Acid Base Titrations and Indicators

Titration is an operation for the measurement of quantities of substances in solution by the method of 'volume analysis'. This process involves adding a solution of the reagent of known concentration (standard solution), taken in a burette (called 'titrant'), to an unknown solution (called analyte), taken in a flask known as titration flask. Titration is continued until the stoichiometric amounts of the two reagents (titrant and analyte) have been mixed. For example a titration of a strong acid such as HCl and a strong base such as NaOH involve the following reaction.

The stoichiometric point corresponds to the neutralization of HCl and NaOH and is called equivalence point or end point. At this stage the number of moles of OH^- ions added to an acid solution becomes equal to number of moles of H⁺ ions present in the solution. Therefore, at equivalence point:

The simplest method for determining the equivalence point is to add a dye to the solution that shows change in colour at the equivalence point. Such dyes are called indicators. For example in the above titration of HCl with NaOH only NaCl is present in water at the equivalence point and the solution will have a pH of 7. Therefore, any indicator, which shows change in colour around pH 7 will be suitable for the titration.