Several problems should be paid attention to in the use of pH electrode
The pH electrode protection solution can effectively protect the glass sensitive membrane of the pH electrode, activate it, and improve the response speed of pH detection.
1. Immersion of the pH glass electrode
The pH glass electrode must be soaked in the solution before use for the following reasons:
a. The sensitive membrane of the pH glass electrode can respond to hydrogen ions normally only when a thin hydrated gel layer is formed on its surface. The relationship between potential and pH can follow the Nernst equation.
b. After the pH glass electrode is soaked in water, its asymmetric potential will drop and tend to be stable, and the internal resistance will decrease at the same time.
c. For composite electrodes, immersion in the solution can keep the liquid junction wet and smooth, and the liquid junction potential remains stable. Different electrodes have different soaking methods.
d. For non-composite PH glass electrodes, generally distilled water (or deionized water), buffer solution with a pH value of 4.00 or 0.01mol/L hydrochloric acid solution can be used for soaking. The soaking time depends on the thickness of the sensitive film, the shape of the sensitive film and the aging degree of the electrode. Thick sensitive film, electrode use time is longer, and its soaking time is also longer, generally 8~24h.
Due to its structural characteristics, the conical electrode has a longer soaking time. Generally speaking, the soaking effect is better when the solution temperature is higher than when the temperature is lower.
But avoid soaking in alkaline solution.
For the composite pH electrode, the soaking solution is different from the above, usually soaked in the same solution as the external reference solution, never soaked in distilled water or deionized water. If the soaking method is wrong, the performance of good electrodes will deteriorate. When the composite pH electrode is immersed in distilled water or deionized water, the external reference solution will leak into distilled water or deionized water through the liquid junction. At this time, the silver complex ions dissolved in the potassium chloride solution will When the ion concentration suddenly drops to close to zero, silver chloride will be regenerated and precipitated at the liquid junction, blocking the liquid junction, preventing the external reference solution from leaking normally, causing the liquid junction potential to be unstable, and the inner electrode If the resistance increases, the performance of the electrode will deteriorate, and in severe cases, the electrode will not work.
pH Electrode Inspection
1 Visual inspection
a. Whether there are nicks and cracks on the sensitive film glass.
b. Whether the internal reference solution of the electrode is cloudy or moldy (flocculent).
c. Whether the liquid junction of the external reference electrode of the composite electrode is blocked can generally be judged from the color change.
d. Whether the lead wire and plug of the electrode are in good condition, especially the plug of the electrode should be dry and clean.
If the above phenomenon occurs, it will affect the performance of the electrode.
2 Performance check
The technical indicators of the pH electrode mainly include: the pH value of zero potential, the internal resistance of the electrode, the alkali error, the response time, and the percent theoretical slope (PTS).
When the percent theoretical slope of the electrode is lower than 90%, it is recommended to replace the electrode. Usually, in the analog circuit pH meter, the "slope calibration" potentiometer is adjusted, and when the displayed value does not reach the standard value required for calibrating the buffer solution, the electrode needs to be replaced. In pH meters with microprocessors, the percent theoretical slope is automatically calculated by the computer. When the percent theoretical slope of the electrode is lower than 90%, in order to ensure the accuracy of the measurement, it is recommended to replace the electrode.
