Maintenance of PH meter (acidity meter)

(I) Maintenance 1. Storage of pH glass electrodes For a short period of time: Store in a buffer solution with pH=4; Long term: Store in a buffer solution with pH=7.

2, pH glass electrode cleaning Glass electrode bulb contamination may make the electrode response time longer. Can use CCl4 or soap to remove dirt, and then dip in distilled water and continue using it overnight. When the pollution is serious, it can be soaked with 5% HF solution for 10-20 minutes, rinsed immediately with water, and then immersed in 0.1N HCl solution for a whole day and night.

3. Treatment of Glass Electrode Aging The aging of glass electrodes is related to the gradual change of the adhesive layer structure. The old electrode responds slowly, the membrane resistance is high, and the slope is low. Exfoliating the outer layer with hydrofluoric acid can often improve the electrode performance. If this method can be used regularly to remove the inner and outer layers, the electrode life is almost unlimited.

4. Storage of the Reference Electrode The best storage solution for silver-silver chloride electrodes is a saturated potassium chloride solution. A high concentration of potassium chloride solution can prevent the precipitation of silver chloride at the liquid junction and maintain the liquid junction at work. status. This method is also suitable for storage of composite electrodes.

5, the reference electrode regeneration of the reference electrode occurs most of the problems caused by the liquid junction blockage, can be used to solve the following methods:

(1) Soaking solution junction: Using a mixture of 10% saturated potassium chloride solution and 90% distilled water, heat to 60-70°C, immerse the electrode in about 5 cm, and soak for 20 minutes to 1 hour. This method dissolves the crystallization of the electrode tip.

(2) Ammonia soaking: When the liquid junction is blocked by silver chloride, it can be soaked with concentrated ammonia. The specific method is to wash the electrode and dip the liquid into ammonia water for 10 to 20 minutes after emptying, but do not allow ammonia to enter the interior of the electrode. Take out the electrode and wash it with distilled water. Continue to use it after adding the internal liquid.

(3) Vacuum method: Put the hose on the junction of the reference electrode and use the water suction pump. The liquid in the suction part passes through the liquid junction to remove the mechanical plug.

(4) Boiler fluid junction: The liquid junction of the silver-silver chloride reference electrode is immersed in boiling water for 10 to 20 seconds. Note that the electrode should be cooled to room temperature before the next boiling.

(5) When the above methods are ineffective, the mechanical method of sandpaper grinding can be used to remove the blockage. This method may cause the grinded sand to plug into the liquid junction. Cause permanent blockages.

(b) Inspection 1. General inspection methods for glass electrodes (1) Check the zero potential setting. pH meter In the mV measurement file, insert the glass electrode and the reference electrode together in a buffer solution of pH = 6.86. The reading of the instrument should be approximately - 50 to 50mV.

(2) Check the slope connection (1), and then measure the mV value of the buffer solution with pH=4.00 or pH=9.18. Calculate the slope of the electrode. The relative slope of the electrode should generally be a composite technical indicator. Note: 1) The electrode zero potential check method is only for glass electrodes with an equipotential point of 7. If the isoelectric point of the glass electrode is not 7, it will be different. 2) For some pH meters, when the calibration adjustment can meet the requirements, the electrode can be used if the above test results are out of range. 3) For some smart pH meters, you can directly check the zero potential and slope values ​​obtained from the instrument calibration results.

2. Inspection method of reference electrode (1) The internal resistance inspection method adopts a laboratory conductivity meter. One end of the electrode socket of the conductivity meter is connected to the reference electrode, the other end is connected to a wire, and the reference electrode and the wire are simultaneously immersed. In solution, the measured internal resistance should be less than 10k. If the internal resistance is too large, it means there is a blockage in the fluid interface and should be handled.

(2) Electrode potential check A good reference electrode of the same type and the reference electrode to be measured are connected to both ends of the input of the pH meter, and then KCl solution (or pH=4.00 buffer solution) is inserted at the same time. The potential difference should be -3 ~ 3mV, and the potential change should be less than 1mV. Otherwise, the reference electrode should be replaced or regenerated.

(3) Visual inspection Silver-silver chloride should be dark brown. If it is gray, it indicates that the silver chloride is partially dissolved.

The pH meter is mainly used to measure the pH value of the solution, so that the chemical experiment personnel can understand the acid-base characteristics of the solution. The accuracy of the pH meter will directly affect the correct acquisition of experimental data. Due to the intrinsic asymmetric potential of the electrode system, the potential of the liquid junction, the ambient temperature and other factors, there is a certain error in the value measured by the pH meter. Different pH meters have different tolerances. Metrological verification personnel generally verify the qualification of a pH meter in terms of indication errors of the meter, input current (A), input impedance, indication error caused by the temperature compensator, and repeatability of the indication of the electrical meter.

In the pH metering verification, we found that the main reason for the failure of the instrument is that the value of the temperature compensator is too poor, resulting in inaccurate pH indications. The following describes the function of the temperature compensator, the effect on the pH indication, the cause of the problem, and the solution.