Accuracy of acidity meter and PH meter
The level of the acidity meter is expressed according to the division value (resolution or minimum display value) of its indicator (electric meter for short), for example: an instrument with a division of 0.1pH is called a 0.1-level instrument; the minimum display value is 0.001pH The instruments are called class 0.001 instruments, and so on. 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. Starting from the actual use requirements, the current division value of the electric meter is 0.1~0.001pH. If necessary, a more precise electric meter can be produced according to the current technological level. However, due to structural and manufacturing reasons, the performance of commonly used electrodes cannot reach a completely ideal level. Neither the repeatability error of the glass electrode nor the stability of the solution junction potential of the reference electrode is better than 0.01pH. Therefore, no matter how high the resolution of the electrometer 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 influence of the electric meter on the test error to the greatest extent.
Since it is no longer a problem to make the electric meter achieve satisfactory accuracy, it is constantly improving and improving in terms of intelligence, humanization, reliability, easy operation and cost performance of the instrument. The pHS-4C+ and other "+" series instruments produced by Chengdu Ark Technology Development Co., Ltd. reflect this trend.
The relationship between the level of the acidity meter and its test accuracy is stipulated in the National Measurement and Verification Regulations for the acidity meter (JJG 119-84) as follows:
Instrument level 0.2 level 0.1 level 0.02 level 0.01 level 0.001 level
Graduation value or minimum display value (pH) 0.2 0.1 0.02 0.01 0.001
Electrometer indication error (pH) ±0.1 ±0.05 ±0.01 ±0.01 ±0.002
The total error of supporting test indication (pH) ±0.2 ±0.1 ±0.02 ±0.02 ±0.01
Note: The test range of supporting test should be controlled within pH3～pH10.
It can be seen from the above that for the acidity meter below the 0.01 level, the total error value of the indication is equal to its level; for the acidity meter of the 0.01 level, the total error of the indication value is 0.02pH; It can only reach ±0.01pH, and at this time, it is necessary to use the best pH standard material to be guaranteed. (Note: The pH national standard material is divided into two types: ** and secondary. Generally, the pH meter commonly used is the secondary pH standard material.)
What also needs to be explained is why the test range of pH3~pH10 should be specified for the detection of the total error of the instrument indication value? It is because the liquid junction potential of the reference electrode is related to the pH of the solution. The liquid junction potential can be stabilized in the range of ±0.01pH in the range of pH3~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 will also produce alkali error and acid error in alkaline and acidic solutions. Therefore, it must be specified to detect within the range of pH3~pH10, otherwise it will be impossible to distinguish the total error of the indication value of various grades of acidity meters.
