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The figure above illustrates the effect for the alginate case by providing
intrinsic viscosities for a range of alginates with different molecular weights, at
three ionic strengths.
Note that a high molecular weight alginate (M
w
= 2.7 mill Da) has more than a
threefold decrease in intrinsic viscosity when going from 0.01 M to 1 M ionic
strength. The change decreases, however, when M
w
decreases: The effect is
largest for the largest molecules.
3.1.6. Charge manipulation: pH and acid-‐base
titration – basic
concepts
The alginate and chitosan examples give above demonstrate that we can use
pH titration to turn charges on and off, and generally manipulate the charge
density or profile of all polyelectrolytes. This may also explain the extensive
use of different buffers, especially in protein chemistry. The purpose of using
a high or low pH buffer is often to adjust the amount and +/- balance of
charges.
The fundamentals of acid-base titration, the concepts of pK
a
and the degree of
dissociation (
α
), are all well covered in Chapters 3.1.3 and 3.2.1 of the
Biopolymer Chemistry textbook. Here, only the main points are given.
Biopolymers may contain one or several ‘titratable’ functional groups, the two
most important being:
Carboxyl: -COOH = -COO
-
+ H
+
Primary amino: - -NH
3
+
= -NH
2
+ H
+
The key parameter to control is the degree of dissociation (
α
), defined as the
fraction of acidic groups being dissociated. If 30 % of the -COOH groups in
alginate, or 30% of the -NH
3
+
groups in chitosan, are dissociated to the
corresponding base form, then
α
= 0.3.
The key equations for an acid-base equilibrium involving a weak acid (HA),
including the relationships between the degree of dissociation (
α
), pH and pK
a
(the Henderson-Hasselbach equation) are:
HA = H
+
+
A
−
(Dissociation of a weak acid)
HA
[ ]
0
=
Initial concentration of acid (M)
K
a
=
H
+
⎡⎣ ⎤⎦
A
−
⎡⎣ ⎤⎦
HA
[ ]
=
H
+
⎡⎣ ⎤⎦
A
−
⎡⎣ ⎤⎦
HA
[ ]
0
−
A
−
⎡⎣ ⎤⎦
(dissociation contant)
α
=
Def
A
−
⎡⎣ ⎤⎦
HA
[ ]
0
Degree of dissociation
(
)
pH
=
pK
a
+
α
1
−
α
Henderson-Hasselbach equation
(
)