acetic acid CH 3 COOH pKa 48 10 08 06 02 04 OH equivalents pH A H A H A HA 1 1000 pH 18 pK a 30 Titration of a weak acid acetic acid CH ID: 594953
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Slide1
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
A
H
A
-
H
+
[A
-
]
: [HA] = 1 : 1000
pH:
1.8
=
pK
a
–3.0Slide2
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
2.8
=
pK
a
–2.0
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 100
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide3
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
3.8
=
pK
a
–1.0
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 10
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide4
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
4.2
=
pK
a
–0.6
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 4
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide5
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
4.4
=
pK
a
–0.4
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 2.3
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide6
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
4.6
=
pK
a
–0.2
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 1.5
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide7
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
4.8
=
pK
a
A
H
A
-
H
+
[A
-
] : [HA] = 1 : 1
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide8
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
5.8
= pK
a
+1.0
A
H
A
-
H
+
[A
-
] : [HA] = 10: 1
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide9
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
6.8
= pK
a
+2.0
A
H
A
-
H
+
[A
-
] : [HA] = 100: 1
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide10
Titration of a weak acidacetic acid, CH3COOH, pKa = 4.8
1.0
0.8
0.6
0.2
0.4
OH
-
equivalents
pH:
7.8
= pK
a
+3.0
A
H
A
-
H
+
[A
-
] : [HA] = 1000: 1
OH
-
+ HA
H
2
O + A
-
OH
-
+ H
+
H
2
OSlide11
~80% of acid dissociation occurs within1 pH unit of pKa
OH- eq. added
pH
= pKa + ___
[A-] : [HA]0.01.8-3.0
1:1000~0.012.8
-2.01:1000.13.8
-1.0
1:100.24.2
-0.61:40.34.4
-0.41:2.30.4
4.6-0.21:1.5
0.54.80.01:1
0.65.00.2
1.5:10.75.2
0.42.3:10.8
5.40.64:1
0.95.81.0
10:1~0.996.8
2.0100:11.0
7.83.01000:1Slide12
In this buffering region (±1 pH unit of
pKa
), dissociation of the weak acid slows the rise in pHSlide13
Titration of other acids gives the same curve, just shifted along the pH axis (based on
pKa
)Slide14
Some weak acids (like amino acids) are
polyprotic
and can buffer over multiple pH ranges
With 2 acidic groups, there are 3 charge states
Isoelectric point (
pI):pH of highest occurrence of zero charge stateand
pH at which average charge on molecules is zeroFor simple cases,pI = (pKai
+ pKaj
)/2Slide15
With 3 acidic groups, there are 4 charge states
The amino acid glutamate has an acidic R group
To calculate
pI
, use
pKa’s that relate to neutral charge state
Neutral charge state predominates in this region of titration
pI
= (pK
1 + pKR)/2= 3.22