What occurs at the end of the point when sulphuric acid neutralises sodium hydroxide solution using phenolphthalein indicator ?
End point shows pale pink colouration.
phenolphthalein is represented by a general formula of HPh, where H stands for Hydrogen and Ph stands for phenolphthalein ion. when phenolphthalein is present in an acidic medium like sulphuric acid, it doesn't dissociate into :
H(+) and Ph(-), because of the pre-existence of H(+) ions in the acidic medium. (Common ion effect taking charge)
But during the titration process, when eventually base like sodium hydroxide is added into the acid, the OH(-) ions furnished by the base, which neutralize all the H(+) ions of the acid( sulphuric acid). when all the H(+) ions of the acid are neutralised , the additional last few drops of the base, added, readily dissociating to give few OH(-) ions are neutralized by the H(+) ions furnished by phenolphthalein on its dissociation (HPh -> H(+) + Ph(-)).
And the dissociated Ph(-) ions is responsible for the pink colour seen at the end point of the titration indicating that all the H(+) ions furnished by the acid are neutralized by the OH(-) ions furnished by the base.
Point to be noted is that , only when all the H(+) ions furnished by the base are netralized by the OH(-) ions of the base, phenolphthalein will dissociate to give H(+) ions and not before that.
At the endpoint of the reaction, the indicator will turn pink because a strong acid has been neutralized by a strong base. The endpoint pH will be dependent on the concentration of each substance.
In a titration lab, you use a known concentration of a substance, an indicator, and equipment (buret, Erlenmeyer flask, graduated cylinder, flasks of the substances, pH paper) to find the concentration of the substance with a concentration that is not known.
Because sulfuric acid is a strong acid and sodium hydroxide is a strong base, the phenolphthalein will turn pink because the solution has become more basic. The pH will become more neutral, but will be dependent on the concentration of each substance.
Source (my own knowledge of having done a titration, plus):