A pH meter (also called a pH meter) is an instrument used to measure the pH of a solution. In this article, a detailed description of the acidity meter from the principles, grades and accuracy of the pH meter, use and maintenance, and the operation of the pH meter will give us a comprehensive and thorough understanding of the pH meter.
First, the basic principle of PH meter
A pH meter (also known as a pH meter) is an instrument used to measure the pH of a solution. The acidity meters commonly used in laboratories are Ray-Magnetic 25, PHS-2 and PHS-3. Although they have many models and different structures, they have the same principle. The panel is constructed with both a scale pointer display and a digital display. The following are the pHS-2C and pHS-29 pH meters.
The method of measuring the pH by a pH meter is potentiometry. In addition to measuring the acidity of the solution, it can also measure the battery electromotive force (mV). It is mainly composed of a reference electrode (cabbage electrode), an indicator electrode (glass electrode) and a precision potentiometer. The glass electrode is used as the indicator electrode and the saturated calomel electrode (SCE) is used as the reference electrode to form the battery.
b) Maintenance of the PH meter
1. Storage of pH glass electrode for a short period of time: stored in a buffer solution of pH=4;
Long-term: Stored in a buffer solution of pH=7.
2. Cleaning of the glass electrode of the pH meter The contamination of the glass electrode bulb may increase the electrode response time. The soil can be decontaminated with CCl4 or soap, and then immersed in distilled water for a day and night to continue use. When the pollution is serious, immerse in 5% HF solution for 10-20 minutes, rinse immediately with water, then immerse in 0.1N HCl solution for a day and night and continue to use.
3. Treatment of the aging of the glass electrode of the acidity meter The aging of the glass electrode is related to the gradual change of the rubber layer structure. The old electrode has a slow response, high membrane resistance, and low slope. Etching the outer layer with hydrofluoric acid often improves electrode performance. If this method is used to periodically remove the inner and outer layers, the life of the electrode is almost unlimited.
4. The best storage solution for the silver-silver chloride electrode of the reference electrode of the acidity meter is saturated potassium chloride solution. The high concentration potassium chloride solution can prevent the precipitation of silver chloride at the liquid junction and maintain the liquid junction. Is working. This method is also applicable to the storage of composite electrodes.
5. The problem of the regeneration reference electrode of the reference electrode of the acidity meter is mostly caused by the blockage of the liquid junction. It can be solved by the following methods:
(1) Soaking liquid junction: using a mixture of 10% saturated potassium chloride solution and 90% distilled water, heating to 60-70 ° C, immersing the electrode in about 5 cm, and soaking for 20 minutes to 1 hour. This method dissolves the crystals at the ends of the electrodes.
(2) Ammonia soaking: When the liquid junction is blocked by silver chloride, it can be diluted with concentrated ammonia. The specific method is to wash the electrode inside, and immerse the liquid in ammonia water for 10 to 20 minutes, but do not let ammonia water enter the inside of the electrode. The electrode was taken out and washed with distilled water, and the internal liquid was refilled and continued to be used.
(3) Vacuum method: The hose is placed around the reference electrode liquid junction, and the water suction pump is used, and the liquid in the suction part passes through the liquid junction to remove the mechanical blockage.
(4) Boiling solution 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 boil.
(5) When the above methods are invalid, the mechanical method of sandpaper grinding can be used to remove the blockage. This method may cause the sand under grinding to be inserted into the liquid junction. Causes permanent blockage.
Acid meter check of three PH meter
1. General inspection method of glass electrode (1) Check the zero potential of the acidity meter. Set the pH meter in the “mV†measurement file. Insert the glass electrode and the reference electrode into the buffer solution with pH=6.86. The reading of the instrument should be about - 50 to 50 mV.
(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, and the relative slope of the electrode should generally be composite technical index.
note:
1) The electrode zero potential value check method is only for the glass electrode with an isoelectric point of 7. If the equipotential point of the glass electrode is not 7, it is different.
2) For some pH meters, when the calibration adjustment can meet the requirements, the electrode can be used when the above inspection result is out of range.
3) For some intelligent pH meters, you can directly check the zero potential and slope values ​​obtained from the calibration results of the instrument.
2, the acidity meter reference electrode inspection method (1) The acidity meter internal resistance inspection method uses a laboratory conductivity meter, the conductivity meter electrode socket is connected to the reference electrode at one end, the other end is connected to a wire, the reference electrode and The wire is simultaneously immersed in the solution and the measured internal resistance should be less than 10 kΩ. If the internal resistance is too large, it indicates that the liquid junction is blocked and should be treated.
(2) Acidity meter electrode potential check Take a good reference electrode of the same model type and the reference electrode to be measured into the input end of the pH meter, and then insert KCl solution (or buffer solution with pH=4.00) and measure The potential difference should be -3 to 3 mV, and the potential change should be less than ±1 mV. Otherwise, the reference electrode should be replaced or regenerated.
(3) The appearance of the acidity meter silver-silver chloride should be dark brown. If it is grayish white, the silver chloride has partially dissolved.
1. Storage of pH glass electrode for a short period of time: stored in a buffer solution of pH=4;
Long-term: Stored in a buffer solution of pH=7.
2. Cleaning of the glass electrode of the pH meter The contamination of the glass electrode bulb may increase the electrode response time. The soil can be decontaminated with CCl4 or soap, and then immersed in distilled water for a day and night to continue use. When the pollution is serious, immerse in 5% HF solution for 10-20 minutes, rinse immediately with water, then immerse in 0.1N HCl solution for a day and night and continue to use.
3. Treatment of the aging of the glass electrode of the acidity meter The aging of the glass electrode is related to the gradual change of the rubber layer structure. The old electrode has a slow response, high membrane resistance, and low slope. Etching the outer layer with hydrofluoric acid often improves electrode performance. If this method is used to periodically remove the inner and outer layers, the life of the electrode is almost unlimited.
4. The best storage solution for the silver-silver chloride electrode of the reference electrode of the acidity meter is saturated potassium chloride solution. The high concentration potassium chloride solution can prevent the precipitation of silver chloride at the liquid junction and maintain the liquid junction. Is working. This method is also applicable to the storage of composite electrodes.
5. The problem of the regeneration reference electrode of the reference electrode of the acidity meter is mostly caused by the blockage of the liquid junction. It can be solved by the following methods:
(1) Soaking liquid junction: using a mixture of 10% saturated potassium chloride solution and 90% distilled water, heating to 60-70 ° C, immersing the electrode in about 5 cm, and soaking for 20 minutes to 1 hour. This method dissolves the crystals at the ends of the electrodes.
(2) Ammonia soaking: When the liquid junction is blocked by silver chloride, it can be diluted with concentrated ammonia. The specific method is to wash the electrode inside, and immerse the liquid in ammonia water for 10 to 20 minutes, but do not let ammonia water enter the inside of the electrode. The electrode was taken out and washed with distilled water, and the internal liquid was refilled and continued to be used.
(3) Vacuum method: The hose is placed around the reference electrode liquid junction, and the water suction pump is used, and the liquid in the suction part passes through the liquid junction to remove the mechanical blockage.
(4) Boiling solution 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 boil.
(5) When the above methods are invalid, the mechanical method of sandpaper grinding can be used to remove the blockage. This method may cause the sand under grinding to be inserted into the liquid junction. Causes permanent blockage.
Acid meter check of three PH meter
1. General inspection method of glass electrode (1) Check the zero potential of the acidity meter. Set the pH meter in the “mV†measurement file. Insert the glass electrode and the reference electrode into the buffer solution with pH=6.86. The reading of the instrument should be about - 50 to 50 mV.
(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, and the relative slope of the electrode should generally be composite technical index.
note:
1) The electrode zero potential value check method is only for the glass electrode with an isoelectric point of 7. If the equipotential point of the glass electrode is not 7, it is different.
2) For some pH meters, when the calibration adjustment can meet the requirements, the electrode can be used when the above inspection result is out of range.
3) For some intelligent pH meters, you can directly check the zero potential and slope values ​​obtained from the calibration results of the instrument.
2, the acidity meter reference electrode inspection method (1) The acidity meter internal resistance inspection method uses a laboratory conductivity meter, the conductivity meter electrode socket is connected to the reference electrode at one end, the other end is connected to a wire, the reference electrode and The wire is simultaneously immersed in the solution and the measured internal resistance should be less than 10 kΩ. If the internal resistance is too large, it indicates that the liquid junction is blocked and should be treated.
(2) Acidity meter electrode potential check Take a good reference electrode of the same model type and the reference electrode to be measured into the input end of the pH meter, and then insert KCl solution (or buffer solution with pH=4.00) and measure The potential difference should be -3 to 3 mV, and the potential change should be less than ±1 mV. Otherwise, the reference electrode should be replaced or regenerated.
(3) The appearance of the acidity meter silver-silver chloride should be dark brown. If it is grayish white, the silver chloride has partially dissolved.
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