Transcript 6. Buffers: The Henderson-Hasselbalch Equation

General, Organic, and
Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 10
Acids and Bases
10.6
Buffers
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Lectures
Buffers
A buffer solution
 maintains pH by neutralizing added acid or base.
 in the body, absorbs H3O+ or OH− from foods and
cellular processes to maintain pH.
Blood contains buffers that maintain a pH close to 7.4.
A change in blood pH can disrupt oxygen absorption
and metabolic processes.
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Chapter 10, Section 6
2
Buffers (continued)
When an acid or base
is added
 to water, the pH
changes drastically.
 to a buffer solution,
the pH does not
change very much;
pH is maintained.
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Chapter 10, Section 6
3
Components of a Buffer
The components of a buffer solution
 are acid–base conjugate pairs that have nearly equal
concentrations.
 can be a weak acid and a salt of its conjugate base.
 can also be a weak base and a salt of its conjugate
acid.
An acid must be present to react with any OH−.
A base must also be available to react with any added
H3O+.
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Chapter 10, Section 6
4
Learning Check
Will each of the following combinations produce a buffer
solution or not? Explain.
A. HCl and KCl
B. H2CO3 and NaHCO3
C. H3PO4 and NaCl
D. HC2H3O2 and KC2H3O2
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Chapter 10, Section 6
5
Solution
Will each of the following combinations produce a buffer
solution or not? Explain.
A. HCl + KCl
No; HCl is a strong acid.
B. H2CO3 + NaHCO3
Yes; this is a weak acid and
its salt.
C. H3PO4 + NaCl
No; NaCl is not a
conjugate base of H3PO4.
D. HC2H3O2 and KC2H3O2 Yes; this is a weak acid and
its salt.
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Chapter 10, Section 6
6
Buffer Action
An acetic acid/acetate buffer contains acetic acid
(HC2H3O2) and the salt of its conjugate base, sodium
acetate (NaC2H3O2).
 Acid dissociation occurs.
 The salt provides a higher concentration of the
conjugate base C2H3O2− than provided by the
dissociation of the weak acid by itself.
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Chapter 10, Section 6
7
Weak Acids React with Bases
 The function of the weak acid in a buffer is to
neutralize added base.
 The acetate ion produced by the neutralization
becomes part of the available acetate.
acetic acid
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base
Chapter 10, Section 6
acetate ion
water
8
Conjugate Bases React with Acids
 The function of the acetate ion C2H3O2− is to
neutralize added H3O+.
 The acetic acid produced by the neutralization
contributes to the available weak acid.
acetate ion
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acid
acetic acid
Chapter 10, Section 6
water
9
Summary of Buffer Action
Buffer action occurs because
 the weak acid in a buffer neutralizes the base.
 the conjugate base in the buffer neutralizes the
acid.
 the pH of the solution is maintained.
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Chapter 10, Section 6
10
Calculating the pH of a Buffer
The [H3O+] in the Ka expression is used to determine
the pH of a buffer.
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Chapter 10, Section 6
11
Guide to Calculating pH of a
Buffer
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Chapter 10, Section 6
12
Calculating Buffer pH
The weak acid H2PO4− in a blood buffer, H2PO4−/HPO42−
has a
. What is the pH of the buffer if
Analyze the Problem.
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Chapter 10, Section 6
13
Calculating Buffer pH
The weak acid H2PO4− in a blood buffer, H2PO4−/HPO42−
has a
. What is the pH of the buffer if
Step 1 Write the Ka expression.
Step 2 Rearrange Ka for [H3O+].
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Chapter 10, Section 6
14
Calculating Buffer pH
The weak acid H2PO4− in a blood buffer, H2PO4−/HPO42−
has a
. What is the pH of the buffer if
Step 3 Substitute Ka, [HA] and [A].
Step 4 Use [H3O+] to calculate pH.
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Chapter 10, Section 6
15
Learning Check
What is the pH of a H2CO3 buffer that is 0.20 M
H2CO3 and 0.10 M HCO3−?
A. 6.66
B. 6.36
C. 6.07
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Chapter 10, Section 6
16
Solution
What is the pH of a H2CO3 buffer that is 0.20 M H2CO3
and 0.10 M HCO3−?
Analyze the Problem.
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Chapter 10, Section 6
17
Solution
What is the pH of a H2CO3 buffer that is 0.20 M H2CO3
and 0.10 M HCO3−?
Step 1 Write the Ka expression.
Step 2 Rearrange Ka for [H3O+].
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Chapter 10, Section 6
18
Solution
What is the pH of a H2CO3 buffer that is 0.20 M H2CO3
and 0.10 M HCO3−?
Step 3 Substitute Ka, [HA] and [A].
Step 4 Use [H3O+] to calculate pH.
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Chapter 10, Section 6
19
Buffer Preparation
A buffer solution is needed to maintain a pH of 3.5 to
3.8 in a urine sample. Which of the following buffers
would you use if 0.1 M solutions of the weak acid and
conjugate base are available?
Formic acid/formate
Carbonic acid/bicarbonate
Ammonium/ammonia
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Chapter 10, Section 6
20
Buffer Preparation
To estimate the pH of a buffer system, we can use the
exponent of the Ka.
pH estimate
Formic acid/formate
Carbonic acid/bicarbonate
Ammonium/ammonia
4
7
10
The formic acid/formate buffer will have a pH close to 4,
and, therefore, will be the best choice.
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Chapter 10, Section 6
21