This document discusses two main types of indicator electrodes: metal electrodes and glass electrodes. Metal electrodes are typically made of an inert or reactive metal and are used to determine the concentration of analytes in solution. Glass electrodes consist of a glass membrane that is sensitive to hydrogen ion concentration and are very selective with a fast response time. Both metal electrodes and glass electrodes play essential roles in electrochemical analysis in various industries such as healthcare, environmental monitoring, and industrial production.