Download Summarized pH pracctical22

pH and Buffers
ILO of the current topic:
By the end of this topic, the student will be able to:
Identify different pH solutions.
1
Our Goals for today
1 .Dissociation of Water:
• Apply Kw Calculations
• pH
• Define the pH scale
2 .Acids & Bases
• Definition
• General properties
• Classification (strong , weak)
3 .The Henderson-Hasselbalch Equation
• Calculation
• Importance
4 .MEASURING THE pH
• Indicators
• PH Meter
• Titration
5 .Buffer
• Definition
• Principles of Buffering
• Applications
Dissociation Of Water
1-Kw
2-pH
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Dissociation Of Water
H2O
H+ + OH-
Water has limited capacity to
dissociate (ionize) into H+ &
OH-
Ka Is The Acid Dissociation Constant Of
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Water
[H+involving
] [OH-] such –ve
Since those values
Ka =
powers of 10 which[His2O]
difficult to use in
Since water is too
little dissociated
calculations
[H+] [OH-]
So, Kw So
Sorensen
(1909)
suggested
that
the
+
Kw = [H ] [OH ]
[H+]concentration
of DILUTE
aqueous
-7
-7
10 mol/L
Kw = 10 X 10
solutions
Kw =beThe
Ionexpressed
Product Of
would
better
asWater
PH value
-14
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
What is the pH?
• The pH of a solution is the negative of the
logarithm to the base 10 of its hydrogen
ion concentration in (mol/L)
pH = -log[H+]
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
pH scale
2
3
4
5
acidic
(H+) > (OH-)
ranges from 0 – 14
pH < 7 acid
pH = 7 neutral
pH > 7 base
6
7
8
neutral @ 25oC
(H+) = (OH-)
distilled water
9
10
11
basic or alkaline
(H+) < (OH-)
12
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
P= -Log
H= H+ Concentration
pH= -log of H+ Concentration
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
[OH-] may be represented as pOH value
P= -Log
OH= [OH- Concentration]
pOH= -log of [OH- Concentration]
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Since [H+] x [OH-] =10 -14
By –log of both sides
–log [H+] + -log [OH-] =14
i.e. pH + pOH =14
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Problem 2
Q- What is the pH of 0.0001 N HCL sol.?
A- [H+] = 0.0001 = 10-4 mol/L
pH = - log 10-4 = 4
Q - What is the pH of 0.001 N NaOH sol.?
A- [OH-] = 0.001 = 10-3 mol/L
pOH = - log 10-3 = 3
Since pH+ pOH=14
So, pH = 14 - pOH= 14 -3= 11
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Problem 3
What is the pOH of a 0.010 M
solution of Ba(OH)2:
?
A. 1.0
B. 2.0
C. 4.0
D. 5.0
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
pH + pOHProblem
= 14.00
4
 The pH of a sample of human blood
pOH = 14.00-7.4 = 6.6
was
measured to be 7.41 at 25°C.
+
[H
]
=
antilog(-pH)=
antilog
(-7.41)
=
+
Calculate pOH, [H ] and [OH ] for
-8M
3.9x10
the sample.
[OH-] = antilog(-pOH) = antilog (-6.6)
Answer
= 2.51 x 10–7 M
Acids and bases
1-Definition
2-Properties
3-Classification
4-Neuteralization
Acids and bases
Acids
• Taste sour
• React w/ metals &
carbonatesproduce H gas
• Turn blue litmus
paper red
• form hydrogen
ions (H+) in water
Bases
• Taste bitter
• Feel slippery
• Don’t react w/
carbonates
• Bases turn red
litmus paper blue.
• form hydroxide
ions in water
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Acids cause
H+
to increase
Bases cause OH- to increase
2-17
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Amphoteric Substance
They can act as
Proton DONORS with bases
&
Proton ACCEPTORS with acids
Eg. Amino acid –H2O
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Problem 5
In dilute aqueous solution, as [H+]
increases:
A. pH decreases
B. pOH increases
C. [OH–] decreases
D. all of the above
Neutralization
HCl + NaOH → H2O + NaCl
acid
base
water
salt
• reaction between an acid & base
• produces water & a salt
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
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
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
HA
What happens to the HA molecules in solution?
Let’s
examine the
behavior of
an acid, HA,
in aqueous
solution.
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
100% dissociation of HA
HA
H+
Strong Acid
A-
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Partial dissociation of HA
HA
H+
Weak Acid
A-
Would the
solution be
conductive?
HA  H+ + A-
HA
H+
Weak Acid
A-
At any one time,
only a fraction of
the molecules are
dissociated.
HENDERSON-HUSSELBALCH
Equation
1-Calculation
2-Uses
Faculty of Medicine
Ain Shams University
How can
you
calculate
the
pH
of
Medical Biochemistry & Molecular Biology Department
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
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
HENDERSON-HUSSELBALCH
Equation
pH = pKa
[A-]
+ Log [HA]
Faculty of Medicine
Ain Shams University
USES OF
Medical Biochemistry & Molecular Biology Department
HENDERSON-HUSSELBALCH
Equation
1. For Calculation of pH of a weak Acid.
2. Preparation Of Buffer Of Known pH
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
When [A-]= [HA]
pH = pKa
[A-]
+Log [HA]
So PKa Of an acid is the pH at which
[The Acid conc.] = [Its Conjugate base conc.]
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Summary
The smaller
the value of
pKa
The stronger
the acid
The greater
the value of Ka
The more it
dissociates
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Strong Acid
Weak Acid
Transfers all of its protons to water
Transfers only a small fraction of its
protons to water
Completely Ionized
Partly Ionized
Ka large
Ka is small
smallerpKA
larger pKA
Eg. HCl and H2SO4
Eg. Organic acids e.g. H2CO3
33
Determining pH
1-Indicators
2-pH Meter
3-Titeration
1-Indicator
• Compound that changes color when in
contact w/ an acid or base
• They are weak acids
• ex. litmus paper
• Red litmus paper
• Blue litmus paper
• universal indicators
•Red Cabbage Juice
Mechanism of action of the litmus
paper indicator
In basic
solutions
• indicator act as acid and loss protons
• ionized form
• Blue color
In acidic
solutions
• indicator act as bases and accept protons
• mostly nonionized form
• Red color
In intermediate
pH
• ionized form+nonionized form
• Intermediate color (green)
Name
of indicator
Colour
acid
Colour
base
pH
range
Red
Yellow
3-4
Bromothymol Blue
Yellow
Blue
6-8
Phenolphthalein
Clear
Red
8 - 10
Methyl Orange
Universal indicators


Universal indicators
made by mixing
several different
indicators
Paper soaked in
universal indicator 
pH paper
2-pH meter


pH meter  determines the pH
of a solution by measuring
voltage between two electrodes
that are placed in solution
Voltage changes as hydronium
ion concentration in solution
changes
BUFFER
1-Definition
2-Principle of buffering
3-Application
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Buffers
It is a solution of A Weak Acid (H2CO3)
& its Conjugate Base (NaHCO3)
Or Weak Base (NH4OH) &
its Conjugate Acid (NH4CL)
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
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+
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Principles of Buffering:
AH ↔ H+ + A-
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
The Extent Of Buffer Capacity
Depends on 3 factors
-]= [HA ] i.e the acid is half
If
[A
Any
buffer
exerts
maximal
dissociated (half-neutralization state)
buffering
capacity
Log [A-]/ [HA
] = log 1= when
zero
pH
=
pKa
+
0
the pH is equal to its pKa.
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
pH= pKa  1
Is Considered as pH range
For
SATISFACTORY Buffering
Capacity
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
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.
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department
Problem 6
• 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-]
Faculty of Medicine
Ain Shams University
Lab activities
include :
Medical Biochemistry & Molecular Biology Department
1.To identify a solution is acidic, basic or neutral in the
laboratory
2.Demonstrate how indicators can be used to test for the
presence of acids or bases and the effect of pH on them and to
understand the concept of neutralization reaction as well
3.Demonstrate neutralization reaction (in home )
Faculty of Medicine
Ain Shams University
Medical Biochemistry & Molecular Biology Department