Introduction
The pH value is one of the most important and therefore also one of the most frequently measured quantities in analytical chemistry. Sometimes, a simple pH test strip is sufficient for making a rapid check; precise pH meters are, however, required whenever accurate measuring results are needed. Such a pH meter is available in nearly every laboratory and is generally ready for use at a moment's notice. The measured pH value can be read off the pH meter directly and can also often be archived immediately in compliance with GLP. However, to verify the accuracy of the measured value, the following points should always be considered before taking a pH measurement.
1. The pH measuring instrument
A pH meter must meet certain prerequisites for optimum accuracy and reproducibility. Depending on the required accuracy, a pH meter should permit multipoint calibration across the pH range in which the pH value of the sample is expected. Additionally, it should enable temperature compensation of the pH value according to the Nernst equation. It should indicate drifting values in the display and also offer various different options to read the pH value automatically, either after a fixed time interval or – for maximum reproducibility – once a predefined drift criterion is fulfilled. Whether or not a sample is stirred, the best way to obtain reproducible results is to use a firmly installed setup (stand, stirrer, position of the electrode in the measuring vessel).
2. The pH electrode
The selection of a pH electrode should be based on the characteristics of the sample (pH value, temperature, consistency, matrix). Particular attention should be paid to selecting a suitable reference electrolyte and diaphragm. The most universal diaphragms are the sleeve diaphragms. They consist of two perfectly ground glass surfaces allowing a well-defined electrolyte outflow and therefore show significant advantages: Besides the optimized, circular geometry, these diaphragms are virtually insensitive to contamination and have a much better signal stability. The highest level of comfort can be achieved with the sleeve diaphragm and easyClean technology. The sleeve diaphragm can be opened for cleaning by simply pushing the electrode head. An internal spring mechanism ensures that the sleeve is pulled back with very high reproducibility to ensure an equal electrolyte outflow – and herewith very reproducible results – for each measurement.
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