Easy to Use pH Meter

The pH meter is an important tool for measuring and reacting the pH of the solution. There are various models and products of the pH meter, and there are two display modes: pointer display and digital display. However, no matter how the type of pH meter changes, its working principle is the same. Likewise, its body is a precision potentiometer.

Structure of pH Meter

1. A reference electrode;
2. A glass electrode whose potential depends on the pH of the surrounding solution;
3. A galvanometer capable of measuring small potential differences in a circuit of great resistance.

The following are the main functions of each component:

The basic function of the reference electrode is to maintain a constant potential as a control for measuring various offset potentials. The silver-silver oxide electrode is currently the most commonly used reference electrode in pH.

The function of the glass electrode is to establish a potential difference that responds to changes in the hydrogen ion activity of the solution being measured. Put the pH-sensitive electrode and the reference electrode in the same solution to form a primary battery, and the potential of the battery is the algebraic sum of the potential of the glass electrode and the reference electrode. E battery = E reference + E glass, if the temperature is constant, the potential of this battery changes with the pH of the solution to be measured, and it is difficult to measure the potential generated by the battery in the pH meter because its electromotive force is very small and the circuit The impedance is very large, 1-100MΩ; therefore, the signal must be amplified enough to drive a standard millivoltmeter or milliamperemeter.

The function of the ammeter is to amplify the potential of the primary battery several times, and the amplified signal is displayed by the ammeter, and the degree of deflection of the pointer of the ammeter indicates the strength of the signal it pushes. For the needs of use, the dial of the pH ammeter is engraved with a corresponding pH value; while the digital pH meter directly displays the pH value in numbers.

How a pH Meter Works

The pH meter uses the potentiometric method to measure the pH value of the solution. Therefore, the working method of the pH meter can not only measure the pH value of the solution, but also measure the electromotive force of the battery. In Latin, pH is the abbreviation of Pondus hydrogenii, which is the activity of hydrogen ions in the substance, and the pH value is the negative number of the logarithm of the concentration of hydrogen ions.

The main measuring parts of the pH meter are the glass electrode and the reference electrode, the glass electrode is sensitive to pH, and the potential of the reference electrode is stable. Put the two electrodes of the pH meter into the same solution together to form a primary battery, and the potential of the primary battery is the algebraic sum of the potentials of the glass electrode and the reference electrode.

The potential of the reference electrode of the pH meter is stable, so when the temperature remains stable, the potential change of the primary battery composed of the solution and the electrode is only related to the potential of the glass electrode, and the potential of the glass electrode depends on the pH of the solution to be measured Therefore, by measuring the change of the potential, the pH value of the pH solution can be obtained.

Error correction

Theoretically, the generation potential difference from 0 to 7 to 14pH is about +414mV to 0 to -414mV at 25°C. In the Nernst equation, the potential difference will change about -59mV, but in fact, the change of 1pH will change about -58mV. In addition, for strong acidity and strong alkalinity, there will be errors due to the difference in the material of the glass membrane and the type of liquid.

Calibration of the pH meter uses a pH standard solution that conforms to JIS standards. pH standard solutions include oxalate (1.68pH), phthalate (4.01pH), neutral phosphate (6.86pH), phosphate (7.41pH), borate (9.18pH), carbonate (10.01pH ).

How to use the pH meter (steps)

Preparations before use of the pH meter

1. Clean the electrode with triple distilled water before using the pH meter, and be careful not to break the glass electrode.
2. Prepare to put the NaOH solution and HCL solution for adjustment next to the platform pH meter.
3. Take out the constant pH solution (pH=7.0) in the refrigerator and put it on the platform.
4. Turn on the pH meter, adjust the pH value, press the ︿﹀ key to select the pH and CAL options, select the CAL item, adjust and insert it into the pH solution (pH=7.0), press the “” key to select the data value to 7.0, A small octopus can appear.
5. Insert the glass electrode into the solution to be tested, then put another electrode, and stir the liquid properly (note: do not break the glass electrode).
6. The electronic unit of the pH meter must pay attention to the protection of the circuit. When the pH value is not measured, the input of the pH meter should be short-circuited to avoid damage to the pH meter.
7. The glass electrode socket of the pH meter must be kept clean, clean and dry, and must not be exposed to harmful gases such as salt spray and acid fog. At the same time, it is strictly forbidden to have any aqueous solution on the glass electrode socket to avoid high input impedance of the pH meter.
8. Be careful to add NaOH solution and HCL solution when the pH value you need is not reached.
9. Be careful not to exceed the required fixed volume when adding liquid.

pH meter usage steps

1. Open the back cover and insert a battery.
2. Install the composite glass electrode Note:
   (1) The lower end of the composite electrode is a fragile glass bulb, so be careful when using and storing it to prevent it from colliding with other items.
   (2) There is a KCl saturated solution in the composite electrode as a conductive medium. If the result of drying up is inaccurate, you must observe whether there is any liquid at any time. If there is a small amount left, go to the laboratory for perfusion.
   (3) No pollution, including water droplets, is allowed at the instrument interface of the composite electrode.
   (4) The composite electrode connection cannot be forcibly pulled to prevent the line connector from breaking.
3. After turning on the power switch, switch to the pH measurement file.
4. Use a thermometer to measure the temperature of the pH6.86 standard solution, and then adjust the temperature compensation knob of the pH meter to the measured temperature value.
5. Rinse the composite electrode with deionized water and dry it with filter paper.
6. Pour 2-5mL of the pH6.86 standard solution into a plastic beaker that has been washed and dried with water, pour it out after washing the beaker and the composite electrode, then add 20mL of the pH6.86 standard solution into the plastic beaker, and insert the composite electrode In the solution, use the instrument positioning knob to adjust to a reading of 6.86 until it stabilizes. The following two points should be noted:
   (1) The pH6.86 standard must be used to adjust the bit.
   (2) After the adjustment, the positioning knob must not be moved again.
7. Wash the composite electrode with deionized water, dry it with filter paper, measure the temperature of the pH4.00 solution with a thermometer, and adjust the temperature compensation knob of the instrument to the measured temperature value.
8. Pour 2-5mL of the pH4.00 standard solution into another plastic beaker, wash the beaker and the composite electrode, and pour it out, then add 20mL of the pH4.00 standard solution, insert the composite electrode into the solution, and when the reading is stable, use the slope knob to Adjust to pH 4.00. It should be noted that after the slope knob is adjusted, it must not be moved again.
9. Measure the temperature of the liquid to be tested with a thermometer, and adjust the temperature compensation of the instrument to the measured temperature.
10. Insert the composite electrode into the solution to be tested, and read the pH value, which is the pH value to be tested. The following two points should be noted:
    (1) The temperature should not be too high during the measurement. If the measurement result is inaccurate over 40°C, take it out with a beaker and cool it slightly.
    (2) Avoid contacting the composite electrode with organic matter, and clean it with anhydrous ethanol once contacted or contaminated.
11. Precautions: The instrument must be calibrated before use, that is, the above 4 to 8 operations. If the instrument is not turned off, it can measure continuously, and it needs to be calibrated once it is turned off. However, it must be calibrated once every 12 hours even if the machine is not turned off.

Three Methods of pH Meter Calibration

One-point calibration of pH meter

Any pH meter must be calibrated with a pH standard solution before measuring the pH value of the sample. For samples with a measurement accuracy below 0.1pH, the instrument can be adjusted with a one-point accurate method. Generally, pH6.86 or pH7. 00 standard buffer. Some instruments have an accuracy of only 0.2pH or 0.lpH, so the instrument only has a ¨positioning¨ adjustment knob. The specific operation steps are as follows:
(1) Measure the temperature of the standard buffer solution, check the table to determine the pH value at the temperature, and adjust the temperature compensation knob to the temperature.
(2) Rinse the electrode with pure water and dry it.
(3) Dip the electrode into the buffer solution and shake it and place it still. After the reading is stable, adjust the positioning knob to make the instrument display the pH value of the standard solution.
(4) Take out the electrodes, rinse and dry them.
(5) Measure the temperature of the sample, and adjust the temperature compensation knob of the pH meter to the temperature value

Two-point calibration of pH meter

For precision grade pH meters, in addition to ¨positioning¨ and ¨temperature compensation¨ adjustments, there is also an electrode ¨slope¨ adjustment, which requires two standard buffer solutions for calibration. Generally, pH6.86 or pH7.00 is used for ¨positioning¨ calibration first, and then according to the acid-base condition of the test solution, pH4.00 (acidic) or pH9.18 and pHI0.0l (alkaline) buffer solutions are selected for ¨slope¨ Correction. The specific operation steps are:
(1) Clean and dry the electrode, immerse in pH6.86 or pH7.00 standard solution, and place the temperature compensation knob of the instrument at the temperature of the solution. After the displayed value is stable, adjust the positioning knob to make the instrument display the pHs value of the standard solution.
(2) Take out the electrode, wash and dry it, and immerse it in the second standard solution. After the displayed value is stable, adjust the slope knob of the instrument so that the displayed value of the instrument is the pHs value of the second standard solution.
(3) Take out the electrode, wash it and dry it, and then immerse it in pH6.86 or pH7.00 buffer solution. If the error exceeds 0.02pH, repeat steps (1) and (2) until the correct pHs can be displayed in the two standard solutions without adjusting the knob.
(4) Take out the electrode and shake it dry, adjust the pH temperature compensation knob to the temperature of the sample solution, immerse the electrode in the sample solution, shake it and place it still, it will display a stable reading.

Third Point Calibration of a pH Meter

No matter what kind of pH meter, the point of pH=7 must be calibrated, and the point of pH=7 must be calibrated first during the two-point calibration. When doing calibration, start from 7.0, and the selected standard solution is related to the pH value of the solution to be measured, so that the pH value of the solution can fall within the calibrated pH range. Generally, two points can be used to meet the requirements. If the requirements are very high, the third point should be considered. Some instruments can calibrate three points, and there is an optional mode, which can be used directly. For some that don’t, two-point two-point proofreading is generally used, that is, proofreading twice.

With the calibration of 7.00 and 4.01, if the third point is needed, should I use 9.21 buffer or 10.01, 9.18, 12.46, 1.68 and other buffers? How to determine?

1. In fact, the pH correction at the third point mainly depends on your sample conditions. As you said, there are many kinds of calibration solutions ranging from pH 1.68 to 12.46, and the appropriate calibration solution should be selected according to the final pH range of the sample. We commonly use 4.00, 6.86, 9.18. If your sample is more alkaline, you need 9.18, 10.01, 12.46. The order of calibration varies with different instruments. Some require calibration in order, while others do not. The instrument will automatically recognize it. Please refer to the relevant instrument manual.
2. No matter what kind of pH meter, the point of pH=7 must be calibrated, and the point of pH=7 must be calibrated first when performing two-point calibration. When doing calibration, start from 7.0, and the selected standard solution is related to the pH value of the solution to be measured, so that the pH value of the solution can fall within the calibrated pH range. Generally, two points can be used to meet the requirements. If the requirements are very high, the third point should be considered. Some instruments can calibrate three points, and there is an optional mode, which can be used directly. For some that don’t, two-point two-point proofreading is generally used, that is, proofreading twice.
3. We usually use the calibration sequence of 7, 4, and 10. Check the acid first and then the base.

Precautions for use of pH meter

1. Under normal circumstances, when the pH meter is used continuously, it should be calibrated once a day; generally, the instrument does not need to be calibrated within 24 hours.
2. Before use, pull off the rubber sleeve on the upper end of the electrode of the pH meter to expose the small hole on the upper end.
3. The calibration buffer solution generally uses a pH=6.86 solution for the first time, and a buffer solution close to the pH value of the measured solution for the second time. If the measured solution is acidic, the buffer should be pH=4.00; When the solution is alkaline, choose a buffer with pH = 9.18.
4. When measuring, the leading wire of the electrode should be kept still, otherwise it will cause unstable measurement.
5. Do not soak the electrode in distilled water. If the electrode used in the pH meter is a new electrode or an electrode that has not been used for a long time, it must be soaked in distilled water for several hours before use, so that the asymmetric potential of the electrode of the pH meter can be reduced to a stable level, thereby reducing the electrode. internal resistance.
6. When the pH meter is measuring the pH value, it is necessary to ensure that the bulb of the electrode completely enters the medium to be measured, so as to obtain more accurate measurement results.
7. When using the pH meter, remove the rubber stopper at the electrolyte filling port of the reference electrode, so that the reference electrolyte can be kept under gravity.

Maintenance of pH Meter

1. Storage of pH meter glass electrode
   When the pH meter is not in use for a short period of time, it can be fully immersed in saturated potassium chloride solution. However, if it is not used for a long time, it should be placed dry, and it should not be soaked with washing liquid or other water-absorbing agents.
2. Cleaning of the pH glass electrode
   Contaminated glass electrode bulbs may increase electrode response time. Wipe off dirt with CCL4 or soap solution, then soak in distilled water for a day and night before continuing to use. When the pollution is serious, soak in 5% HF solution for 10-20 minutes, rinse with water immediately, and then continue to use after soaking in 0.1N HCL solution for a day and night.
3. Treatment of glass electrode aging
   The aging of the glass electrode is related to the gradual change of the glue layer structure. Older electrodes have sluggish response, high membrane resistance, and low slope. Etching off the outer adhesive layer with hydrofluoric acid often improves electrode performance. If this method can be used to regularly remove the inner and outer adhesive layers, the life of the electrode is almost unlimited.
4. Storage of the reference electrode
   The best storage solution for silver-silver chloride electrode is saturated potassium chloride solution. High-concentration potassium chloride solution can prevent silver chloride from precipitating at the liquid junction and maintain the liquid junction in working condition. This method is also applicable to the storage of composite electrodes.

Common Problems and Solutions

1. The same sample, the pH value of the two measurements are not the same?
   A change in pH can be caused by a change in temperature or by a chemical reaction in the sample itself. Therefore, try to keep the temperature consistent and avoid chemical reactions.
2. The same sample is measured on two pH meters at the same time, but the readings are inconsistent?
   Due to the different calibration conditions of the two pH meters (for example, calibration done at different times), the measured values are different. Therefore, the pH meter should be calibrated at the same time with the same buffer solution, and then measured at the same time.
3. Why the buffer solution has deteriorated within the validity period and cannot be used?
   The validity period of the buffer solution refers to the storage period in the unopened state of use. Once opened and used, the buffer solution is more likely to deteriorate due to the action of various molds in the air.
   Note: The used buffer solution must not be poured back into the original bottle!
4. How often do electrodes need to be calibrated?
   The frequency of electrode calibration depends on specific conditions such as electrode use, maintenance, sample properties, and measurement accuracy. It is recommended to calibrate once a day, at most no more than once a week. When replacing electrodes or not using for a long time, it must be calibrated before use.
5. How to maintain the pH electrode?
   After using the electrode for a period of time, if you find that the slope becomes lower and the response speed becomes slower, you can try the following methods:
   ① If the measured sample contains protein, the electrode membrane can be cleaned with pepsin/hydrochloric acid lotion.
   ② If the measured sample is oily/organic liquid, it can be washed with acetone or ethanol.
   ③ If the junction of the electrode is found to be dirty and black, the junction can be cleaned with mercaptan cleaning solution.
   ④ Activate the electrode membrane, activation method: Soak in electrode regeneration solution for 30 seconds, then soak in 3mol/L KCL solution for 5 hours.
6. The sample temperature is 10°C. At this time, does the meter display the pH value at 10°C or 25°C?
   The acidity meter shows the pH value of the solution at the current temperature. If it is measured at 10°C, the meter displays the value of the solution at 10°C. If it is necessary to obtain a pH of 25°C, the temperature of the solution must be raised/cooled to 25°C, and then Take measurements. The temperature compensation of the acidity meter refers to compensating the influence of temperature on the pH electrode, but it cannot compensate the pH value at any temperature to 25°C.
7. Why is the electrode displayed as 7.02 after being calibrated in the pH7.00 buffer?
   At this time, the temperature of the buffer solution is about 20°C. Since the pH of the buffer varies slightly with temperature, 7.00 is only the value of the buffer at 25°C, and the value of the buffer at 20°C should be 7.02. The pH meter can automatically compensate the influence of temperature on the buffer to ensure measurement accuracy. the
8. How long is the life of the pH electrode?
   The life of the pH electrode is related to the nature of the measured sample, the temperature of the sample, the frequency of use, and the maintenance. Under normal use and proper maintenance, the life of the pH electrode is 1 to 2 years.
9. Check whether the pH meter is accurate?
   Is the pH meter accurate? The only reliable and easiest method is to use the pH standard buffer solution for verification. Take three pH standard buffer solutions: pH6.86, pH4.00, pH9.18 (preferably freshly prepared and at the same temperature), use pH6.86 for positioning calibration, pH4.00 for slope calibration, and then test pH9. 18. Check whether the pH meter is accurate, and see whether it is qualified or not. If the accuracy is unqualified, you can further judge whether there is a problem with the pH meter or the pH electrode.
10. Summary of the reasons for the digital instability of the pH meter:
    ① Check whether the electrode is damaged;
    ②It should be that the electrode has been used for too long, first calibrate to see if it is effective;
    ③ You can try soaking the probe with 2.5mmoL/L KCL solution;
    ④ Clean the glass ball, is it a long time, some organic matter is attached to it, resulting in insensitive reaction;
    ⑤There is a chemical level CO2+H2O→H++HCO3- in water, and the pH will always rise because the balance moves to the positive reaction direction because the general pure water or surface water is weakly alkaline;
    ⑥Add a neutral salt (such as KCL) to the water sample to be tested as an ionic strength regulator to change the total ionic strength in the solution, increase the conductivity, and make the measurement fast and stable. The national standard GB/T6904.3-1993 of this method stipulates: “In order to reduce the influence of liquid junction potential and quickly achieve stability when measuring water samples, add a drop of neutral 0.1moL/L KCL solution to every 50mL water sample.” Although this The method changes the ionic strength in the water sample, which causes the change of its pH value to a certain extent, but the experiment proves that this change only changes about 0.01pH in value, which is completely acceptable. However, when using this method, it must be noted that the added KCL solution should not contain any alkaline or acidic impurities. Therefore, the KCL reagent should be of high purity, and the water quality of the prepared solution should also be high-purity neutral water.