These common operating misunderstandings of online pH detectors

The online pH detector uses a glass electrode as the measuring electrode and a calomel electrode as the reference electrode. The change of the potential of the glass electrode caused by the change of the hydrogen ion concentration caused the change of the potential between it and the calomel electrode. It is an instrument for measuring H+ activity in solution by potentiometric method. The value measured by the pH meter is a method used to express the pH of the solution, which is expressed by the negative logarithm of the H+ ion concentration in the solution, that is: pH= ー lg[H+]. The pH meter we use is composed of two parts: an electric meter and an electrode. In the actual measurement, the electrode is immersed in the solution to be measured, and the concentration of H+ ions in the solution is converted into a millivolt level voltage signal, which is input to the electric meter. The electric meter amplifies the signal and converts it into pH through logarithm, and then the pH is displayed by the millivolt level display instrument. How many of these common operating errors of online pH detectors do you know? Misunderstanding 1: Regarding the accuracy level of the PH meter and its allowable error Some people think that in conventional instruments, the accuracy level of the instrument is expressed by the reference error, and the accuracy level of the PH meter is also the same. For example, a PH meter with an accuracy grade of 0.01 and a measurement range of 0-14PH has a maximum allowable error of ±0.0014PH, but in fact this is a wrong understanding. The level of the online PH detector and the accuracy of the instrument are two different concepts. Its level is expressed by the score (resolution or minimum display value) of its indicator (referred to as the electric meter), while the accuracy of the instrument It is the comprehensive error of the electrometer and the electrode matching test standard solution. It is not only related to the electrometer, but also more related to the glass electrode and the reference electrode. According to the National Metrology Verification Regulations (JJG 119-2005), the indication errors of the electric meter and the whole machine of the PH meter of each level are shown in Table 2. Misunderstanding 2: About the measurement parameters of the online PH detector Since the online PH detector is used to measure the acidity and alkalinity of the solution, it can also be used to measure the concentration of acid and alkali solutions. In fact, the pH meter measures the ion activity in the solution, not the concentration of the solution, because the concentration and activity of the solution are different. Misunderstanding 3: About the calibration of the online PH detector Many people think that the calibration of the meter should be carried out in the full range, and so should the pH meter. In fact, it is unrealistic to obtain the same accuracy in the full range of the electrode of the online pH detector in use. The calibration of the pH meter generally adopts two-point calibration. One point is a standard buffer solution around PH7 to position the pH meter; the other point is to use a standard buffer solution around PH4 or PH9 for calibration according to the use environment (acidic or alkaline). In addition, special attention should be paid to the temperature of the solution to be tested before calibration. In order to correctly select the standard buffer solution, and adjust the temperature compensation knob on the electrometer panel to make it consistent with the temperature of the solution to be tested. After the calibration work is finished, the frequently used pH meter generally does not need to be re-calibrated within 48 hours. In case of any of the following situations, the instrument needs to be re-calibrated: (1) There is a large difference between the solution temperature and the calibration temperature; (2) The electrode is exposed to the air for too long, such as more than 0.5h; (3) The positioning or slope regulator is misoperated; (4) Measuring overacid (pH<2) or overly alkaline (pH>12) After the solution; (5) After changing the electrode; (6) When the pH value of the measured solution is not in the middle of the solution selected during the two-point calibration, and it is far away from 7PH. Misunderstanding 4: Regarding the automatic compensation function of the online pH detector In industrial pH measurement, people often think that since a pH meter with temperature compensation function is used, no matter how the solution temperature changes, what the pH meter measures should be PH value at standard temperature (25°C). In fact, it is not the case. The online pH detector always measures the pH value at the current temperature. The pH meter with temperature compensation function corrects the slope of the electrode potential with the pH value according to the measured temperature, so that it can be measured accurately.