BioPHYSICAL chemistry

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.