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F
A
refers to the fraction of N-acetylated residues, i.e. neutral sugars not
involved in acid-base titration (see Section 1.3 for chitosan chemistry). At pH
3, where all remaining GlcN residues are positively charged, we clearly see
the electrophoretic mobility decreases as F
A
increases. Roughly, the mobility
at F
A
= 0.49 is about half of that at F
A
= 0.01 which is reasonable as the
charge density of the latter is twice the former.
The pK
a
value is in this case taken as the midpoint of the curve since 50% of
electrophoretic mobility corresponds to 50% ionization. The figure shows that
pK
A
is the same for all chitosans at an ionic strength of 0.1 M, namely 6.5.
Can you predict the situation at a much lower ionic strength?
Below is titration curve for chitosan on the basis of NMR data combined with
pH measurements: