Biomedical Engineering Reference
In-Depth Information
7.2 Plot for Buffer Calculations
Figure 7.4 is the graphical expression of the Henderson-Hassel-
balch equation. The graph may be used for estimation of buffer
composition, if the pK of the buffering substance is known. For
example, the base A
−
of an acetate buffer is the anion CH
3
COO
−
,
theacidHAisCH
3
COOH, and in a Tris buffer, TrisH
+
is the acid HA
andunprotonatedTrisisthebaseA
−
(according to the Brönsted
definition, the acid is that substance which has bound the hydrogen
cation):
A
−
pK +
[
]
=
pH
[
HA
]
Difference between wanted pH and pK
Abscissa
Quotient of concentrations of base and acid (e.g., HPO
4
/H
2
PO
4
)
Ordinate
Tris buffer, pH 8.2, 0.1 M,4
◦
C
i.
wanted:
Examples
=
given:
pK
4
8.51
∆
=
=
solution:
pH
(8.2 − 8.51)
−0.31
∆
=
=
−0.31 in Fig. 7.4 gives [A
−
] : [HA]
The abscissa value
pH
0.48,
i.e., the ratio of Tris base to Tris
·
HClis0.48.
=
=
0.48
(moles Tris − moles HCl) : moles HCl
(0.1 −
x
):
x
=
From this follows:
x
0.0676
To prepare the wanted buffer, dissolve 12.11 g Tris (0.1 mol)in
about 900 ml water, add 67.6 ml 1 N HCl (0.0676 moles), and bring
up to 1000 ml.
Fig. 7.4.
Graph for buffer estimation
