How does an online pH meter work?
The working principle of the online pH meter is based on the galvanic cell. This cell consists of two electrodes, and the electromotive force between them follows the Nernst equation.
This electromotive force is influenced not only by the properties of the electrodes themselves but also by the concentration of hydrogen ions in the solution.
In this system, there is a direct correlation: the electromotive force of the galvanic cell is specifically related to the hydrogen ion concentration.
The negative logarithm of this hydrogen ion concentration corresponds exactly to the pH value we commonly use.
When the electrode of the online pH meter is immersed in the solution to be tested, the hydrogen ions in the solution interact with the electrode.
For example, the glass electrode has a sensitive membrane that selectively responds to hydrogen ions. When hydrogen ions from the solution exchange with ions on the surface of the glass membrane, a potential difference is generated across the membrane.
The reference electrode provides a stable potential reference to ensure accurate measurement. By measuring the potential difference between the glass electrode and the reference electrode and accurately calculating it according to the Nernst equation, the hydrogen ion concentration in the solution can be precisely converted into the corresponding pH value.
For instance, when the hydrogen ion concentration in the solution changes, the potential difference between the electrodes changes accordingly. The online pH meter can then accurately display the solution’s pH value in real time through precise measurement and complex computation of this potential difference.
Analysis of key components of online pH meter
Electrode:
The electrode is the core component of the online pH meter, mainly including glass electrodes and reference electrodes. The front end of the glass electrode is a thin film made of special glass, which has a highly selective response to hydrogen ions. When the glass electrode is immersed in a solution, the hydrogen ions in the solution will exchange with the ions on the surface of the film, thereby generating a potential difference. The reference electrode provides a stable potential reference for the measurement to ensure the accuracy and reliability of the measurement results. Different types of electrodes are suitable for different measurement scenarios. For example, when measuring solutions containing special substances, electrodes with special materials or structures may need to be used to ensure the accuracy of the measurement and the service life of the electrode.
Display:
The display is used to visually display the measured pH value. Today, most online pH meters are equipped with clear, easy-to-read digital displays that can quickly and accurately present measurement results. Some advanced displays also have multiple functions, such as displaying temperature, measurement time, and other related information at the same time, providing operators with more comprehensive data references. In addition, parameters such as the brightness and contrast of the display may also affect the accuracy and convenience of the reading. When selecting and using an online pH meter, appropriate adjustments need to be made according to actual needs and environmental conditions.
Temperature compensation device:
Temperature has a significant effect on the measurement results of pH value. In order to eliminate the error caused by temperature changes, online pH meters are usually equipped with temperature compensation devices. This device can automatically adjust the measurement results according to the actual temperature of the solution. This is because, according to the Nernst equation, the electromotive force of the electrode is proportional to the temperature. When the temperature of the solution changes, even if the concentration of hydrogen ions in the solution remains unchanged, the electromotive force of the electrode will change, resulting in a deviation in the measured pH value. For example, at high temperatures, the electromotive force of the electrode may increase, and if temperature compensation is not performed, the measured pH value may be low. With the temperature compensation device, the online pH meter can ensure that the pH value of the solution can be accurately measured under different temperature conditions.
How to calibrate an online pH meter？
Two-point calibration method:
Prepare two standard buffer solutions with known pH values, such as pH 4.00 and pH 7.00 solutions. Rinse the electrode with distilled water, immerse it in the first standard buffer solution, and gently stir the solution to make the electrode fully contact with the solution. After the pH value displayed by the instrument stabilizes, adjust the positioning knob of the instrument so that the displayed value is consistent with the pH value of the standard buffer solution. Next, take out the electrode, rinse it with distilled water, and immerse it in the second standard buffer solution. Stir the solution in the same way. After the displayed value stabilizes, adjust the slope knob so that the displayed value is consistent with the pH value of the second standard buffer solution. After completing these two steps, the two-point calibration of the instrument is basically completed. During the calibration process, ensure the accuracy and reliability of the standard buffer solution and avoid using expired or contaminated solutions.
Three-point calibration method:
The three-point calibration method is relatively more accurate and requires the preparation of three standard buffer solutions with different pH values, such as pH 4.00, pH 7.00, and pH 10.00. First, follow the steps of the two-point calibration method and immerse the electrode in pH 4.00 and pH 7.00 standard buffer solutions for positioning and slope adjustment. Then, wash the electrode, immerse it in pH 10.00 standard buffer solution, stir it evenly, and after the displayed value stabilizes, check again whether the displayed value of the instrument is consistent with the pH value of the standard buffer solution. If there is a deviation, the slope knob can be fine-tuned to make the displayed value consistent with the standard value. Through three-point calibration, the measurement accuracy of the online pH meter in different pH ranges can be further improved. When selecting the standard buffer solution, it is necessary to make a reasonable choice according to the actual measured pH range to ensure that the pH value of the calibration solution can cover the pH range of the solution to be tested.
Common Problems and Solutions of Online pH Meters
Calibration:
Failure to calibrate or large errors after calibration are difficult problems that may be encountered when using online pH meters. Electrode aging is one of the common reasons. Long-term use will reduce the performance of the electrode's sensitive membrane and affect its ability to respond to hydrogen ions. To determine whether the electrode is aged, you can observe its response time and stability in the standard buffer solution. If the response is slow and the reading fluctuates greatly, it is likely that the electrode has aged and needs to be replaced in time.
Problems with the calibration solution should not be ignored. If the solution is not stored properly, evaporates, or deteriorates, its pH value will change, resulting in inaccurate calibration. Check the solution carefully before use. If there is turbidity, precipitation, discoloration, etc., it should be replaced immediately. At the same time, ensure the validity period of the calibration solution and store it strictly in accordance with the prescribed storage conditions.
Abnormal measurement value handling:
Unstable and large deviations in measured values will seriously affect the judgment of experimental results or production processes. Electrode contamination is a common factor. Oil stains and impurities in the solution will adhere to the electrode surface, hindering the normal reaction of hydrogen ions with the electrode.
The electrode can be soaked in a suitable cleaning solution, such as detergent solution for oil stains and dilute hydrochloric acid solution for calcium and magnesium plasma precipitation, and then rinsed with distilled water. The presence of interfering substances in the solution will also affect the measurement accuracy.
For example, if it contains strong oxidizing substances, it may oxidize the electrode surface and reduce its performance. At this time, the solution needs to be pretreated, such as by filtering, diluting, etc., to remove interfering substances. In addition, drastic changes in temperature and electromagnetic interference in the measurement environment may also cause abnormal measurement values. The measurement environment needs to be kept stable, and shielding measures should be taken if necessary.
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