Why does pink colour appear on gradually adding sodium hydroxide to a solution of acid having 2-3 drops of phenolphthalein?
Why does pink colour appear on gradually adding sodium hydroxide to a solution of acid having 2-3 drops of phenolphthalein?
Phenolphthalein (HIn) is weakly acidic in nature. And in aqueous solution, it dissociates into H+H+ and In−In− ions.
The pink colour of the solution is due to the concentration of In−In− ions in the solution.
Under acidic conditions, the concentration of In−In− in the solution is very low and concentration of H+H+ is high, hence it iscolourless.
Similarly, under basic conditions, the concentration of H+H+ ions is very low and concentration of In−In− is high, hence the solution is pink coloured.
For example Titration of HCl (0.1N) against NaOH (0.1N) in the presence ofphenolphthalein indicator.
10 ml Titrant ( HCl ) is taken in a conical flask and 2–3 drops of phenolphthalein is added to it.
At this point, no Titrand (NaOH) is added to the solution.
Therefore, Phenolphthalein is under acidic conditions and hence it is colourless.
This solution is now titrated against Titrand ( NaOH).
As soon as we add 10 ml of NaOH, complete Neutralisation takes place.
HCl + NaOH ====> NaCl + H2H2O.
At this point, Phenolphthalein indicator is under neutral conditions and it imparts veryvery light pink colour to the solution. This point is called as Equivalence point.
Now, after the equivalence point when we add 0.1 ml of NaOH.
After this addition phenolphthalein comes under basic conditions and imparts Pink colour to the solution. This point is called as the End point.
End point tells us that the reaction is completed.
Phenolphthalein (HIn) is weakly acidic in nature. And in aqueous solution, it dissociates into H+H+ and In−In− ions.
The pink colour of the solution is due to the concentration of In−In− ions in the solution.
Under acidic conditions, the concentration of In−In− in the solution is very low and concentration of H+H+ is high, hence it iscolourless.
Similarly, under basic conditions, the concentration of H+H+ ions is very low and concentration of In−In− is high, hence the solution is pink coloured.
For example Titration of HCl (0.1N) against NaOH (0.1N) in the presence ofphenolphthalein indicator.
10 ml Titrant ( HCl ) is taken in a conical flask and 2–3 drops of phenolphthalein is added to it.
At this point, no Titrand (NaOH) is added to the solution.
Therefore, Phenolphthalein is under acidic conditions and hence it is colourless.
This solution is now titrated against Titrand ( NaOH).
As soon as we add 10 ml of NaOH, complete Neutralisation takes place.
HCl + NaOH ====> NaCl + H2H2O.
At this point, Phenolphthalein indicator is under neutral conditions and it imparts veryvery light pink colour to the solution. This point is called as Equivalence point.
Now, after the equivalence point when we add 0.1 ml of NaOH.
After this addition phenolphthalein comes under basic conditions and imparts Pink colour to the solution. This point is called as the End point.
End point tells us that the reaction is completed.
