Biomedical Engineering Reference
In-Depth Information
Table 7.2.
(continued)
Substance
pK
dpK/dT
M
r
CAPS (3-(cyclohexylamino)-1-
propanesulfonic acid)
10.40
0.032
221.3
Ethylamine
10.62
45.1
Triethylamine
10.72
101.2
Piperidine
11.12
−0.031
85.2
Phosphate (phosphoric acid) (pK
3
)
12.33
−0.026
98.0
Important:
Consider concentration and temperature dependency
of buffering substances when you prepare and use a buffered solu-
tion.
The capability to stabilize a given pH by a buffer depends on its
capacity. Figure 7.3 shows as an example for Tris buffer, that the
buffer capacity
β
(amount of acid or base which causes only a little
change in pH) is highest in the range of
0.5 units around the pK
and that e.g. at one pH unit distant from pK addition of a small
amount of acid or base causes a huge change in pH.
Figure 7.2 illustrates that the ability of a buffering substance
(this example: 0.1 M Tris) is highest at pH
±
=
pK and decreases with
distance to pK.
The dissociation constant of a weak acid or base is altered by
ionic strength, too. Ionic strength I is a measure for total content
of ions within a solution and depends on concentration as well as
on charge of ions:
n
1
2
·
=
z
i
)
I
(c
i
·
=
i
1
Fig. 7.1.
Change of pHof some 0.1-Mbuffers depending on temperature
