Titration is a method of analysis that will allow you to determine the precise endpoint of a reaction and therefore the precise quantity of reactant in titration flask.

Acid-Base Titration

An acid-base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor the progress of the acid-base reaction. The chemical reaction involved in acid-base titration is also called as neutralization reaction.

Acid-Base Indicators

An indicator is a substance which is used to determine the end point in a titration. In acid base titrations, organic substances (weak acids or weak bases) are generally used as indicators. They change their colour within a certain pH range.

An acid-base indicator is a weak acid or a weak base. Some of the indicators used in acid-base reactions include – Litmus, Phenolpthalein, Methyl Orange, thymol blue, methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red. The pH range and colour of indicators are given in table below.


pH range

Color for weak acid

Color  for conjugated base

Methyl orange




Bromphenol blue




Thymol blue








Alizarin yellow














Theory of Acid-Base Indicators

Two theories have been proposed to explain the change of colour of acid-base indicators with change in pH.

Ostwald's theory:

According to this theory, the colour change is due to ionization of the acid-base indicator. The unionized form has different colour than the ionized form. The ionization of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionization is very much low in acids due to common H+ ions while it is fairly ionized in alkalized. Similarly if the indicator is a weak base, its ionization is large in acids and low in alkalizes due to common OH- ions.

Considering two important indicators phenolphthalein (a weak acid) and methyl orange (a weak base),

Ostwald theory can be illustrated as follows:

Phenolphthalein: It can be represented as HPh.

It ionises in solution to a small extent as:

HPh ↔ H++ Ph

Colourless Pink Applying law of mass action,

K = [H+][Ph- ]/[HpH]

The un-dissociated molecules of phenolphthalein are colourless while Ph- ions are pink in colour. In presence of an acid the ionization of HPh is practically negligible as the equilibrium shifts to left hand side due to high concentration of H+ ions. Thus, the solution would remain colourless. On addition of alkali, hydrogen ions are removed by OH- ions in the form of water molecules and the equilibrium shifts to right hand side. Thus, the concentration of Ph- ions increases in solution and they impart pink colour to the solution.

Let us derive Henderson equation for an indicator

HIn + H2O ↔ H3O + + In-      Acid form' 'Base form'

Methyl orange:

 It is a very weak base and can be represented as MeOH. It is ionized in solution to give Me+ and OH- ions.

MeOH ↔ Me+ + OH (Yellow Red)

Applying law of mass action,

K = [Me+ ][OH- ]/[MeOH]

In presence of an acid, OH- ions are removed in the form of water molecules and the above equilibrium shifts to right hand side. Thus, sufficient Me+ ions are produced which impart red colour to the solution. On addition of alkali, the concentration of OH- ions increases in the solution and the equilibrium shifts to left hand side, i.e., the ionisation of MeOH is practically negligible. Thus, the solution acquires the colour of unionised methyl orange molecules, i.e., yellow. This theory also explains the reason why phenolphthalein is not a suitable indicator for titrating a weak base against strong acid. The OH- ions furnished by a weak base are not sufficient to shift the equilibrium towards right hand side considerably, i.e., pH is not reached to 8.3. Thus, the solution does not attain pink colour. Similarly, it can be explained why methyl orange is not a suitable indicator for the titration of weak acid with strong base.