The PH meter used the unit to ask the following questions: The PH meter requires three points of correction, and 2 points are not enough.
With the correction made by 7.004.01, if the third point is to use the 9.21 buffer or which of the other buffers such as 10.01, 9.18, 12.46, 1.68, etc.? How to determine?
1, in fact, the third point of pH calibration is mainly depends on your sample. As you said, there are many kinds of calibration solutions from pH 1.68 to 12.46. Depending on the final pH range of the sample, choose the appropriate calibration solution. We usually use 4.00, 6.86, 9.18. If your sample is more alkaline, you need 9.18, 10.01, 12.46. The calibration sequence is also different according to different instrument conditions. Some requirements are calibrated in sequence, while others are not required. The instrument will automatically recognize it. You need to refer to the instruction manual of the relevant instrument.
2. Regardless of the pH meter, the pH=7 point must be corrected, and the pH=7 should be corrected first when the two points are corrected. When making the calibration, starting from 7.0, the selected standard solution is related to the pH of the solution to be measured, so that the pH of the solution can fall within the corrected pH range. Generally, two points can be used to meet the requirements. If the requirements are high, the third point is considered. Some instruments can be calibrated at three points, and there are modes available, which can be used directly. Some of them are not proofed by two points and two points, that is, proofreading twice.
3, we usually use the calibration order of 7, 4, 10. The first acid is the base.
Then the PH meter that has been idle for a long time, and the electrode is not placed in the protective liquid, how to activate the electrode and calibration? What should be paid attention to? How to match the standard calibration solution? What details should I pay attention to when using the PH meter?
(1) Maintenance
1. Storage of pH glass electrode
Short-term: stored in a buffer solution of pH=4;
Long-term: Stored in a buffer solution of pH=7.
2. Cleaning of pH glass electrode
Contamination of the glass electrode bulb may lengthen the electrode response time. Decontaminate can be removed with CCl4 or soap, 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, the treatment of glass electrode aging
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. Storage of reference electrode
The best storage solution for the silver-silver chloride electrode is a saturated potassium chloride solution. The high concentration potassium chloride solution prevents the precipitation of silver chloride at the liquid junction and maintains the liquid junction in a working state. This method is also applicable to the storage of composite electrodes.
5, the regeneration of the reference electrode
Most of the problems with the reference electrode are caused by blockage of the liquid junction, which 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 to 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 the 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 then used.
(3) Vacuum method: the hose is placed around the reference electrode liquid junction, and a water flow suction pump is used, and the liquid in the suction portion passes through the liquid junction to remove the mechanical blockage.
(4) Boiling liquid 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 all 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.
(two) use
First, the use of pH meter
1. Remove the electrode from the electrode protection solution and rinse it out. Drain it with dust-free paper and place it in the solution to be tested (the sample to be tested must have not passed the electrode bubble). Press the power button to turn on the pH meter. The pH meter automatically enters the measurement. Press the “measure save/print” button and wait for the reading to stabilize before reading.
2. After the pH meter is used, rinse the electrode clean and dry it with dust-free paper, and fully soak the electrode protection solution. The electrode protection fluid should be replaced in time and replaced once a week.
Second, the calibration of the pH meter
1. Transfer standard buffers with pH values of 4.01, 7.00, 10.01 to a clean, dry 50 mL beaker.
2. Press the power button to turn on the pH meter, rinse the electrode clean, dry it with a dust-free paper, and place it in a standard buffer of pH 4.01. Press the “calibrate” button to the CAL.1 interface, wait for the reading to stabilize and the cursor in front of the reading flashes, press the “Digital Edit” button to adjust the pH meter reading to the pH value of the standard. Then press the “calibrate” button to enter the CAL.2 interface.
3. Rinse the electrode and dry it with dust-free paper. Place it in standard buffer with a pH of 7.00. Wait for the reading to stabilize and the cursor in front of the reading flashes. Press the “Digital Edit” button to adjust the pH meter reading to the standard solution. pH value. Then press the “calibrate” button to enter the CAL.3 interface.
4. Rinse the electrode and dry it with dust-free paper. Place it in standard buffer with a pH of 10.01. Wait for the reading to stabilize and the cursor in front of the reading flashes. Press the “Digital Edit” button to adjust the pH meter reading to the standard solution. pH value.
5. Press the “measure save/print” button to save the calibration result and get the slope of the line after the three-point calibration. This calibration is valid if the slope of the line is in the range of 100 ± 3 and the pH of the other two standard buffers is measured within ±0.3. Otherwise you need to recalibrate.
6. After using the standard buffer, seal it with a parafilm and put it in a dry place for multiple use.
When the pH of the solution is measured in a small range (eg 3-8), it can be calibrated with only two standard buffers at pH 4.01 and 7.00.
After the calibration is completed, if the pH meter is used frequently, it is calibrated every 2 days. The pH meter needs to be recalibrated if:
(1) The electrode is exposed to air for too long, such as more than half an hour.
(2) After measuring a solution of peracid (pH < 2) or over-alkali (pH > 12).
(3) After changing the electrode.
(three), pay attention
1. When the electrode is not in use, fully soak the electrode protection solution. The electrode protection fluid should be replaced in time and replaced once a week. Do not dip or soak in pure water with washing liquid or other water-absorbing agent.
2. When measuring a solution with a large concentration, try to shorten the measurement time and carefully clean it after use to prevent the test solution from adhering to the electrode and contaminating the electrode.
3. After cleaning the electrode, do not wipe the glass film with dust-free paper, and use dust-free paper to dry, avoid damage to the glass film, prevent cross-contamination, and affect measurement accuracy.
4. In the measurement, note that the silver-silver chloride internal reference electrode should be immersed in the chloride buffer solution in the bulb. When the external reference solution is less than 1/3, it should be added in time to avoid the digital display in the electrode display. Jump phenomenon. When using, be careful to gently rub the electrode a few times.
5. The electrode should not be used in strong acids, strong bases or other corrosive solutions.
6. It is strictly forbidden to use in dehydrating medium such as absolute ethanol, potassium dichromate and the like.
7. pH standard buffer should be sealed and stored in a dry place.
8. The standard buffer that has been transferred must be stored in a clean and dry container. Rinse the electrode and dry it with dust-free paper for each calibration. Prevent the standard buffer from being contaminated and diluted. After using the standard buffer, use the parafilm. Seal in a dry place for multiple use. When the transferred standard buffer solution is found to be turbid, moldy or precipitated, it cannot be used any more.
