Chem. 31 – 5/6 Lecture
Announcements
• Lab Stuff
– Reminder that resubmissions must have original
report attached
– IC and Formal Lab Reports due 5/11
• Today’s Lecture
– Chapter 10: Acid – Base Titrations
• Strong Acid – Strong Base Titrations
• Qualitative Aspects of Acid Base Titrations
• Weak Acid – Strong Base Titrations
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
• Strong Acid – Strong
Base Titration
– How does pH Change as
NaOH is added?
– 3 regions to titrations
(different calculations in
each region):
• before equivalence point
• at equivalence point
• after equivalence point
– Go to Board to show pH at
5 mL, 12.5 mL, and 15 mL
on this one
0.100 M NaOH
0.050 M HCl, 25 mL
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
Titration Plot
0.100 M NaOH
Titration Plot
14.00
12.00
pH
10.00
8.00
0.050 M HCl, 25 mL
6.00
4.00
2.00
0.00
0
5
10
15
V(HCl)
20
25
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
• What affects shape of
curve?
Titration Plot
14.00
12.00
10.00
pH
– Concentration of acid and
base
– Curves are normally sharp.
Only exception occurs
when intial concentrations
are very low (e.g. [H+]o,
[OH-]o < 10-4 M)
– Note: At low
concentrations, one also
needs to ensure CO2
doesn’t affect NaOH
solutions
50 mM acid
8.00
1 mM acid
6.00
50 uM acid
4.00
2.00
0.00
0
10
20
30
V(HCl)
Note: Base conc. changed by same
factor
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
– Depends on method for determining equivalence point
– End point = “measured” volume of titrant
– Titration error (systematic) = difference between end
point and equivalence point
– With indicators, systematic errors occur when pH(end
point) ≠ pKa(indicator) – This causes the titration error
– With indicators, random errors occur from difficulty in
determining correct color shade which depends on
[In-]/[HIn]
– A factor of 2 uncertainty in [In-]/[HIn] (e.g. 0.5 to 2)
translates to pHmeas = pHtrue + 0.3
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 1 – precise + accurate titration
Uncertainty from
judging indicator
color (e.g. Methyl
orange)
pH
Vol. Base
sV is small
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 2 – mainly imprecise – due to
unsharp titration (e.g. low concentrations)
Uncertainty from
judging indicator
color (e.g. Methyl
orange)
pH
Vol. Base
sV is large
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 3 – mainly inaccurate titration
(due to selection of wrong indicator
equiv. pt. pH
pH
indicator color change
region (pH < pKa)
Vol. Base
small unc. in V but significant error
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• Overview:
– What affects accuracy and precision?
• Best titrations are sharp
• Best titrations measure pH change near actual
equivalence point pH value
Chapter 10 – Acid Base Titrations
Some Questions
1. 25.0 mL of 0.200 M NaOH is being titrated with
0.120 M HNO3. The student wants to estimate the
pH after adding 45 mL of HNO3. To do the
calculation, should the student calculate [H+] or
[OH-] first?
2. For a titration in which HCl is added to a weak base,
the pH at the equivalence point is calculated to be
5.21. A student chooses to use Congo Red as an
indicator. This indicator has a pKa of 4.0.
a) Will selection of this indicator result in a systematic error?
b) Besides, the pKa, what other factor will affect errors in
estimating the equivalence point volume?
Weak Acid – Strong Base Titration
• Weak Acid – Strong Base
Titration
– How does pH Change as NaOH
is added?
– Reaction:
HA + OH- ↔ A- + H2O K = 1/Kb
Example – acetic acid:
Ka = 1.74 x 10-5
– 4 regions to titrations (different
calculations in each region):
•
•
•
•
initial pH
before equivalence point
at equivalence point
after equivalence point
– Go to Board to show pH at 5 mL,
12.5 mL, and 15 mL
0.100 M NaOH
0.050 M HA, 25 mL
Chapter 11 – Acid Base Titrations
Weak Acid – Strong Base Titrations
– pKa values (low pKa or
stronger weak acid gives
sharper titration)
– pKa affects position of curve
before and at equivalence
point
– Concentration of acid and
base (higher concentrations
give sharper titrations)
– Concentration affects curve
shape at and after
equivalence point
– 50 mM solution will also have
greater calculation errors due
to poor assumptions (e.g.
buffer calculations)
50 mM
pKexample
= 4.76 example
a
TitrationPlot
Plot
Titration
14.00
14.00
12.00
12.00
10.00
10.00
pH
pH
• What affects shape of curve?
pKa50= mM
4.76acid
8.00
8.00
pKa1=mM
3 acid
6.00
6.00
50 uM acid
pKa = 7
4.00
4.00
2.00
2.00
0.00
0.00
00
55
1010
15 15
V(HCl)
V(HCl)
20 20
25 25