Titration is the simple operation to find the concentration of a solution by finding out the volumes of reacting solutions needed at the equivalent point of reaction with the help of suitable indicator.
Equivalent Point of a reaction:
Equivalent point of a reaction is defined as the point where two or more reagents react completely in the ratio of their equivalent masses.
The equivalent point of acid-base reaction is called neutralization point. When the ph at the neutralization point is 7, the point is called Neutral point.
It is the suitable chemical substance used in course of titration that helps to find out the equivalent point of the reaction by the change in its color.
Eg. Methyl orange is an indicator used in the titration of strong acid against strong base.
End Point of Titration:
It is the equivalent point of reaction determined by titration by the sharp change of color of the indicator.
The determination of concentration of a solution by titration becomes accurate only when the end point of the titration coincide with the equivalent point of reaction.
For the coincident, the indicator should be selected properly by studying the nature of reaction involved in titration.
Classification of Titration
Based on the nature of reaction involved titration can be classified into different groups:
1. Acid-base titration (Neutralization titration)
Acid-Base titration involve acid-base reaction
Eg. HCl + NaOH → NaCl + H2O
The indicator used to find out the equivalent point of acid-base reaction in titration by the sharp change in its color is acid-base indicator or pH-indicator.
Phenolphthalein, methyl-orange, etc. are the common pH indicator.
2. Redox Titration
Redox titration involves redox reaction.
Eg. KMnO4 + H2SO4 → K2SO4 + MnSO4 + H2O + CO2
The titration between oxalic acid solution and acidified KMnO4 solution is a redox titration because the reaction involved is redox reaction.
The indicator used in redox titration is called redox indicator. In the above titration, KMnO4 itself acts as a redox indicator.
Here, oxalic acid solution is a primary standard solution and KMnO4 is a secondary standard solution and the concentration of secondary standard (KMnO4) solution is determined by titration of it with primary standard oxalic acid solution.
The determination of concentration of other given reducing agent solution by titration with the standard KMnO4 solution is called permanganometric titration.
3. Color change of acid-base indicators (pH-indicators)
Acid-base indicators are weak organic acid or organic base having different distinct color in acidic and basic medium.
Eg. Phenolphthalein is a weak organic acid (HIn) acting as a pH indicator which is colorless in acid and pink in alkali medium.
Similarly, methyl-orange is a weak organic base (InOH) acting as an indicator which is pink in acid and yellow in alkali medium.
In order to explain the color change of pH indicator consider the ionization equilibrium of a weak acid indicator (HIn) in aqueous solution.
HIn (unionizaed form) ⇄ H+ + In- (Ionized form)
There exists an equilibrium between ionized and unionized form of indicators. The unionized and ionized form of indicators must have different colors.
Eg. Unionized form of phenolphthalein is colorless and ionized form of phenolphthalein is pink.
The color of one form of an indicator is seen from the solution only which the concentration of this form is many times greater than the concentration of its other form.
When acid (H+) is added in this equilibrium, the concentration of H+ increases and the equilibrium distributes and then retain the equilibrium, the equilibrium shifts to the backward direction (Lechatelies Principle). Consequently the concentration of the unionized form of the indictor becomes greater than the ionized form. In this case the color of unionized form of the indicator is seen in the solution.
The unionized form of the phenolphthalein is colorless. This is the reason why phenolphthalein is colorless in acid.
Similarly, when base (OH) is added in this equilibrium, the concentration of H+ decreases because it reacts with OH to form H2O. It disturbs the equilibrium and to retain the equilibrium, the equilibrium should shift to the forward direction (Lechatelies Principle). Here, the concentration of ionized form of the indicator is greater than unionized form and the color of ionized form persists in the solution.
The ionized form of phenolphthalein is pink. This is why phenolphthalein is pink in alkaline medium.