Correct use and maintenance of the PH meter

2019-09-14 07:05:09

The acidity meter is abbreviated as a pH meter and consists of two parts: an electrode and an electric meter. If the electrode can be properly maintained during use, the standard buffer is prepared as required, and the meter is operated correctly, the pH indication error can be greatly reduced, thereby improving the reliability of chemical experiments and medical test data.

First, the correct use and maintenance of the electrode

At present, the electrodes used in the laboratory are composite electrodes, which have the advantages of being convenient to use, not affected by oxidizing or reducing substances, and having a fast balancing speed. When in use, the rubber sleeve and the rubber sleeve at the lower end of the electrode filling port are completely removed to maintain the hydraulic pressure difference of the potassium chloride solution in the electrode. The following is a brief introduction to the use and maintenance of the electrode:

1. When the composite electrode is not used, it can be fully immersed in 3M potassium chloride solution. Do not dip with washing liquid or other water-absorbing reagents.

2. Check the bulb at the front end of the glass electrode before use. Under normal circumstances, the electrode should be transparent without cracks; the bulb should be filled with solution and no bubbles should exist.

3. When measuring a solution with a large concentration, shorten the measurement time as much as possible, and carefully clean it after use to prevent the test solution from adhering to the electrode and contaminating the electrode.

4. After cleaning the electrode, do not wipe the glass film with filter paper, and use filter paper to dry, avoid damage to the glass film, prevent cross-contamination, and affect measurement accuracy.

5. In the measurement, pay attention to the silver-silver chloride internal reference electrode of the electrode should be immersed in the chloride buffer solution in the bulb to avoid digital jumping phenomenon in the display part of the electricity meter. When using, be careful to gently rub the electrode a few times.

6. The electrode should not be used in strong acids, strong bases or other corrosive solutions.

7. It is strictly forbidden to use in dehydrating media such as absolute ethanol, potassium dichromate and the like.

Second, the preparation and preservation of standard buffer

1. The pH standard substance should be stored in a dry place. For example, the mixed phosphate pH standard substance will deliquesce when the air humidity is high. Once deliquescent occurs, the pH standard substance cannot be used.

2. Prepare the pH standard solution using double distilled water or deionized water. If it is used for a 0.1-stage pH meter measurement, ordinary distilled water can be used.

3. The pH standard solution should be diluted with a smaller beaker to reduce the pH standard solution on the beaker wall. Plastic bags or other containers for storing pH standards should be rinsed several times with distilled water, and then poured into the prepared pH standard solution to ensure that the prepared pH standard solution is accurate.

4. The prepared standard buffer solution can be stored for 2-3 months. If it is found to be turbid, moldy or precipitated, it cannot be used any more.

5. The alkaline standard solution should be stored in a polyethylene bottle and sealed. Prevent carbon dioxide from entering the standard solution to form carbonic acid and lower its pH.

Third, the correct calibration of the pH meter

The pH meter has many types depending on the design of the electric meter, and its operation steps are different. Therefore, the operation of the pH meter should be carried out in strict accordance with the instruction manual. In a specific operation, calibration is an important step in the operation of the pH meter. The data in Table 1 is the measured value of the pH meter with accuracy of 0.01 grade and passed the calibration, and the calibration value can be seen from the calibration.
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”
Standard pH â”„â”„â”„ pre-calibration error (pH) â”„â”„â”„ post-calibration error (pH)
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”
13.000â”…â”…â”… 00.0600â”…â”…â”…â”… 00.0000
12.000â”…â”…â”… 00.0450â”…â”…â”…â”… 00.0005
11.000â”…â”…â”… 00.0500â”…â”…â”…â”… 00.0010
10.000â”…â”…â”… 00.0300â”…â”…â”…â”… 00.0000
9.000 â”…â”…â”… 00.0200â”…â”…â”…â”… 00.0005
8.000 â”…â”…â”… 00.010 â”…â”…â”…â”… 00.0005
7.000â”…â”…â”… 00.0015â”…â”…â”…â”… 00.0000
6.000â”…â”…â”… -00.0100â”…â”…â”…â”… -00.0005
5.000â”…â”…â”… -00.0105â”…â”…â”…â”… 00.0005
4.000â”…â”…â”… 00.0150â”…â”…â”…â”… 00.0000
3.000â”…â”…â”… -00.0300â”…â”…â”…â”… 00.0000
2.000â”…â”…â”… -00.0200â”…â”…â”…â”… -00.0003
1.000â”…â”…â”… -00.0350â”…â”…â”…â”… -00.0001
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”

Although there are many types of pH meters, the calibration method uses a two-point calibration method, that is, two standard buffers are selected: one is pH7 standard buffer, and the second is pH9 standard buffer or pH4 standard buffer. First, the meter is positioned with pH 7 standard buffer, and then the second standard buffer is selected according to the acidity and alkalinity of the solution to be tested. If the solution to be tested is acidic, pH 4 standard buffer is used; if the solution to be tested is alkaline, pH 9 standard buffer is used. For manually adjusted pH meters, repeat the operation several times between the two standard buffers until the zero point and positioning (slope) knobs are adjusted, and the pH meter accurately displays the pH values â€‹â€‹of the two standard buffers. The calibration process ends. Thereafter, the zero point and positioning knob should not be moved during the measurement. In the case of a smart pH meter, there is no need to repeat the adjustment because the pH of several standard buffers stored therein is available for selection and can be automatically identified and automatically calibrated. However, attention should be paid to the selection of standard buffers and the accuracy of their formulation. The intelligent 0.01 pH meter generally has three to five standard buffer pH values, such as KL-016 pH meter from Kelilong.

Second, pay special attention to the temperature of the solution to be tested before calibration. In order to properly select the standard buffer, and adjust the temperature compensation knob on the meter panel to match the temperature of the solution to be tested. The pH of the standard buffer solution is different at different temperatures. As shown in table 2:
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”
Temperature (Â°C) â”„â”„ pH7 â”„â”„ pH4 â”„â”„ pH9.2
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”
10â”„â”„â”„â”„â”„â”„6.92 â”„â”„ 4.00 â”„â”„9.33
15â”„â”„â”„â”„â”„â”„6.90 â”„â”„ 4.00 â”„â”„9.28
20â”„â”„â”„â”„â”„â”„6.88 â”„â”„ 4.00 â”„â”„9.23
25â”„â”„â”„â”„â”„â”„6.86 â”„â”„ 4.00 â”„â”„9.18
30â”„â”„â”„â”„â”„â”„6.85 â”„â”„ 4.01 â”„â”„9.14
40â”„â”„â”„â”„â”„â”„6.84 â”„â”„ 4.03 â”„â”„9.01
50â”„â”„â”„â”„â”„â”„6.83 â”„â”„ 4.06 â”„â”„9.02
50â”„â”„â”„â”„â”„â”„6.83 â”„â”„ 4.06 â”„â”„9.02
â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”â”

After the calibration is completed, the instrument does not need to be recalibrated within 48 hours for a frequently used pH meter. The instrument needs to be recalibrated if one of the following conditions is encountered:
(1) When there is a big difference between the solution temperature and the calibration temperature.
(2) The electrode is exposed to the air for too long, such as more than half an hour.
(3) The positioning or slope adjuster is mis-operated;
(4) After measuring a solution of peracid (pH<2) or overbase (pH>12);
(5) after changing the electrode;
(6) When the pH of the measured solution is not in the middle of the selected solution at the time of two-point calibration, and is further away from 7 pH.

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