The characteristics of the pH electrode and how to use the pH electrode correctly?

The characteristics of pH electrode (PH meter electrode, pH sensor): ⑴ Asymmetric potential: When the internal and external reference electrodes of the electrode and the internal and external reference solutions are the same, theoretically the electromotive force of the battery should be equal to zero, but in fact there are always several millivolts to The potential difference of tens of millivolts exists, which shows that the two interfaces inside and outside the glass bulb are asymmetrical, and this potential difference is called asymmetric potential. The asymmetric potential is related to the bulb blowing process, and also related to the erosion or contamination of the bulb surface during use. In actual use, it can be eliminated by the special positioning regulator of the electric meter. (2) Zero potential: The zero potential of the pH electrode refers to the pH value of the solution when the potential of the pH measuring battery is zero. It depends on the pH value and chloride ion concentration of the internal reference solution. If the internal reference solution is a mixed phosphate solution of 0.025mol/L, and the zero potential pH value of the electrode is greater than or lower than the zero potential pH value of the electrode, the polarity of the electrode will change. (3) Internal resistance: The internal resistance of the electrode is mainly determined by the internal resistance of the bulb glass film, which depends on the composition and thickness of the glass, and the resistance value is generally tens of megohms. And it increases exponentially as the temperature drops (for every 7°C drop, the internal resistance doubles, for example, it is 50 megohms at 28°C, and 800 megohms at 0°C). Electrodes with high internal resistance have high requirements on the input impedance and insulation shielding of the electrometer, so it is better to have a lower internal resistance of the electrode. ⑷ Alkali error and acid error: When the acidity and alkalinity of the measured solution increase, the electrode potential and the pH value of the solution will deviate from the linear relationship, and this deviation is called alkali error and acid error. The alkali error is due to the fact that in a solution with a very low concentration of hydrogen ions, the response of the electrode membrane is not only related to the concentration of hydrogen ions, but also related to the concentration of alkali metal ions in the solution. The base error makes the measured pH value lower than the actual value. acid errors in the lower pH range (pH<1~2) appears, the acid error makes the pH measurement value higher than the actual value. Correctly use the pH meter electrode, the posture of the pH sensor: (1) There should be no air bubbles at the front of the bulb, if there are air bubbles, they should be thrown away with force. (2) After the electrode is taken out of the soaking bottle, it should be shaken in deionized water and dried. Do not wipe the bulb with a paper towel, otherwise the time for potential stabilization will be prolonged due to the electrostatic induction of charge transfer to the glass membrane. A better method is to use The solution to be measured rinses the electrodes. (3) After the pH composite electrode is inserted into the solution to be tested, it should be stirred and shaken for a few times and then placed statically, which will speed up the response of the electrode. Especially when using a plastic shell pH composite electrode, the stirring and shaking is more severe, because there will be a small cavity between the bulb and the plastic shell, and after the electrode is immersed in the solution, sometimes the gas in the cavity will produce bubbles if it is too late to get rid of it, making the ball Bubbles or liquid junctions are in poor contact with the solution, so vigorous stirring and shaking is necessary to remove air bubbles. (4) After testing in viscous samples, the electrodes must be rinsed repeatedly with deionized water several times to remove the samples adhered to the glass membrane. Sometimes it is necessary to wash the sample with other solvents first, then wash with water to remove the solvent, and then immerse in the soaking solution for activation. ⑸Avoid contact with strong alkali or corrosive solutions. If you test such solutions, you should minimize the immersion time and clean them carefully after use. ⑹ Avoid using in dehydrating media such as absolute ethanol and concentrated sulfuric acid, which will damage the hydrated gel layer on the surface of the bulb. ⑺The shell material of the plastic shell pH composite electrode is polycarbonate plastic (PC). PC plastic will dissolve in some solvents, such as carbon tetrachloride, trichloroethylene, tetrahydrofuran, etc. If the above solvents are contained in the test, it will If the electrode casing is damaged, a pH composite electrode with a glass casing should be used instead.