Classification and level accuracy of pH meters
The level of the acidometer and the accuracy of the instrument are two different concepts, and the level of the instrument is not entirely consistent with its accuracy. The level of an acidometer is expressed by the graduation value (resolution or minimum display value) of its indicator (referred to as an electric meter). For example, an instrument with a graduation of 0.1pH is called a 0.1-level instrument; the minimum display value is 0.001pH. The instruments are called 0.001 level instruments, etc. The accuracy of the instrument is the comprehensive error of the test standard solution of the meter and the electrode. It is not only related to the meter, but also to the glass electrode and reference electrode. Based on the actual use requirements, the scale value of the electric meter is 0.1~0.001pH. If necessary, according to the current level of science and technology, a more precise electric meter can be produced. However, due to structural and manufacturing reasons, the performance of commonly used electrodes cannot yet reach a completely ideal level.
The repeatability error of the glass electrode and the stability of the solution junction potential of the reference electrode are not better than 0.01pH. Therefore, no matter how high the resolution of the electric meter is, the test accuracy of the instrument is difficult to be better than 0.01pH. However, choosing a high-resolution instrument can overcome or eliminate the impact of electrical meter on test errors to the greatest extent. Since it is no longer a problem to achieve satisfactory accuracy of electric meters, they are constantly innovating and improving in terms of instrument intelligence, user-friendliness, reliability, ease of operation and cost-effectiveness. The relationship between the level of an acidometer and its test accuracy is stipulated in the National Measurement Verification Regulations for Acidometers (JJG119-84) as follows:
The levels of pH acidity meter instruments are 0.2, 0.1, 0.02, 0.01, and 0.001
Graduation value or minimum display value (pH) 0.2, 0.1, 0.02, 0.01, 0.001
Electric meter indication error (pH) ±0.1, ±0.05, ±0.01, ±0.01, ±0.002
The total error of supporting test indication value (pH) is ±0.2, ±0.1, ±0.02, ±0.02, ±0.01
Note: The test range during supporting testing should be controlled within pH3 ~ pH10. As can be seen from the above table, for acidimeters below level 0.01, the total error value of the indication value is equal to its level. For acidity meters of level 0.01, the total error value of the indication value is 0.02pH. For acidity meters of level 0.001, the total error value of the indication value is 0.02pH. It can only reach ±0.01pH, and at this time, it is necessary to use a first-level pH standard material to ensure it. (Note: National pH standard materials are divided into first-level and second-level standards. Generally, the second-level pH standard materials are commonly used in acidimeters.)
