Titrating Polyfunctional Acids and Bases 920207 1 http:\\ asadipour.kmu.ac.ir 40 slides
Apr 01, 2015
Titrating Polyfunctional Acids and Bases
920207 1http:\\asadipour.kmu.ac.ir 40 slides
1. Treating Complex Acid-Base Systems
• Complex systems are defined as solutions made up of:
(1) An acid or base that has two or more acidic protons or basic functional groups
H3PO4
Ca(OH)2
(2) Two acids or bases of different strengths
HCl + CH3COOH
NaOH + CH3COO-920207 2http:\\asadipour.kmu.ac.ir 40
slides
)3) An amphiprotic substance that is capable of acting as both acid and base
HCO3- + H2O CO3
2- + H3O+
HCO3- + H2O H2CO3 + OH-
NH3+CH2COO- + H2O NH2CH2COO- + H3O+
NH3+CH2COO- + H2O NH3
+CH2COOH + OH-
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•
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34
324
24243
1 POHPOPOHPOH KaKaK a
Kb3Kb2 Kb1
Ka1×Kb3=Kw Ka2×Kb2=Kw Ka3×Kb1=Kw
Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12
Ktotal=Ka1×Ka2× Ka3=1×10-21
pH of H3PO4
1. Calculate the pH of 0.100M H3PO4 solution.
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negligibleisKK
Ka
a
a ..1000 22
1
H
POHHKa 100.0
]].[[101 422
1
H+ is not negligible
pH of HA-
pH of HA- solution
HA- A2- + H+
HA- H2A + OH-
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Ka2
Kb2
Ka1 1
2
1][
a
HA
waHA
K
CKKC
H
pH of HA-
Calculate the pH of 0.100M NaHCO3 solution.
Ka2×CHA- =1×10-10 ×1.00>>Kw…….Kw is negligible
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Ka1=1×10-6 >Ka2=1×10-10
66
a1
HA 101101
00.1
K
C
82
..........1 21
aa pKpKpHleisnegligib
1
2
1][
a
HA
waHA
K
CKKC
H
pH of HA-
Calculate the pH of 0.0100M NaH2PO4 solution.
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Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12
Ka2×CHA- =1×10-7 ×0.01>>Kw…….Kw is negligible
1101
01.0
K
C2
a1
HA
negligiblenotis ......1
1
2
1][
a
HA
waHA
K
CKKC
H
1
2
1][
a
HA
aHA
K
CKC
H
pH of HA-
Calculate the pH of 1.00×10-3M Na2HPO4 solution.
Ka2×CHA- =1×10-10 ×0.001=1×10-13 Kw isnot negligible
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Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12
4
a1
HA 101101
001.0
K
C7
negligibleis.....1
1
2
1][
a
HA
waHA
K
CKKC
H
1
2][
a
HA
waHA
K
CKKC
H
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Sulfuric acid is unusual in that one of its protons behaves as a strong acid in water and the other as a weak acid (Ka2 = 1.02 X 10-2). Let us consider how the hydronium ion concentration of sulfuric acid solutions is computed using a 0.0400M solution as an example.
H2SO4 →H+ +HSO4- SO42- + H+
We will first assume that the dissociation of HSO4 is negligible because of the large excess of H30+ resulting from the complete dissociation of H2SO4. Therefore,
This result shows that [SO4- ] is not small relative to [HSO4 ], and a more rigorous so lution is required.
From stoichiometric considerations, it is necessary that
Mixture of weak and strong acids
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[SO[SO44] = [H] = [H++] - 0.0400 ] - 0.0400 [H[H++] = 0.0400 + ] = 0.0400 + [SO[SO442-2-]] CCHH22SOSO44, = 0.0400 = [HS0, = 0.0400 = [HS044
-- ] + [SO ] + [SO442-2-] ]
[HSO[HSO44--] = 0.0800 - [H] = 0.0800 - [H33OO++]]
[H[H++] ≈ [HSO] ≈ [HSO44 ] ≈ 0.0400 M ] ≈ 0.0400 M 04.0
][04.0 24
SOKa
Sulfuric acid is unusual in that one of its protons behaves as a strong acid in water and the other as a weak acid (Ka2 = 1.02 X 10-2). Let us consider how the hydronium ion concentration of sulfuric acid solutions is computed using a 0.0400M solution as an example.
H2SO4 →H+ +HSO4- SO42- + H+
We will first assume that the dissociation of HSO4 is negligible because of the large excess of H30+ resulting from the complete dissociation of H2SO4. Therefore,
Mixture of weak and strong acids
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])[04.0(
][])[04.0(
][
][][2
4
24
24
4
24
SO
SOSO
HSO
SOHKa
[H[H++] = 0.0400 + ] = 0.0400 + [SO[SO442-2-]] [HSO[HSO44
--] = 0.0400 - ] = 0.0400 - [SO[SO442-2-]]
Curves for the titration of strong acid / weak acid mixture with 0.1000 M NaOH. Each titration is on 25.00 ml of a solution that is 0.1200 M in HCl and 0.0800 M in HA.920207 27http:\\asadipour.kmu.ac.ir 40
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Curves for the titration of 25.00 ml of polyprotic acid with 0.1000M NaOH solution .
A)0.1000 M H3PO4,
B) 0.1000M oxalic acid,
C) 0.1000 M H2SO4
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Ka1 =5.6 × 10-2 and Ka2 = 5.4 x 10-5
Ka1=1×10-2 >Ka2=1×10-7> Ka3=1×10-12
Ka2 = 1.02 × 10-2
Titration of 20.00 ml of 0.1000 M H2A with 0.1000 M NaOH. For H2A, Ka1= 1.00 × 10–3 and Ka2 = 1.00 × 10–7 .
Titration curves for polyfunctional acids
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Titration of 25.00 ml of 0.1000M maleic acid with 0.1000M NaOH.
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HOOC-C=C-COOHpKa1=1.89 ,pKa2=6.23
Fractional composition diagram for fumaric acid (trans-butenedioic acid).
Fractional composition diagram for maleic acid (Cis-butenedioic acid).
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Z-HOOC-C=C-COOHE-HOOC-C=C-COOH
pKa1=3.05 ,pKa2=4.49 pKa1=1.89 ,pKa2=6.23
amino acids
alanine
The amine group behaves as a base, while the carboxyl group acts as an acid.
Aspartic acid920207 32http:\\asadipour.kmu.ac.ir 40
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1-Determining the pK values for amino acidsAmino acids contain both an acidic and a basic group.
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NH2-CH2-COOH +NH3-CH2-COO- Zwitterion formation
+NH3-CH2-COO- + H2O NH2-CH2-COO- + H3O+
+NH3-CH2-COO- + H2O +NH3-CH2-COOH + OH-
]COO-CH2-NH3[
]H3O [] COO-CH2-[NH2-
-
Ka
]COO-CH-NH[
]OH []COOH-CH-NH[-
23
23
Kb
Ka×Kb= ??!!!!
2-Determining the pK values for amino acidsAmino acids contain both an acidic and a basic group.
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NH2-CH2-COOH +NH3-CH2-COO- Zwitterion formation
+NH3-CH2-COOH +NH3-CH2-COO- NH2-CH2-COO-Kb Ka
Ka1=5×10-3 Ka2=2×10-10
Curves for the titration of 20.00ml of 0.1000M alanine with
A) 0.1000 M NaOH
B) 0.1000M HCl.
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AB
• The zwitterion of an amino acid, containing as it does a positive and a negative charge, has no tendency to migrate in an electric field,
• whereas the singly charged anionic and cationic species are attracted to electrodes of opposite charge.
• NH2-CH2-COO- +NH3-CH2-COOH
• No net migration of the amino acid occurs in an electric field when the pH of the solvent is such that [anionic] = [cationic], which is pH dependent.
• The pH at which no net migration occurs is called the isoelectric point; this point is an important physical constant for characterizing amino acids. The isoelectric point is readily related to the ionization constants for the species. Thus, for glycine,
Iso electric point:The pH at which the average charge of the polyprotic acid is zero
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+NH3-CH2-COO-
1-Determining iso electric point for amino acids
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+NH3-CH2-COO- Zwitterion formation
]COO-CH-NH[
]OH [] COO-CH-[NH-
23
3-
22
Ka ]COO-CH-NH[
]OH []COOH-CH-NH[-
23
23
Kb
]COOH-CH-NH[] COO-CH-[NHpoint electric isoIn 23-
22
][
][ 3
OH
OH
K
K
b
a
b
a
K
OHKOH
][][ 3
b
wa
K
KOHKOH
][][ 3
3
b
wa
K
KKOH
23 ][
b
wa
K
KKOH
][ 3
2-Determining iso electric point for amino acids
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+NH3-CH2-COOH +NH3-CH2-COO- NH2-CH2-COO-
pKa1=2.35 Ka2=9.87
221 aa pKpK
pH
Method1=method2
AHAAH bb KK 12
22aK1aK
HAHAAHOH ab KK2
Kb
KwKaOH 3
2
21213
pKapKaKaKaOH
Ka=Ka2,,,,,,,,,,,,Kb=Kb2 1b2
w
K
Ka
b
w KK
K
213 KaKaKb
KwKaOH
For simple amino acids, Ka and Kb are generally so small that their quantitative determination by neutralization titrations is impos sible.
Amino acids that contain more than one carboxyl or amine group can sometimes be determined. If the Ka values are different enough (104 or more), stepwise end points can be obtained just like other polyfunctional acids or bases as long as the Ka values
Formol titration
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+NH3-CH2-COO- + OH- Product
+NH3-CH2-COO- + HCOH CH2=NCH2COOH