of Strongly Alkaline and Strongly Acidic Samples
Lithium glass electrodes shall be used for alkaline solutions with pH＞10 to reduce alkaline error. Ordinary pH glass electrodes have relatively small alkaline error at present; when pH＞12, alkaline error increases. Long-term immersion of the electrode in alkaline solution may cause potential drift and slow response. In this case, thoroughly rinse the electrode with water and immerse it in 0.1 mol/L HCl for 24 hours. The electrode performance can be restored after such treatment. In strongly alkaline solutions, the outer reference liquid junction materials, ceramic core and asbestos thread are vulnerable to corrosion and damage by alkaline liquid.
For acidic solutions (pH＜2), acid error may occur. Lithium glass electrodes are preferred. If potential drift occurs in acidic solutions, the electrode can usually recover after immersion in distilled water.
For pH measurement in alkaline and acidic media, the pH value of the calibration standard solution should be as close as possible to that of the test solution to reduce residual liquid junction potential. When calibrating with alkaline standard solutions, pay attention to the interference of atmospheric CO₂ on standard solutions. After 12 hours of exposure to air, the pH of pH=10 standard solution changes by 0.22 units due to CO₂ absorption, while the pH of pH=9.18 standard solution changes by 0.02 units. Therefore, sealed containers are recommended for alkaline calibration solutions. Open the container only during use and seal it after measurement, and keep the electrode immersed in the solution.
pH Measurement of Hydrofluoric Acid (HF) Containing Solutions
In acidic media, fluoride concentrations higher than 10⁻⁶ mol/L will cause severe corrosion to glass electrodes. Two common solutions are available: ① Rapid measurement with glass electrodes. ② Adoption of metal antimony electrodes.
The PHG8006 online pH meter and P8T3B hydrofluoric acid resistant pH electrode manufactured by Shanghai Woyu Instrument Technology Co., Ltd. are recommended.
