![<?echo $_SERVER['SERVER_NAME'];?>](/template/twentyseventeen/skin/images/header.jpg)
A pH meter (also called a pH meter) is an instrument used to measure the pH of a solution. This article, from the pH meter principle, level and accuracy, use and maintenance and operation of the pH meter and other details of the acidity meter, will allow us to have a comprehensive and thorough understanding of the acidity meter.
1. The basic principle of the pH meter The pH meter (also called pH meter) is an instrument for measuring the pH of a solution. Frequently used in the laboratory acidometer Lei magnetic type 25, PHS-2 type and PHS-3 type and so on. Although there are many models and different structures, their principles are the same. The panel structure has two kinds of scale pointer display and digital display. The pHS-2C and pHS-29 acidometers are described below.
The method of measuring pH by acidity is potential measurement. In addition to measuring the acidity of the solution, it can also measure the cell's electromotive force (mV). Mainly consists of three parts: reference electrode (calcium electrode), indicator electrode (glass electrode) and precision potentiometer. The glass electrode was used as the indicator electrode for measurement, and the saturated calomel electrode (SCE) was used as the reference electrode to form the battery:
(I) Glass Electrode I Measurement pH Solution IISCE ()
Since the electrode potential of the calomel electrode does not change with the pH value of the solution, it is a certain value at a certain temperature and the electrode potential of the glass electrode changes with the change of the pH value of the solution, so the battery electromotive force of their composition only depends on the pH of the solution. Variety.
The glass electrode (Fig. 1) consists of an Ag-AgCl electrode, hydrochloric acid and a special spherical glass film. It is inserted into the test solution. The electrode potential and the solution pH have the following relation: the glass electrode standard potential. R is the gas constant, T is the Kelvin temperature scale, and F is the Faraday constant. The saturated calomel electrode (SCE) (Fig. 2) consists of mercury, calomel paste, and saturated KCl solution. The concentration of saturated KCl solution at a certain temperature is a certain value, so the potential of the saturated calomel electrode is also a certain value of 0.2412 V at 298K. The glass electrode and the saturated calomel electrode are inserted into the solution to form a primary battery. The electromotive force of the battery is:
From the above equation, E shows a linear relationship with pH. As long as E is measured, the pH can be obtained. Since the 4乇 is usually unknown, the actual assay should be located with a standard solution that is close to the pH of the solution being tested. The electromotive force given by the standard solution in the primary battery is:
(b) When using glass electrodes, pay attention to:
(1) The glass film at the lower end of the glass electrode is easily broken and should not be in contact with hard objects.
(2) For the initial use, the spherical glass film should be soaked in distilled water and stay up all night. When not in use, it should also be soaked in distilled water to simplify the soaking process for the next use.
(3) When the strong alkaline solution is measured, it should be operated as soon as possible, and immediately after the test is finished, it should be washed with water to prevent the alkali solution from corroding the glass film.
(4) The glass film must not be stained with oil. If this happens, the mango is first immersed in the distiller's grains and then in ether or carbon tetrachloride. Then it is moved to alcohol. Finally it is rinsed with water and immersed in water.
(5) The Plexiglass tube on the electrode plug has excellent insulation properties, and should not be in contact with chemicals or oil.
2. The level of the pH meter and the accuracy of the instrument The level of the pH meter and the accuracy of the instrument are two different concepts. The instrument level is not exactly the same as its accuracy. The level of the pH meter is expressed in terms of the index (resolution or minimum display value) of its indicator (electrometer). For example: an instrument with a division of 0.1 pH is called a class 0.1 instrument; the minimum display value is 0.001 pH The instrument is called a 0.001 instrument, and so on. The accuracy of the instrument is the combined error of the electrometer and the electrode matching test standard solution. It is not only related to the electrometer, but also more relevant to the glass electrode and the reference electrode.
From the actual use requirements, the current electrical division value is 0.1 ~ 0.001pH, if necessary, according to the current level of technology, can produce a more precise electrical meter. However, due to structural and manufacturing reasons, the performance of commonly used electrodes has not yet reached a completely ideal level. Both the repeatability error of the glass electrode and the solution junction potential stability of the reference electrode are not better than 0.01 pH. Therefore, the resolution of the electricity meter is higher, and the accuracy of the instrument test is hardly superior to 0.01 pH. However, selecting a high-resolution instrument can maximally overcome or eliminate the effect of the meter on test errors. Because it is no longer a problem to achieve a satisfactory accuracy of the electricity meter, the innovation, improvement and improvement of the instrument's intelligence, user-friendliness, reliability, easy operation, and cost-effectiveness are all ongoing. The "+" series models such as the pHS-4C+ produced by the Ark Technology Development Corporation of Chengdu reflect this trend.
The relationship between the level of the pH meter and its test accuracy is specified in the National Meter Verification Regulations (JJG119-84) of the acidometer as follows.
Instrument grade; 0.2, 0.1, 0.02, 0.01, 0.001 graded or minimum display (pH); 0.2, 0.1, 0.02, 0.01, 0.001
Electrometer indication error (pH); ±0.1, ±0.05, ±0.01, ±0.01, ±0.002
Total test indication error (pH); ±0.2, ±0.1, ±0.02, ±0.02, ±0.01
For example, the company's pH meter; KL-009I, KL-009II/KL-03I, KL-016, PC-2
Note: The scope of the test when supporting the test should be controlled within the pH 3 ~ pH10.
It can be seen from the above table that for a pH meter below 0.01, the value of the total error of the indication is equal to its level. For a 0.01 level acidometer, the total error of the indication is 0.02 pH. For a 0.001 acidity meter, the total error of the indication is shown. Can only reach ± ​​0.01pH, and at this time the need to use a first-level pH reference material can be guaranteed. (Note: pH national standard substances are divided into two grades: first grade and second grade, general pH meter is commonly used as secondary pH standard material.)
It also needs to be explained why the total error of the instrument indicates why the test range is pH3 to pH10. It is due to the fact that the junction potential of the reference electrode is related to the pH of the solution. The liquid junction potential in the range of pH3 to pH10 can be stabilized within ±0.01pH. If 10<pH<3, the error caused by the junction potential of the junction Up to ± 0.07 pH can be achieved. At the same time, glass electrodes can also cause alkali and acid errors in alkaline and acidic solutions. Therefore, it must be specified in the range of pH3 ~ pH10, otherwise it will not be able to distinguish the indication error of various levels of pH meter.
3, PH meter correct use and maintenance of pH meter referred to as pH meter, composed of electrodes and electricity meter. If the electrode can be properly maintained during use, the standard buffer can be prepared as required and the electricity meter can be operated correctly, the pH indication error can be greatly reduced, thereby improving the reliability of the chemical experiment and medical test data.
3.1 Proper use and maintenance of electrodes Electrodes are currently used in the laboratory as composite electrodes, which have the advantages of being easy to use, free of oxidizing or reducing substances, and having a faster equilibrium. In use, the rubber sleeve and the rubber sleeve at the bottom of the electrode filling port are completely removed to maintain the hydraulic pressure difference of the potassium chloride solution in the electrode. Here's a simple 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. Never wash with wash solution or other absorbent agents.
2. Before use, check the bulb at the front of the glass electrode. Under normal conditions, the electrode should be transparent and free from cracks; the bulb should be filled with a solution and no bubbles should be present.
3. When measuring a solution with a large concentration, try to shorten the measurement time as much as possible and carefully clean it after use to prevent the measured liquid from adhering to the electrode and contaminating the electrode.
4. After cleaning the electrode, do not wipe the glass film with filter paper, but use filter paper to dry it to avoid damage to the glass film and prevent cross-contamination, affecting the measurement accuracy.
5. The silver-silver chloride internal reference electrode of the measurement electrode should be immersed in the chloride buffer solution in the bulb to avoid the phenomenon of digital chatter in the display part of the electricity meter. When using, pay attention to gently swing the electrode.
6. The electrode cannot be used for strong acids, alkalis or other corrosive solutions.
7. It is strictly prohibited to use in dehydrating medium such as absolute ethanol, potassium dichromate, etc.
3.2 Preparation and storage of standard buffers 1. The pH reference material should be stored in a dry place. For example, the mixed phosphate pH standard substance will deliquesce when the air humidity is high. Once the deliquescence occurs, the pH reference substance cannot be used.
2. The preparation of pH standard solution should use double distilled or deionized water. If you are using a 0.1 pH meter, use distilled water.
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 storage of the pH standard substance should be rinsed with distilled water several times in addition to the plastic bags or other containers, and then poured into the formulated pH standard solution to ensure that the prepared pH standard solution is accurate.
4. The prepared standard buffer solution can generally be preserved for 2-3 months. If any turbidity, mildew or sedimentation is found, it cannot be used.
5. Alkaline standard solution should be sealed in a polyethylene bottle. Prevent carbon dioxide from entering the standard solution to form carbonic acid and reduce its pH.
3.3. Proper calibration of the pH meter The pH meter has many different types due to the different design of the electricity meter. The operation steps are different. Therefore, the operation of the pH meter should be carried out correctly in accordance with the instructions for use. In a specific operation, calibration is an important step in the use of a pH meter. The data in Table 1 shows the accuracy of a 0.01-level, pH meter that has been qualified for metrological verification and the measured values ​​after calibration, from which the importance of calibration can be seen.
Standard pH; Pre-Calibration Error (pH); Calibration Error (pH)
13.000; 00.0600; 00.000
12.000;00.0450;00.0005
11.000;00.0500;00.0010
10.000;00.0300;00.0000
9.000;00.0200;00.0000
8.000;00.010;00.0005
7.000; 00.0015; 00.0000
6.000;-00.0100;-00.0005
5.000;-00.0105;00.0005
4.000; 00.0150; 00.0000
3.000;-00.0300;00.0000
2.000;-00.0200;-00.0003
1.000;-00.0350;-00.0001
Although there are many types of pH meters, the calibration method uses a two-point calibration method, which is to choose two standard buffers: one is a pH 7 standard buffer, and the second is a pH 9 standard buffer or a pH 4 standard buffer. The pH meter standard buffer was used to locate the electricity meter, and a second standard buffer was selected according to the acidity and alkalinity of the solution to be tested. If the test solution is acidic, use pH4 standard buffer; if the test solution is alkaline, use pH9 standard buffer. If the pH meter is manually adjusted, it should be repeated several times between the two standard buffers until there is no need to adjust its zero and position (slope) knobs. The pH meter can accurately display the two standard buffer pH values. The calibration process ends. After this, the zero point and positioning knob should not be moved again during the measurement. If it is a smart pH meter, it does not need to be adjusted repeatedly, because the pH value of several standard buffers has been stored in the inside, and it can be automatically identified and automatically calibrated. But pay attention to the standard buffer selection and the accuracy of its preparation. The intelligent 0.01-level pH meter typically has three to five standard buffer pH values, such as the KL-016 pH meter from Colecron.
Second, special attention should be paid to the temperature of the solution to be tested before calibrating. In order to correctly select the standard buffer, and adjust the temperature compensation knob on the power meter panel to make it consistent with the temperature of the solution to be tested. The pH of the standard buffer solution is different at different temperatures. The following table shows:
Temperature (°C); pH7; pH4; pH9.2
10;6.92;4.00;9.33
15;6.90;4.00;9.28
20; 6.86; 4.00; 9.18
25; 6.86; 4.00; 9.18
30; 6.85; 4.01; 9.14
After calibration is completed, the instrument does not need to be calibrated again within 48 hours for frequently used pH meters. If one of the following conditions is encountered, the instrument needs to be recalibrated:
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, positioning or slope adjuster is misoperation;
4. After measuring the 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 when the two points are calibrated, and when the pH is further away from 7pH.
1. The basic principle of the pH meter The pH meter (also called pH meter) is an instrument for measuring the pH of a solution. Frequently used in the laboratory acidometer Lei magnetic type 25, PHS-2 type and PHS-3 type and so on. Although there are many models and different structures, their principles are the same. The panel structure has two kinds of scale pointer display and digital display. The pHS-2C and pHS-29 acidometers are described below.
The method of measuring pH by acidity is potential measurement. In addition to measuring the acidity of the solution, it can also measure the cell's electromotive force (mV). Mainly consists of three parts: reference electrode (calcium electrode), indicator electrode (glass electrode) and precision potentiometer. The glass electrode was used as the indicator electrode for measurement, and the saturated calomel electrode (SCE) was used as the reference electrode to form the battery:
(I) Glass Electrode I Measurement pH Solution IISCE ()
Since the electrode potential of the calomel electrode does not change with the pH value of the solution, it is a certain value at a certain temperature and the electrode potential of the glass electrode changes with the change of the pH value of the solution, so the battery electromotive force of their composition only depends on the pH of the solution. Variety.
The glass electrode (Fig. 1) consists of an Ag-AgCl electrode, hydrochloric acid and a special spherical glass film. It is inserted into the test solution. The electrode potential and the solution pH have the following relation: the glass electrode standard potential. R is the gas constant, T is the Kelvin temperature scale, and F is the Faraday constant. The saturated calomel electrode (SCE) (Fig. 2) consists of mercury, calomel paste, and saturated KCl solution. The concentration of saturated KCl solution at a certain temperature is a certain value, so the potential of the saturated calomel electrode is also a certain value of 0.2412 V at 298K. The glass electrode and the saturated calomel electrode are inserted into the solution to form a primary battery. The electromotive force of the battery is:
From the above equation, E shows a linear relationship with pH. As long as E is measured, the pH can be obtained. Since the 4乇 is usually unknown, the actual assay should be located with a standard solution that is close to the pH of the solution being tested. The electromotive force given by the standard solution in the primary battery is:
(b) When using glass electrodes, pay attention to:
(1) The glass film at the lower end of the glass electrode is easily broken and should not be in contact with hard objects.
(2) For the initial use, the spherical glass film should be soaked in distilled water and stay up all night. When not in use, it should also be soaked in distilled water to simplify the soaking process for the next use.
(3) When the strong alkaline solution is measured, it should be operated as soon as possible, and immediately after the test is finished, it should be washed with water to prevent the alkali solution from corroding the glass film.
(4) The glass film must not be stained with oil. If this happens, the mango is first immersed in the distiller's grains and then in ether or carbon tetrachloride. Then it is moved to alcohol. Finally it is rinsed with water and immersed in water.
(5) The Plexiglass tube on the electrode plug has excellent insulation properties, and should not be in contact with chemicals or oil.
2. The level of the pH meter and the accuracy of the instrument The level of the pH meter and the accuracy of the instrument are two different concepts. The instrument level is not exactly the same as its accuracy. The level of the pH meter is expressed in terms of the index (resolution or minimum display value) of its indicator (electrometer). For example: an instrument with a division of 0.1 pH is called a class 0.1 instrument; the minimum display value is 0.001 pH The instrument is called a 0.001 instrument, and so on. The accuracy of the instrument is the combined error of the electrometer and the electrode matching test standard solution. It is not only related to the electrometer, but also more relevant to the glass electrode and the reference electrode.
From the actual use requirements, the current electrical division value is 0.1 ~ 0.001pH, if necessary, according to the current level of technology, can produce a more precise electrical meter. However, due to structural and manufacturing reasons, the performance of commonly used electrodes has not yet reached a completely ideal level. Both the repeatability error of the glass electrode and the solution junction potential stability of the reference electrode are not better than 0.01 pH. Therefore, the resolution of the electricity meter is higher, and the accuracy of the instrument test is hardly superior to 0.01 pH. However, selecting a high-resolution instrument can maximally overcome or eliminate the effect of the meter on test errors. Because it is no longer a problem to achieve a satisfactory accuracy of the electricity meter, the innovation, improvement and improvement of the instrument's intelligence, user-friendliness, reliability, easy operation, and cost-effectiveness are all ongoing. The "+" series models such as the pHS-4C+ produced by the Ark Technology Development Corporation of Chengdu reflect this trend.
The relationship between the level of the pH meter and its test accuracy is specified in the National Meter Verification Regulations (JJG119-84) of the acidometer as follows.
Instrument grade; 0.2, 0.1, 0.02, 0.01, 0.001 graded or minimum display (pH); 0.2, 0.1, 0.02, 0.01, 0.001
Electrometer indication error (pH); ±0.1, ±0.05, ±0.01, ±0.01, ±0.002
Total test indication error (pH); ±0.2, ±0.1, ±0.02, ±0.02, ±0.01
For example, the company's pH meter; KL-009I, KL-009II/KL-03I, KL-016, PC-2
Note: The scope of the test when supporting the test should be controlled within the pH 3 ~ pH10.
It can be seen from the above table that for a pH meter below 0.01, the value of the total error of the indication is equal to its level. For a 0.01 level acidometer, the total error of the indication is 0.02 pH. For a 0.001 acidity meter, the total error of the indication is shown. Can only reach ± ​​0.01pH, and at this time the need to use a first-level pH reference material can be guaranteed. (Note: pH national standard substances are divided into two grades: first grade and second grade, general pH meter is commonly used as secondary pH standard material.)
It also needs to be explained why the total error of the instrument indicates why the test range is pH3 to pH10. It is due to the fact that the junction potential of the reference electrode is related to the pH of the solution. The liquid junction potential in the range of pH3 to pH10 can be stabilized within ±0.01pH. If 10<pH<3, the error caused by the junction potential of the junction Up to ± 0.07 pH can be achieved. At the same time, glass electrodes can also cause alkali and acid errors in alkaline and acidic solutions. Therefore, it must be specified in the range of pH3 ~ pH10, otherwise it will not be able to distinguish the indication error of various levels of pH meter.
3, PH meter correct use and maintenance of pH meter referred to as pH meter, composed of electrodes and electricity meter. If the electrode can be properly maintained during use, the standard buffer can be prepared as required and the electricity meter can be operated correctly, the pH indication error can be greatly reduced, thereby improving the reliability of the chemical experiment and medical test data.
3.1 Proper use and maintenance of electrodes Electrodes are currently used in the laboratory as composite electrodes, which have the advantages of being easy to use, free of oxidizing or reducing substances, and having a faster equilibrium. In use, the rubber sleeve and the rubber sleeve at the bottom of the electrode filling port are completely removed to maintain the hydraulic pressure difference of the potassium chloride solution in the electrode. Here's a simple 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. Never wash with wash solution or other absorbent agents.
2. Before use, check the bulb at the front of the glass electrode. Under normal conditions, the electrode should be transparent and free from cracks; the bulb should be filled with a solution and no bubbles should be present.
3. When measuring a solution with a large concentration, try to shorten the measurement time as much as possible and carefully clean it after use to prevent the measured liquid from adhering to the electrode and contaminating the electrode.
4. After cleaning the electrode, do not wipe the glass film with filter paper, but use filter paper to dry it to avoid damage to the glass film and prevent cross-contamination, affecting the measurement accuracy.
5. The silver-silver chloride internal reference electrode of the measurement electrode should be immersed in the chloride buffer solution in the bulb to avoid the phenomenon of digital chatter in the display part of the electricity meter. When using, pay attention to gently swing the electrode.
6. The electrode cannot be used for strong acids, alkalis or other corrosive solutions.
7. It is strictly prohibited to use in dehydrating medium such as absolute ethanol, potassium dichromate, etc.
3.2 Preparation and storage of standard buffers 1. The pH reference material should be stored in a dry place. For example, the mixed phosphate pH standard substance will deliquesce when the air humidity is high. Once the deliquescence occurs, the pH reference substance cannot be used.
2. The preparation of pH standard solution should use double distilled or deionized water. If you are using a 0.1 pH meter, use distilled water.
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 storage of the pH standard substance should be rinsed with distilled water several times in addition to the plastic bags or other containers, and then poured into the formulated pH standard solution to ensure that the prepared pH standard solution is accurate.
4. The prepared standard buffer solution can generally be preserved for 2-3 months. If any turbidity, mildew or sedimentation is found, it cannot be used.
5. Alkaline standard solution should be sealed in a polyethylene bottle. Prevent carbon dioxide from entering the standard solution to form carbonic acid and reduce its pH.
3.3. Proper calibration of the pH meter The pH meter has many different types due to the different design of the electricity meter. The operation steps are different. Therefore, the operation of the pH meter should be carried out correctly in accordance with the instructions for use. In a specific operation, calibration is an important step in the use of a pH meter. The data in Table 1 shows the accuracy of a 0.01-level, pH meter that has been qualified for metrological verification and the measured values ​​after calibration, from which the importance of calibration can be seen.
Standard pH; Pre-Calibration Error (pH); Calibration Error (pH)
13.000; 00.0600; 00.000
12.000;00.0450;00.0005
11.000;00.0500;00.0010
10.000;00.0300;00.0000
9.000;00.0200;00.0000
8.000;00.010;00.0005
7.000; 00.0015; 00.0000
6.000;-00.0100;-00.0005
5.000;-00.0105;00.0005
4.000; 00.0150; 00.0000
3.000;-00.0300;00.0000
2.000;-00.0200;-00.0003
1.000;-00.0350;-00.0001
Although there are many types of pH meters, the calibration method uses a two-point calibration method, which is to choose two standard buffers: one is a pH 7 standard buffer, and the second is a pH 9 standard buffer or a pH 4 standard buffer. The pH meter standard buffer was used to locate the electricity meter, and a second standard buffer was selected according to the acidity and alkalinity of the solution to be tested. If the test solution is acidic, use pH4 standard buffer; if the test solution is alkaline, use pH9 standard buffer. If the pH meter is manually adjusted, it should be repeated several times between the two standard buffers until there is no need to adjust its zero and position (slope) knobs. The pH meter can accurately display the two standard buffer pH values. The calibration process ends. After this, the zero point and positioning knob should not be moved again during the measurement. If it is a smart pH meter, it does not need to be adjusted repeatedly, because the pH value of several standard buffers has been stored in the inside, and it can be automatically identified and automatically calibrated. But pay attention to the standard buffer selection and the accuracy of its preparation. The intelligent 0.01-level pH meter typically has three to five standard buffer pH values, such as the KL-016 pH meter from Colecron.
Second, special attention should be paid to the temperature of the solution to be tested before calibrating. In order to correctly select the standard buffer, and adjust the temperature compensation knob on the power meter panel to make it consistent with the temperature of the solution to be tested. The pH of the standard buffer solution is different at different temperatures. The following table shows:
Temperature (°C); pH7; pH4; pH9.2
10;6.92;4.00;9.33
15;6.90;4.00;9.28
20; 6.86; 4.00; 9.18
25; 6.86; 4.00; 9.18
30; 6.85; 4.01; 9.14
After calibration is completed, the instrument does not need to be calibrated again within 48 hours for frequently used pH meters. If one of the following conditions is encountered, the instrument needs to be recalibrated:
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, positioning or slope adjuster is misoperation;
4. After measuring the 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 when the two points are calibrated, and when the pH is further away from 7pH.
Metal Base Plate,Metal Terminal Box,Sheet Metal Base Plate,Oem Sheet Metal Baseplate
FCX Metal Structure Co., Ltd. , https://www.fcxmetal.com