ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 11

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 11
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 11 POLYPROTIC ACIDS AND BASES
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POLYPROTIC ACIDS
- Have more than one acidic proton Examples
phosphoric acid, carbonic acid, amino acids
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POLYPROTIC ACIDS
Tooth Decay - Bacteria on teeth metabolize sugar
into lactic acid CH3CH(OH)CO2H - Tooth enamel
contains hydroxyapatite (calcium
hydroxyphosphate) Ca10(PO4)6(OH)2 - Tooth decay
is the result of reaction between lactic acid
and hydroxyapatite to produce phosphoric
acid Ca10(PO4)6(OH)2 14H ? 10Ca2 6H2PO4-
2H2O
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POLYPROTIC ACIDS
Erosion of limestone or marble (calcium
carbonate) - Calcite (calcium carbonate) is
soluble in acidic solutions (insoluble in neutral
or basic solutions) - Calcite dissolves in acid
rain causing erosion on buildings CaCO3(s) ?
Ca2 CO32- CO32- H ? HCO3- Acid Rain -
SO2, NOx, and CO2 combine with water vapor and
sunlight to produce sulfuric acid, nitric acid,
and carbonic acid
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POLYPROTIC ACIDS
Amino Acids - Building blocks of proteins -
Have acidic carboxylic acid group and basic amino
group - The acidic proton resides on the N of
the amino group - Have positive site (amino
group) and negative site (acid group) - Called
zwitterion - Both groups are protonated at low
pH and depotonated at high pH
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DIPROTIC SYSTEMS
- Contain two acidic protons H2A ? HA-
H (Ka1) HA- ? A2- H (Ka2) - Acid
dissociation constants Ka1 gt Ka2 A2- H2O
? HA- OH- (Kb1) HA- H2O ? H2A
OH- (Kb2) - Base association constants Kb1 gt
Kb2
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DIPROTIC SYSTEMS
H2A ? HA- H (Ka1) HA- H2O ?
H2A OH- (Kb2) H2O ? H OH- Ka1
x Kb2 Kw Ka2 x Kb1 Kw
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DIPROTIC SYSTEMS
Ka1 gtgtgtgt Ka2 - A solution of a diprotic acid
behaves like a solution of a monoprotic acid with
Ka Ka1 Kb1 gtgtgtgt Kb2 - The fully basic form
of a diprotic acid can be considered as monobasic
with Kb Kb1
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DIPROTIC SYSTEMS
The Intermediate Form - Is both an acid and a
base - Can donate or accept a proton - Called
amphiprotic
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TRIPROTIC SYSTEMS
Ka1 x Kb3 Kw Ka2 x Kb2 Kw Ka3 x Kb1
Kw
First Intermediate (H2A-)
Second Intermediate (HA2-)
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PREDOMINANT SPECIES
- From the Henderson-hasselbalch equation - pH
changes by 1 if the ratio changes by a factor of
10 pH pKa 1 if A-/HA 10 pH pKa - 1
if A-/HA 0.10
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PREDOMINANT SPECIES
Monoprotic Systems A- HA when pH pKa A-
is the predominant form when pH gt pKa HA is the
predominant form when pH lt pKa
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PREDOMINANT SPECIES
Diprotic Systems There are two pKa values H2A
HA- when pH pKa1 HA- A2- when pH
pKa2 H2A is the predominant form when pH lt
pKa1 HA- is the predominant form when pKa1 lt pH lt
pKa2 A2- is the predominant form when pH gt pKa2
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PREDOMINANT SPECIES
Triprotic Systems There are three pKa
values H3A H2A- when pH pKa1 H2A-
HA2- when pH pKa2 HA2- A3- when pH
pKa3 H3A is the predominant form when pH lt
pKa1 H2A- is the predominant form when pKa1 lt pH
lt pKa2 HA2- is the predominant form when pKa2 lt
pH lt pKa3 A3- is the predominant form when pH gt
pKa3
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TITRATION CURVES
Diprotic acids (two equivalence points)
pH
H2A/HA-
HA-/A2-
Excess OH-
pKa2
pKa1
volume of OH- added