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 measured values are needed. A pH meter of this type is available in practically every laboratory and, as a rule, is 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 conformance with GLP. However, in order to verify the accuracy of the measured value, the points described in this article should always be considered before taking a pH measurement.