CHAPTER 5
EQUILIBRIA AND REACTIONS INVOLVING PROTONS
107
14
2
3
4
5
6
7
8
9
10
11
12
13
1
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
pH
OH
Ϫ added (equivalents)
NH4
ϩ
NH3
H2PO4
Ϫ
HPO4
2Ϫ
CH3COOH
CH3COO
Ϫ
Midpoint of titration
pK A ϭ 9.25
pK A ϭ 6.86
pK A ϭ 4.76
[NH4
ϩ
] ϭ [NH3]
[H2PO4
2 ] ϭ [HPO4
2Ϫ
]
[CH3COOH] ϭ [CH3COO
Ϫ
]
Table 5.2
A partial listing of some common buffers and their pK A values at 20°C.
Buffer
pK A
2-N-(morpholino)Ethanesulfonic acid (MES)
6.15
Piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES)
6.76
Imidazole
7.00
3-(N-morpholino)Propanesulfonic acid (MOPS)
7.20
Phosphate*
7.21
N-2-Hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES)
7.55
Tris(hydroxymethyl)aminomethane
8.08
*Phosphate buffers are normally prepared from a combination of monobasic and
dibasic salts mixed together in proportion to yield the desired pH. Phosphoric acid
has three pK A values: 2.12, 6.86, and 12.32.
Figure 5.8 The titration curves for acetic acid, phosphate, and ammonia.
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