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.
The colour change and the pH range of some common indicators are tabulated below:
Indicator pH Range Over Which Colour Change Occurs
Colour of Acidi Form Colour of Base Form
Methyl Orange 3.2-4.5 Orange Yellow
Methyl Red 4.4 – 6.5 Red Yellow
Bromothymol blue 6.0 -7.8 Yellow Blue
Phenolphthalein 8.3- 10.0 Colourless Pink
Alizarin Yellow 10.1 – 12.1 Yellow Red
Litmus 5.5-7.5 Red Blue
Phenol red 6.8-8.4 Yellow Red
Refer to the following video for acid-base indicators
Two theories…show more content… So phenolphthalein is the suitable indicator for such a titration.
(iii) Strong acid vs. weak base:
pH curve of strong acid (say HCl or H2SO4 or HNO3) with a weak base (say NH4OH) is vertical over the pH range of 4 to 7. So the indicators methyl red and methyl orange are suitable for such a titration.
(iii) Weak acid vs. weak base:
pH curve of weak acid and weak base indicates that there is no vertical part and hence, no suitable indicator can be used for such a titration.
Titration of soluble carbonate with strong acid.
pH curve of sodium carbonate with HCI shows two inflection points (Fig. 10.5). First inflection point (pH 8.5) indicates conversion of carbonate into bicarbonate.
Na2CO3 + HCI → NaHCO3 + NaCl
As the inflection point lies in the pH range 8 to 10, phenolphthalein can be used to indicate the above conversion. The second inflection point (pH 4.3) indicates the following reaction:
NaHCO3 + HCI → NaCl + CO2 + H2O
As the point lies between 3 to 5, methyl orange can be used.
You can also refer
JEE Physical Chemistry Syllabus,
Reference books of