What misunderstandings exist when using pH meter

A pH meter is an instrument used to determine the pH value of a solution. The pH meter is based on the principle of galvanic cell. The electromotive force between the two electrodes of the galvanic cell is based on the specific law of energy, which is related to the properties of the electrodes and the concentration of hydrogen ions in the solution.

Errors in the use of pH meter

Myth 1: The measurement parameters of pH meter

Since the pH meter is to measure the acidity and basicity of the solution, it can also be used to measure the concentration of the acid and base solution.

Actually, the pH meter measures the activity of ions in the solution, not the concentration of the solution, which is different from the activity.

Myth 2: The automatic compensation function of pH meter

In industrial pH measurements, it is often assumed that since a PH meter with temperature compensation is used, the pH meter should measure the PH value at the standard temperature (25 ° C) regardless of the change in solution temperature.

No, the pH meter always measures the PH at the current temperature. The PH meter with temperature compensation function can correct the slope of electrode potential change with pH value according to the measured temperature, so that it can be accurately measured.

Mistake 3: Calibration of pH meter

Many people believe that meters should be calibrated at full scale, as should PH meters.

In practice, it is not practical for PH meter electrodes to achieve the same accuracy at full scale in use. The calibration of PH meter generally adopts two point calibration. One is the standard buffer of PH7 or so, to position the PH meter; The other point is calibrated according to the use of environment (acidic or alkaline) with PH4 or PH9 or so standard buffer.

In addition, special attention should be paid to the temperature of the solution to be measured before calibration. In order to correctly select the standard buffer, and adjust the temperature compensation knob on the meter panel to make it consistent with the temperature of the solution to be measured.

After the calibration, frequently used PH meters generally do not need to be calibrated again within 48 hours. If one of the following situations occurs, 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 in the air for too long, such as more than 0.5h;

(3) The positioning or slope regulator is misactivated;

(4) After measuring an acid (pH<2) or="" a="" hyperbase="" ph="">12) solution;

(5) After changing the electrode;

(6) When the pH value of the measured solution is not in the middle of the selected solution at two point calibration, and the distance from 7PH is far.

Mistake 4: the accuracy level of PH meter and its allowable error

It has been suggested that in conventional meters, the accuracy level of the meter is expressed in terms of the reference error, as is the accuracy level of the PH meter. For example, the maximum allowable error of a PH meter with an accuracy of class 0.01 and a measurement range of 0-14PH is ±0.0014PH, but this is actually a wrong understanding.

The level of PH meter and the accuracy of the instrument are two different concepts, its level is according to its indicator (abbreviated as the meter) score value (resolution or minimum display value), and the accuracy of the instrument is the integrated error of the standard solution of the meter and the electrode, which is not only related to the meter, but also more related to the glass electrode and the reference electrode.