[Erichsen]

Hello I have question that I cant find answers too in any of my chemistry books. Hope you might help me on this.

Some days ago i made an acid-base titration where HCl was titrated with NaOH and phenoltalein as indicator (colorless to pink). In the manual it says that I must titrate until the pink color remains for at least 30 secs. and then fade back to colorless. Can someone explain why the colorshift from pink to colorless occur?
I mean when I reach the equivalence point the solution is pH=7, then i add a tiny bit of NaOH and the indicator change to pink. If this is true, how on earth can the OH^- be consumed again (which result in colorless solution)? Is it because the solution is not total homogeneous when the solution is pink, and when like the 30 secs. has pasted it is total homogenous and therefore not enough OH^- to give the pink color?

Thanks for reading

[enahs]

There is a lot going on here, however, one simple answer is carbonic acid.

CO₂ + H₂O ⇌ H₂CO₃
The CO₂ being supplied by air; H₂CO₃ being a weak acid.

H₂CO₃ ⇌ HCO₃⁻ + H⁺

[AWK]

Pink colour means that a very small excess of NaOH exist in the solution.
When this excess will be neutralized by H2CO3
NaOH + H2CO3 = NaHCO3 + H2O
then pink colour disappears since phenolphtalein with NaHCO3 is colourless (NaHCO3 shows pH slightly lower that is needed for a color change of phenolphtalein)

[Erichsen]

Ahh thanks for this answer. Now I understand

Does that mean that you have to be fast to do the titration? I think no ( I mean if the CO₂ dissolves and make some H₃O⁺. The longer time the titration takes, the more CO₂ will dissolve )

Am I right if I think it only has a great effect near the equivalence point, because it will dissociate more here because H₃O⁺ is now smaller?

[AWK]

Does that mean that you have to be fast to do the titration? I think no ( I mean if the CO2 dissolves and make some H3O+. The longer time the titration takes, the more CO2 will dissolve )

The eqivalence point is at pH~10. At this pH CO2 dissolves much faster than in acidic conditions.
Dissolving CO2 increase H3O+ by remowing OH- anions from the solution
OH- + CO2 = HCO3-
This has an influence on the equilibrium (ion product for water)
2H2O = H3O+ + OH-