A titration is a laboratory technique used to determine the concentration of a species in a solution. This is achieved by adding a second species of a known concentration (the titrant) that will react with the species whose concentration is to be determined (the analyte). There must be some indication of when the titration is complete, or, in other words, there must be some indication of when the unknown species has reacted completely with the titrant. A common example is the titration of a base by an acid. In these cases, a small amount of a pH-sensitive colored indicator can be added, and the end of the titration can be determined by observing a sudden large change in pH via a change in the color of the solution.