Analysis techniques in detail- CLASSICAL METHODS- Titrimetric Analysis, Standard solution, ‘end point’ of the titration, indicator, Free Energy Change

Titrimetric analysis refers the quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration which is required to react quantitatively with the solution of the substance to be determined. The solution of accurately known strength, the Standard solution, is called the titrant and the substance being titrated is termed the titrand. A titration is a reaction between the titrant in the burette and the titrand in the flask. The process of adding the standard solution until the reaction is just complete is termed a titration, and the substance to be determined is titrated.

All chemical reactions can not be considered as titrations. A reaction can serve as a basis of a titration procedure, only if the following conditions are satisfied.

a) The reaction must be a fast one
b) It must proceed stoichiometrically
c) The change in free energy (ΔG) during the reaction must be sufficiently large for spontaneity of the reaction.
d) There should be a way to detect the completion of the reaction.

The point at which an equivalent amount of the titrant has been added is called the ‘equivalence point’ or ‘theoretical or stoichiometric end point’. This is detected by some physical change, produced by the solution by it self or more usually by the addition of an auxillary reagent known as an ‘indicator’. After the reaction is practically complete, the indicator should give a clear visual change (either a colour change or a formation of turbidity) in the solution being titrated. The point at which this occurs is called the ‘end point’ of the titration. In the idea titration the visible end point will coincide with the stoichiometric or theoretical end point. In practice, however a very small difference usually occurs; this represents the titration error. Therefore the indicator and the experimental conditions should be selected so that this difference becomes as small as possible.

When a titration is carried out the free energy change for the reaction is always negative.
e.g. During the initial stages of the reaction between A & B, when the titrant A is added to B the following reaction takes place.

a = activity

Large values of the equilibrium constant K implies that the equilibrium concentration of A & B are very small at the equivalence point. It also indicates that the reverse reaction is negligible and the product C & D are very much more stable than the reactants A & B. Greater the value of K the larger the magnitude of the negative free energy change for the reaction between A & B. Since

Free Energy Change = ΔG = -RT ln K

Where R = Universal gas Constant = 8.314 JK-1mol-1

T = Absolute Temperature.

The reaction of the concentration of A & B leads to the reduction of the total free energy change. If the concentrations of A & B are too low the magnitude of the total free energy change becomes so small and the use of the reaction for titration will not be feasible.