Titration instruments are used to determine the concentration of dissolved substances. Titration is based on a complete chemical reaction between the analyte and a reagent or titrant of known concentration that is added to the sample. The analyte is the substance which a laboratory test is designed to measure. The reagent or titrant is the substance that produces a chemical reaction in order to determine the presence of the analyte. There are two basic titration methods: manual and automatic. Manual titration is performed with a burette, a piece of laboratory glassware that has volumetric graduations along its length and a tap or stopcock on its bottom. Automatic titration is performed with an auto-titrator, an electrically-powered laboratory instrument that can be interfaced to a personal computer (PC). Typically, automatic titration instruments are used for repetitive titrations.