What is the difference between the various types of membrane glass for pH electrodes

As pH is measured in a wide variety of solutions at varying temperatures it is not possible to formulate a single pH membrane glass which can give accurate and reliable pH measurements in all conditions. For this reason different types of membrane glass have been developed to give good performance in a variety of measurement conditions. The most common membrane glass types found in Schott electrodes are as outlined below:

If the wrong membrane type is selected for a particular application it may be rendered unusable without any obvious signs of change.