Acid-base titration. Image 3 of 3. Titrations are a method of volumetric analysis, in which a volume of one reagent is added to a known volume of another reagent until an end point. This end point can be measured by using an indicator, a substance that changes colour depending on its environment. This reaction is an acid-base titration, and the indicator used is methyl orange. The solution in the conical flask (bottom) has gone beyond the end point, due to the addition of excess alkali from the burette (upper centre). The indicator is orange here, showing it is an alkali. For a sequence showing the change in colour at the end point, see images A500/348-350.