Acid / Base Titrations
Volumetric analysis is a technique which employs the measurement of volumes to quantitatively determine the amount of a substance in solution. In any reaction between two or more species, the reaction equation will show the stoichiometric ratio of reacting species. Take the reaction between solutions of KOH and HNO3 (see Example 1). The reaction equation is:
HNO3 + KOH KNO3 + H2O
In a volumetric analysis, if one of these species is present in known molar concentration, then by taking a fixed volume of one solution and progressively adding the other solution, it is possible to find a point at which complete reaction of the substances has occurred. This is called the equivalence point . The incremental process is called titration and enables the concentration of a solution to be determined from the ratio of reacting volumes, the stoichiometric equation and the one known concentration.
Up to the equivalence point the reacting solution contains excess of one reactant; after the equivalence point the solution contains excess of the other reactant. In order to observe this change, some type of indicator is required, for example by a change in colour. The point at which this observed change occurs is called the end point of the reaction and an appropriate indicator is one in which the end point and the equivalence point are as close together as possible.
Not all chemical reactions are suitable for titrations. An appropriate reaction must fulfil several requirements, these being:
- the reaction must be fast, so that the titration can be performed in a convenient time
- the reaction must essentially go to completion; that is, it must have a large equilibrium constant
- the reaction must be free from side reactions so that it can be represented by a single equation
- it must be possible to determine through observation the equivalence point of the reaction
In this experiment a series of acid/base titrations is performed, with the aim of becoming familiar with the technique of titration, the reaction response and the calculations associated with volumetric analysis (see Example 1).
Appendix 5 of the Laboratory Manual details the techniques used in titrations and should be read before the practical session.