Biomedical Engineering Reference
In-Depth Information
acetate buffer, pH 4.96, 0.1 M,20
◦
C
ii. wanted:
=
given:
pK
20
◦
4.76
∆
=
=
solution:
pH
(4.96 − 4.76)
0.2
For abscissa 0.2, the ordinate [A
−
] : [HA] is 1.6.
If acetic acid and sodium hydroxide solution are used for prepa-
ration of the buffer, the amounts are calculated as follows:
=
1.6
(moles NaOH) : (moles CH
3
COOH − moles NaOH)
=
x
: (0.1 −
x
)
=
From this follows:
x
0.061
/
/
Mix 0.1 mol
l acetic acid with 0.061 mol
l sodium hydroxide to
get the expected buffer.
If using acetic acid and sodium acetate, the following results
are obtained:
=
1.6
(moles Na acetate) : (moles acetic acid)
and
=
0.1
(moles Na acetate) + (moles acetic acid) .
Per liter buffer, 0.0615 mol sodium acetate and 0.0385 mol acetic
acid are needed.
7.3 pH Indicators
Solutions of indicators are out for pH measuring, but they are useful
for monitoring some processes, such as tissue cultures (indicator:
phenol red) or electrophoresis (electrophoresis front and pH of
sample; indicator: bromophenol blue).
Figure 7.5 gives the transition intervals and colors of some
selected pH indicators. Because the application of theses indicators
in the context of the biochemical protocol is not volumetric analysis,
the concentration of stock solution is mostly 0.1% (w/v) in ethanol
or propanol, and the final dilution is 100-fold lower.
For application of the indicators in volumetric analysis and of
redox and fluorescent indicators, check specialized literature.
References
Covington AK (2001) In: Lide DR (ed.) CRC Handbook of chemistry and
physics, 82nd ed. CRC Press, Boca Raton, p 8-16
7.4 Buffer Recipes
The selection of buffer depends on several factors:
-
The first aspect is the pH range that shall be buffered. The
buffer capacity has to be large enough to catch all H
+
and OH
−
ions liberated during the experiment; therefore, the pK of the
