Analytical Chemistry

1
Analytical Chemistry
2
Chapters 10 11Acid/Base Equilibrium
3
pH of a Dilute Strong Acid or Base
What is the pH of a dilute (1.00 nM) solution of
NaOH?
pH 5
Wait a minute! That means it is acidic. This
cannot be correct.
4
Solution
Must consider OH- in pure water.
Autoprotolysis
Use systematic approach to equilibrium
5
General Rules for Strong Acids Bases
1) High concentration gt10-6 M
Use concentration given
2) Low concentration lt10-8 M
pH 7.00
3) Intermediate concentration 10-6 to 10-8 M
Use systematic approach
6
Fraction of Dissociation (a)
General Monoprotic Acid
F is the formal concentration
7
Nitrobenzoic Acid
pKa 2.179
pKa 3.449
8
Essence of Weak-Acid Problem
If we assume x ltlt F, then expression reduces to
the following
If x is lt 1 of formal concentration then it is a
good assumption.
9
(CH3)3NHCl
Find the pH of 0.100 M trimethylammonium chloride.
10
Essence of Weak Base Equilibria
If we assume x ltlt F, then expression reduces to
the following
11
Hydroxylamine
What is the pH of a 0.100 M solution of the weak
base hydroxylamine (Kb 9.55 x 10-9)?
12
Buffers
A buffered solution resists changes in pH when
acids or bases are added or dilution occurs.
A buffer is a mixture of an acid and its
conjugate base.
Conjugate Base Neutralizes Acid
Weak Acid Neutralizes Base
13
Henderson Hasselbalch
Henderson discovered the buffering power of CO2.
(1908)
Hasselbalch expressed Henderson's equation in
logarithmic form. (1916)
14
H-H Equation
Equation can be applied to any buffer system.
Prove relationship
15
Blood
Blood is more than red blood cells!
Why is the pH of blood important?
What is the pH of your blood?
How is it maintained?
16
Blood
There are two buffering systems in blood
Bicarbonate
pKa 6.4
Phosphate
pKa 7.2
A healthy persons blood pH is 7.4. At this pH,
calculate the ratio of HCO3-/H2CO3
17
Preparing a Buffer
1) Weigh out either HA or A- and add to water
2) Monitor pH
3) Add either strong acid or strong base to
adjust pH to near the pKa of the buffer acid
4) Dilute to final volume
18
Polyprotic Acid-Base Equilibrium
General Diprotic Weak Acid
Ka1 K1
Ka2 K2
How do we determine the concentration of the
various species?
19
General Rules for a Diprotic Acid
1) Solution H2A
A) Treat H2A as monoprotic. Find H, HA-,
and H2A using K1.
B) Use K2 equilibrium to solve for A2-.
HA- H
20
General Rules for Diprotic Acid
2) Solution of HA-
A) Use HA- F and find pH with
Derived using systematic approach.
B) Solve for H2A and A2- using K1 and K2 .

21
General Rules for Diprotic Acid
3) Solution of A2-
A) Treat A2- as monobasic. Find A2-, H,
and HA-.
B) Use Ka1 equilibrium to solve for H2A.
HA- OH-
22
Principle Species
Monoprotic Acid
a
pKa
pH
HA
A-
pH
23
Principle Species
Polyprotic Acid
pKa1
pKa2

H2A
A2-
HA-
Special case when H2A A-2-
24
Isoionic Isoelectric pH
Isoionic pH or point is the pH obtained when
the pure, neutral polyprotic acid is dissolved in
water.
Isoelectric pH or point is the pH at which the
average charge of the polyprotic acid is zero.
25
Isoelectric Focusing (IEF)
Separate based on pI of analyte
1) Apply electric field (E).
2) Create a pH gradient (not a trivial task).
3) Analyte migrates toward corresponding
electrode.
Important bioanalytical separation method because
biomolecules are often amphoteric.
26
Protein pI or IpH
85 of proteins have IpH between 4.5 and 8.5
27
IEF Study of Salmon
Atlantic Salmon
Coho Salmon