Chapter 15:
Acid / Base Chemistry
15.3 pH Changes in Acid-Base
Reaction Systems
pH curves

A pH curve is a graph showing the continuous change in pH
during a titration (an acid-base reaction)
Inv. 15.3 Demonstration – pH curves

Titration of NaOH and Na2CO3 with HCl

14
pH
7
0
vol. of titrant (HCl) added (mL)
Titration Terminology (Gr.11)

Titration – the progressive transfer of a solution from a
buret (called the titrant) into a measured volume of
another solution (called the sample).

Equivalence point – the volume of titrant required to

Endpoint – the pH at the equivalence point of a titration.

Indicator – a chemical which is added to the sample that

Buffering region – a horizontal region of the pH curve
neutralize the sample (# mol acid = # mol base).
changes colour at the equivalence point of a titration.
where pH is not changing significantly.
Interpreting pH curves
Titration of NaOH with HCl

14
pH
buffer region
endpoint
7
0
X
titrant
equivalence point
equivalence point volume
buffer region
0
10
20
30
volume of titrant added (mL)
40
Which is the titrant?

Take 2 titrations involving NaOH and HCl
pH
pH
vol. of titrant


HCl is titrant
(pH high to low)
vol. of titrant
NaOH is titrant
(pH low to high)
Choosing an Indicator


pH curves are used to determine which indicator(s) are suitable for
a particular titration
The goal is to get an indicator that changes colour in the same
region as the endpoint of your titration

14
pH
7
Titration of NaOH with HCl
unsuitable: alizarin yellow (too early)
good indicators: bromothymol blue,
litmus and phenol red
unsuitable: orange IV (too late)
0
vol. of titrant (HCl) added (mL)
Multiple Endpoints
Titration of Na2CO3 with HCl

14
endpoint 1
pH
8
X
endpoint 2
equivalence point volume 1
4
0
X equivalence point volume 2
0
10
20
30
volume of titrant added (mL)
40
Choosing Indicators for
Multiple Endpoints
Titration of Na2CO3 with HCl

14
endpoint 1 - metacresol purple (7.4-9.0)
pH
8
X
endpoint 2 – methyl orange (3.2-4.4)
4
0
X
0
10
20
30
volume of titrant added (mL)
40
Interpreting Endpoints

The Bronsted-Lowry 5-step method can be used to write
proton-transfer reactions that explain the endpoints on a
pH curve

In general, we only observe distinct endpoints on a pH
curve when a proton has been quantitatively transferred
from an acid to a base; incomplete reactions are not
observed

The number of endpoints that are observed represent
the number of proton transfer reactions between B-L
acids and B-L bases in a titration
Using the B-L 5-Step Method
to Explain Endpoints


Titration of NaOH with HCl
14
pH
7
X
endpoint
0
0
we can write a proton transfer
reaction to explain this
10
20
30
volume of titrant added (mL)
40
Writing B-L Equations
to Explain Endpoints
To explain the endpoint observed for the
titration of NaOH with HCl, we use the B-L
5-step method:

SA
A
 Na + OH H3O +
Cl - H2O

SB
B
B



H
Endpoint: H3O
+
+
+ OH
-
2 H2O
Using the B-L 5-Step Method
to Explain Endpoints


Titration of NaOH with HCl
14
pH
7
X
H3 O
+
+ OH
-
2 H2O
0
0
10
20
30
volume of titrant added (mL)
40
Polyprotic Acids and Bases

Polyprotic acids are those which can donate
more than one proton; in most cases, it is equal
to the number of hydrogens in the chemical
formula
– E.g. H2SO4 : can donate 2 protons
– E.g. H3PO4 : can donate 3 protons

Polyprotic bases are those which can accept
more than one proton; in most cases it is equal
to the magnitude of the charge on the anion
– E.g. CO3 2- : can accept 2 protons
– E.g. PO4 3- : can accept 3 protons
Using the B-L 5-Step Method
to Explain Polyprotic Endpoints
Titration of Na2CO3 with HCl

14
pH
8
X
4
0
endpoint 1
X
0
endpoint 2
10
20
30
volume of titrant added (mL)
40
Writing B-L Equations
to Explain Polyprotic Endpoints
To explain the first endpoint observed for
the titration of Na2CO3 with HCl, we will
use the B-L 5-step method:

SA
A
 Na + CO3 2H3O +
Cl - H2O

SB
B
B

H

Endpoint 1: H3O
+
+
+ CO3
2-
H2O + HCO3 -
Writing B-L Equations
to Explain Polyprotic Endpoints

To explain the second endpoint, we must consider not
just the original entities, but those entities that were
consumed in the first reaction and entities that were
created in the first reaction
SA


Na
+
CO3
2-
H3O
+
Cl
B

H

A
Endpoint 2:
H3 O
+
-
A
H2O
HCO3
B
SB
-
+
+ HCO3
-
H2O + H2CO3
Using the B-L 5-Step Method
to Explain Polyprotic Endpoints
Titration of Na2CO3 with HCl

14
8

X
H3O+ + CO3
2-
H2O + HCO3 -
pH
0
4
0
X
H3O+ + HCO3
10
20
30
volume of titrant added (mL)
-
40
H2O + H2CO3
Other Polyprotic Systems

Consider the titration of phosphoric acid, H3PO4,
with sodium hydroxide, NaOH:
11
X
pH
X
endpoint 1
0
Vol. of NaOH (mL)
endpoint 2
Other Polyprotic Systems
To explain the first endpoint observed for
the titration of H3PO4 with NaOH, we use
the B-L 5-step method:

SA
A
 H3PO4
Na +
OH - H2O

SB
B

H

+
Endpoint 1: H3PO4 + OH
-
H2O + H2PO4
-
Other Polyprotic Systems

To explain the second endpoint, we must consider not
just the original entities, but those entities that were
consumed in the first reaction and entities that were
created in the first reaction


H3PO4
Na
+
OH
SB

H

Endpoint 2:
-
A
SA
H2O
H2PO4
B
-
B
+
H2PO4 - + OH
-
H2O + HPO4 2-
Other Polyprotic Systems

The B-L method suggests there is the possibility of a third endpoint:
A


H3PO4
Na
+
OH
SB

-
H2O
B
SA
H2PO4
B
-
HPO4
2-
B

However, because it was not observed in the pH curve, it must
mean the reaction was not quantitative, therefore we must write it
with a double arrow
H+
> 50%

Endpoint 3:

(not observed)
HPO4
2-
+ OH
-
H2O + PO4 3-
Other Polyprotic Systems

Consider the titration of phosphoric acid, H3PO4, with
sodium hydroxide, NaOH:
11
HPO42- + OH
H2PO4- + OH-
-
H2O + PO43- (not observed)
H2O + HPO42-
X
X
-
pH
H3PO4 + OH
0
Vol. of NaOH (mL)
H2O + H2PO4 -
Homework:

Answer Q. 24-29 p.539-540.