Ask A Scientist Chemistry Archive

name         Julie N.
status       student
age          19

Question -   Why does the faint-pink end point resulting from
phenolphthalein indicator always fades to colorless if the flask is
swirled vigorously each time the end point is reached?
------------------------------------------------
I am assuming you are titrating from lower pH to a higher pH. When you swirl
the flask you are dissolving CO2 from the atmosphere which lowers the pH
just enough when you are just at the end point to cause the phenolphthalein
to fade.

There could be other reasons, but this is the most common culprit.

Vince Calder
=========================================================
Julie,

The color fades because carbon dioxide from the air dissolves in the
solution. When this happens, carbonic acid (a very weak acid) is formed.
That slight acid contribution drives the pH down enough to cause the color
to fade.

You can demonstrate this effect by exhaling into the flask as it is swirled.
So doing, you will be surprised at the depth of color that your breath can
"neutralize."

Regards,
ProfHoff 414
========================================================
Well, when doing a titration like you describe, drops of base solution are
added to an acidic solution containing the indicator phenolphthalein.  When
the acid is nearly neutralized, adding base will increase the pH to above
the pKa of the phenolphthalein, turning it pink.  Mixing between the acidic
solution and the added drop of base is not instantaneous, however.
Initially, just the solution near where the drop was added is basic.  As
this region mixes with the rest of the bulk solution, either the base
overwhelms the acid, in which case the whole solution turns pink, or the
acid overwhelms the base, in which case the pink color disappears.  When the
endpoint is actually reached, the entire solution will turn pink.

Over time (hours to months), carbon dioxide from the atmosphere will diffuse
into the basic solution, making it acidic.  So, eventually the pink
phenolphthalein color will fade, but this is a much slower process.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
=========================================================