What are some typical ph meter usage misconceptions?
First, the measurement parameters of the ph meter
Since the pH meter is used to measure the acidity and alkalinity of a 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.
Second, the automatic compensation function of the ph meter
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, the ph value measured by the ph meter should be the ph value at the standard temperature (25°C). In fact, the ph meter 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.
3. Calibration of ph meter
Many people believe that a meter should be calibrated over the full scale range, and so should a pH meter. In fact, it is unrealistic for the electrode of the pH meter to obtain the same accuracy in the full scale range 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 measured.
After the calibration work is completed, 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. When there is a large difference between the solution temperature and the calibration temperature;
2. When the electrode is exposed to the air for too long;
3. The positioning or slope regulator is misoperated;
4. After measuring the solution of peracid (ph<2) or overalkaline (ph>12);
5. After changing the electrodes;
6. When the pH value of the measured solution is not in the middle of the solution selected during two-point calibration, and it is far away from 7ph.
4. 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 reference error, and the same is true for the accuracy level of the pH meter. In fact, this is a wrong understanding. The level of the ph meter and the accuracy of the instrument are two different concepts. Its level is based on the score (resolution or minimum display value) of its indicator (electric meter for short). Indicated, and the accuracy of the instrument is the comprehensive error of the electrometer and the electrode matching test standard solution, which is not only related to the electrometer, but also more related to the glass electrode and the reference electrode.
