Transcript 9-pH and buffer_ part B 2014-2015 (1)

Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
pH and Buffers (B)
1
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
By the end of this topic, the student will be able
to:
• Identify buffers
• Demonstrate how buffer can
resist changes in pH.
2
1-Acid and Bases
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•Definition
•General properties
•Classification (strong , weak)
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2-Henderson-Hasselbalch Equation
• Calculation
• Importance
3-Buffer
• Definition
• Principles of Buffering
• Applications
Strength Of
Acids & Bases
It depends on the degree of Ionization in
an aqueous solution
i.e.
The efficacy with which
an acid acts as a proton donor
& a base acts as a proton acceptor
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Medical Biochemistry & Molecular Biology Department
HA
Let’s
examine the
behavior of
an acid, HA,
in aqueous
solution.
What happens to the HA molecules in solution?
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Ain Shams University
Medical Biochemistry & Molecular Biology Department
HA
100%
dissociation of
HA
H+
Strong Acid
A-
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Medical Biochemistry & Molecular Biology Department
HA
H+
Partial
dissociation
of HA
Weak Acid
A-
HA 
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+
H
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+
A
Medical Biochemistry & Molecular Biology Department
HA
H+
Weak Acid
A-
At any one
time, only a
fraction of the
molecules are
dissociated.
HENDERSON-HUSSELBALCH
EQUATION
1-Calculation
2-Uses
How can you calculate the pH of Strong
Acids & Bases ??
From the Molarity or Normality of them
i.e.
pH of Acid = - log of acid conc [H+]
pH of Base = 14 - pOH
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Ain Shams University
Medical Biochemistry & Molecular Biology Department
HA ↔ H+ + AKa =
-Log
Ka
[H+] [A-]
[HA]
-]
[A
+] +-Log
[H
-Log
=
[HA]
Values Of Ka like [H+] Involve –Ve Powers Of 10
& They are therefore expressed as
pKa values
pKa= -log Ka
HENDERSON-HUSSELBALCH
Equation
pH = pKa
[A-]
+ Log [HA]
USES OF
HENDERSON-HUSSELBALCH
Equation
1. For Calculation of pH of a weak Acid.
2. Preparation Of Buffer Of Known pH
When
[A ]=
pH = pKa
[HA]
[A-]
+Log [HA]
So PKa Of an acid is the pH at which
[The Acid conc.] = [Its Conjugate base conc.]
Summary
The smaller the
value of pKa
The stronger
the acid
The greater the
value of Ka
The more it
dissociates
Strong Acid
Transfers all of its
protons to water
Weak Acid
Transfers only a small
fraction of its
protons to water
Completely Ionized
Partially Ionized
Ka large
Ka is small
Smaller pKA
larger pKA
Eg. HCl and H2SO4
Eg. Organic acids e.g.
H2CO3
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BUFFER
1-Definition
2-Principle of buffering
3-Application
Buffers
It is a solution of A Weak Acid (H2CO3)
& its Conjugate Base (NaHCO3)
Or Weak Base (NH4OH) &
its Conjugate Acid (NH4CL)
Buffers
It resists changes in pH in a solution
when moderate amount of strong acids
OHOH-
or bases are added
H+
Acid
“H2CO3”
OH-
OH-
Conjugate base
“NaHCO3”
H+
H+
H+
Principles of Buffering:
AH ↔ H+ + A-
The Extent Of Buffer Capacity Depends on
3 factors
-]= [HA ] i.e the acid is half
If [A
Any
buffer
exerts
dissociated (half-neutralization state)
maximal
Log
[A-]/ [HA ] =buffering
log 1= zero
pH
=
pKa
+
0
capacity when the pH is
equal to its pKa.
pH= pKa  1
Is Considered as pH range
For
SATISFACTORY Buffering Capacity
Physiological buffers
1. H2CO3/ HCO3 (pKa= 6.1) This is the most important
buffer system in the body
2. H2PO4/ HPO4 (pKa= 6.8).
3. Plasm Proteins (20% of buffering capacity “NH2+ , COO-”)
4. Hb. (60% of buffering capacity “Histidine”)
5. Free amino acids
But Theoretically
Bicarbonate buffer is < efficient than Phosphate buffer in
intracellular fluid.
The pKa of acetic acid is 4.76,
its buffer mixture is most
effective at pH:
a) 2.3
b) 4.5
c) 3.7
pH = -log[H+]
pOH= -log of [OH]
pH + pOH =14
pH= pKa +log [A-]
[HA-]
Lab activities
Demonstrate how buffer can resist changes
in pH.
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
27
Extended Modular Program
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