May 01, 2013
Indicator
-a conjugate acid base pair, where the acid and
base form have different colours
the pigment in red cabbage is an indicator
Ever used these?
May 01, 2013
General Formula
HIn
In-
Example: Phenolphthalein
HPh
Ph-
May 01, 2013
Equilibrium
HIn(aq) + H2O(l)
HPh(aq) + H2O(l)
H3O+(aq) + In-(aq)
H3O+(aq) + Ph-(aq)
May 01, 2013
Indicators are weak acids, so they have a Ka value.
This means it is possible to predict their reactions
using the five step method.
What reaction will take place when methyl orange is
placed in a solution of acetic acid? (Use the five
step method)
Acid base indicators are useful for showing the end
point of a titration
It is important to choose an indicator that will have a
colour change near the equivalence point of the titration
Acid base indicators are useful for showing the end point of a titration
It is important to choose an indicator that will have a colour change near the equivalence point of the titration
End point:
Equivalence point:
The goal in a titration is to have the end point and the equivalence point the same
End point:
Equivalence point:
The goal in a titration is to have the end point and the
equivalence point the same
May 01, 2013
Titration Curve
A graph that shows the change in pH during an acid
base titration
They have characteristic shapes depending on the
reactants
Indicators are weak acids, so they have a Ka value. This means it is possible to predict their reactions using the five step method.
What reaction will take place when phenol red (HPr/Pr-) is placed in a solution of acetic acid? (Use the five step method)
Strong Acid Strong Base Titrations
May 01, 2013
Buffering Region: the region at the beginning of a
titration curve where the pH doesn’t change very much
with the addition of a strong acid or base (doesn’t occur
in strong acid-strong base titrations)
Titration of a weak acid with a strong base
Example: acetic acid titrated with NaOH
pH at EQ point is
What would be a suitable indicator for this reaction?
What would be the indicator reaction at the end point?
May 01, 2013
Titration of a weak base with a strong acid
Example: HNO3(aq) with sodium cyanide
pH at EQ point is
What would be a suitable indicator for this
reaction?
What would be the indicator reaction at the
end point?
Titration of a polyprotic acid with a strong base
There is an EQ point for each proton donated by
the acid
May 01, 2013
Titration of a polybasic base with a strong acid
There is an EQ point for each proton accepted by the
base
May 01, 2013