Use of Indicators
• Indicators are weak acids that have a different color than their
conjugate base form.
• Each indicator has its own pH range over which it changes
color. Most have a useful range of about 2 pH units
• An indicator can be used to estimate the equivalence point in
a titration as long as it changes color in the small volume
change region where the pH rapidly changes.
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
Aqueous
Equilibria
© 2015 Pearson Education
Titration of a strong base and a strong acid
• Rapid change occurs from pH 3 – 11
• Any indicator that changes colour in this pH region can be used
• Both phenolphthalein and methyl red change color in the rapid-rise portion
of the titration curve.
Is methyl red a suitable
indicator when you are
titrating a strong acid with
a strong base?
Explain your answer.
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
Titration of a weak acid with a strong base
• Choice of an indicator is important when titrating a weak with a
strong base and vice-versa. Why?
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
Titrating a weak base with a strong acid
Why is phenolphthalein not a suitable indicator when titrating a weak base
with a strong acid?
© 2015 Pearson Education,
Education Inc.
Aqueous
Equilibria
What is important about Thymol blue?
Out of Interest
Which compound is responsible for the colour change in litmus paper?
Identify the structure of the compounds responsible for colour change in
each indicator
Aqueous
Equilibria
© 2015 Pearson Education
Titrations of Polyprotic Acids
•
A polyprotic acid is an acid that has more than one
ionizable hydrogen (H) atom
Examples of polyprotic acids include H2SO4 , H2CO3 and H3PO4.
Dissociation of H3PO4.
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
Dissociation of H3PO4.
Simplified version:
What can you observe about Ka1, Ka2 and Ka3 values?
© 2015 Pearson Education,
Education Inc.
Aqueous
Equilibria
Titrations of Polyprotic Acids
When a polyprotic acid is titrated with a base, there is an
equivalence point for each dissociation or for each acidic H.
Why?
- the neutralization reaction occurs in stages.
The most acidic proton (Ka1) is titrated first, followed by the
next most acidic (Ka2), and so forth
The pH titration curve will show various equivalence points
corresponding to each neutralization step.
For example, H2SO4 will have 2 equivalence points.
How about H3PO4?
And H2CO3?
© 2015 Pearson Education,
Education Inc.
Aqueous
Equilibria
Titrations of H2SO4 with NaOH
Aqueous
Equilibria
© 2015 Pearson Education
Titrations of H2CO3 with NaOH
Aqueous
Equilibria
© 2015 Pearson Education
Titrations of H3PO4 with NaOH
3rd equivalence
point is not easy
to detect via
titration with 0.1M
NaOH. Why?
Aqueous
Equilibria
© 2015 Pearson Education
Important information from a pH titration curve of a
weak polyprotic acid
The pH titration curves of the weak polyprotic acid helps us predict the pKa
values of the acid. What is pKa?
Lets consider the pH titration curve of H3PO3 .
Why are we considering only two equivalence points when H3PO3 is suppose dto
have 3 equivalence points?
Write the pH expression for each equilibrium system
Aqueous
Equilibria
© 2015 Pearson Education
What would be the expression
when [H2PO3-] = H3PO3]?
What would be the expression
when [HPO32-] = H2PO3-]?
When [H2PO3-] = H3PO3], pH = pKa1
When [HPO32-] = H2PO3-], pH = pKa2
At which stage does this occur?
- half way before the equivalence point. This point is
Aqueous
known as the half-equivalence
Equilibria
© 2015 Pearson Education
Identifying the mid-points
pH = pKa3
Why is the 3rd
equivalence point difficult
to identify using titration
with NaOH?
pH = pKa2
pH = pKa1
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
What are the implications?
We can estimate the pKa values from the pH titration curve
Estimate the pKa values of H3PO4 using the pH titration curve against NaOH
Compare with the actual values (find the actual values on Google)
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.
Estimate the pKa values of H2SO3 using the pH titration curve against NaOH
Aqueous
Equilibria
© 2015 Pearson Education,
Education Inc.