Level and accuracy of the pH meter
The level of the acidity meter and the accuracy of the instrument are two different concepts, and the instrument level is not exactly the same as its accuracy. The level of the acidity meter is expressed by the index value (resolution or minimum display value) of its indicator (referred to as the electricity meter). For example, an instrument with a division of 0.1 pH is called a 0.1-level instrument; the minimum display value is 0.001 pH. The instrument is called a 0.001 class instrument, and so on. The accuracy of the instrument is the comprehensive error of the standard solution of the electric meter and the electrode. It is not only related to the electric meter, but also related to the glass electrode and the reference electrode. Starting from the actual use requirements, the current electricity meter's graduation value is 0.1 to 0.001 pH. If necessary, according to the current state of the art, a more precise electric meter can be produced. However, due to structural and manufacturing reasons, the performance of commonly used electrodes has not yet reached a fully desirable level. The repeatability error of the glass electrode and the solution junction potential stability of the reference electrode were not better than 0.01 pH. Therefore, the resolution of the electric meter is higher, and the accuracy of the instrument test is hard to be better than 0.01 pH. However, choosing a high-resolution instrument can maximize or eliminate the impact of the meter on test errors. Since it is no problem to make the electric meter achieve satisfactory accuracy, it is constantly innovating and improving in terms of intelligence, humanization, reliability, ease of operation, and cost performance.
The relationship between the level of the acidity meter and its test accuracy is specified in the National Metrology Verification Protocol (JJG 119-84) of the acidity meter as follows:
Instrument level | Level 0.2 | 0.1 level | 0.02 level | 0.01 level | 0.001 level |
| Grading value or minimum display value (pH) | 0.2 | 0.1 | 0.02 | 0.01 | 0.001 |
| Electrical metering error (pH) | ±0.1 | ±0.05 | ±0.01 | ±0.01 | ±0.002 |
| Total test error (pH) | ±0.2 | ±0.1 | ±0.02 | ±0.02 | ±0.01 |
Note: The test range for supporting tests should be controlled within pH 3 to pH 10.
As can be seen from the above table, for a pH meter below 0.01, the total error value of the indication is equal to its grade. For a pH meter of 0.01 grade, the total error of indication is 0.02 pH. For the 0.001 grade acidity meter, the total error of indication is It can only reach ±0.01pH, and it is necessary to use the primary pH standard material to be guaranteed. (Note: pH national standard materials are divided into primary and secondary. Generally, acidity meters are commonly used as secondary pH standards.)
   It should also be noted that the detection of the total error of the instrument indication should specify the test range from pH3 to pH10. Because the liquid junction potential of the reference electrode is related to the pH of the solution, the liquid junction potential can be stabilized within ±0.01pH in the range of pH3 to pH10. If 10<pH<3, the error caused by the liquid junction potential Can be as high as ±0.07pH. At the same time, the glass electrode also produces alkali errors and acid errors in alkaline and acidic solutions. Therefore, it is necessary to specify the detection in the range of pH 3 to pH 10, otherwise the total error of the indication values ​​of the various grades of the acidity meter cannot be distinguished.
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