Indicator and Types of Indicator
The chemical substances which are used to indicate the end point during titration are called indicators.
Types of indicator:
a. Self-indicator: A substance is said to be self indicator if it itself acts as a indicator in titration.
Example: KMnO4 acts as a self indicator during the titration of Potassium permanganate (KMnO4) and
oxalic acid (COOH)2.
b. Internal indicator: The indicator which is used within the solution is called internal indicator.
Example: Methyl orange and phenolphthalein can acts as indicator in acid base titration.
c. External indicator: The indicator which are not added to the solution are called external indicator.
Potassium Ferro cyanide K3[Fe(CN)6] can acts as external indicator during titration of ammonium or
ferrous sulphate (FeSO4) with Potassium dichromate (K2Cr2O7).
Theory of Indicator and Selection of Indicator
Theory of indicator: The main purpose of selecting a suitable indicator is to find out the end point as much
nearer as possible to the equivalence point. This will minimize the titration error. As indicator indicates the
end point by change in its color but all of them do not show at the same pH range. For example:
Phenolphthalein changes its color at pH range of 8 to 9.5 and methyl orange at pH range 3-4.5. The pH at
the end point of a titration depends upon the nature of titrand and titrant used. So the indicator should be
selected properly so as to coincide the end point with equivalent point.
Indicators as weak acids
Litmus
Litmus is a weak acid and is one of the oldest forms of a pH indicator and is used to test materials for
acidity.
Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components.
It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can
be given away to something else. The "Lit" is the rest of the weak acid molecule. There will be an
equilibrium established when this acid dissolves in water. Taking the simplified version of this equilibrium:
The un-ionised litmus is red, whereas the ion is blue. Now use Le Chatelier's Principle to work out what
would happen if you added hydroxide ions or some more hydrogen ions to this equilibrium.
, Adding hydroxide ions:
Adding hydrogen ions:
If the concentrations of HLit and Lit - are equal: At some point during the movement of the position of
equilibrium, the concentrations of the two colors will become equal. The color you see will be a mixture of
the two.
The reason for the inverted commas around "neutral" is that there is no reason why the two concentrations
should become equal at pH 7. For litmus, it so happens that the color does occur at close to pH 7 -
that's why litmus is commonly used to test for acids and alkalis. As you will see below, that isn't true for
other indicators.
Methyl orange
Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange
is yellow and the structure is:
Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively
charged oxygen. That's the obvious place for it to go. Not so!
The chemical substances which are used to indicate the end point during titration are called indicators.
Types of indicator:
a. Self-indicator: A substance is said to be self indicator if it itself acts as a indicator in titration.
Example: KMnO4 acts as a self indicator during the titration of Potassium permanganate (KMnO4) and
oxalic acid (COOH)2.
b. Internal indicator: The indicator which is used within the solution is called internal indicator.
Example: Methyl orange and phenolphthalein can acts as indicator in acid base titration.
c. External indicator: The indicator which are not added to the solution are called external indicator.
Potassium Ferro cyanide K3[Fe(CN)6] can acts as external indicator during titration of ammonium or
ferrous sulphate (FeSO4) with Potassium dichromate (K2Cr2O7).
Theory of Indicator and Selection of Indicator
Theory of indicator: The main purpose of selecting a suitable indicator is to find out the end point as much
nearer as possible to the equivalence point. This will minimize the titration error. As indicator indicates the
end point by change in its color but all of them do not show at the same pH range. For example:
Phenolphthalein changes its color at pH range of 8 to 9.5 and methyl orange at pH range 3-4.5. The pH at
the end point of a titration depends upon the nature of titrand and titrant used. So the indicator should be
selected properly so as to coincide the end point with equivalent point.
Indicators as weak acids
Litmus
Litmus is a weak acid and is one of the oldest forms of a pH indicator and is used to test materials for
acidity.
Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components.
It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can
be given away to something else. The "Lit" is the rest of the weak acid molecule. There will be an
equilibrium established when this acid dissolves in water. Taking the simplified version of this equilibrium:
The un-ionised litmus is red, whereas the ion is blue. Now use Le Chatelier's Principle to work out what
would happen if you added hydroxide ions or some more hydrogen ions to this equilibrium.
, Adding hydroxide ions:
Adding hydrogen ions:
If the concentrations of HLit and Lit - are equal: At some point during the movement of the position of
equilibrium, the concentrations of the two colors will become equal. The color you see will be a mixture of
the two.
The reason for the inverted commas around "neutral" is that there is no reason why the two concentrations
should become equal at pH 7. For litmus, it so happens that the color does occur at close to pH 7 -
that's why litmus is commonly used to test for acids and alkalis. As you will see below, that isn't true for
other indicators.
Methyl orange
Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange
is yellow and the structure is:
Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively
charged oxygen. That's the obvious place for it to go. Not so!
