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 added (equivalents)
NH 4
NH 3
H 2 PO 4
HPO 42
CH 3 COOH
CH 3 COO
Midpoint of titration
pKA 9.25
pKA 6.86
pKA 4.76
[NH 4 ] [NH 3 ]
[H 2 PO 42 ] [HPO 42 ]
[CH 3 COOH] [CH 3 COO]
Table 5.2
A partial listing of some common buffers and their pKAvalues at 20°C.
Buffer pKA
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 pKAvalues: 2.12, 6.86, and 12.32.
Figure 5.8The titration curves for acetic acid, phosphate, and ammonia.