Chemistry Reference
In-Depth Information
potentiometrically. This gives a titration curve with two inflection points, the
difference between the two along the
x
abscissa (
a b
) corresponding to the
amount of acid required to protonate the amino group, and thus, the amino group
concentration is determined according to the following equation:
:
ða bÞF
W
6
1
NH
2
¼
%
where
F
is the molarity of NaOH solution and
W
is the weight of the sample in
grams.
This technique has been used by a number of workers, but its precision has been
questioned by Domard and Rinaudo [
24
] because of the tendency toward precipita-
tion of the Ch in the neutralization pH range. These workers reported an improved
technique in which the Ch is progressively titrated with HCl, while the conductivity
is monitored to determine the stoichiometry of the interaction. This procedure gave
an equivalent weight of 16
1 for the amino group of the Ch sample. Prior to this,
Moore [
25
] examined a back-titration method in which a sample is steeped in a
known excess of 0.5 M HCl, in which Ch is insoluble, followed by titration of
aliquots of the supernatant liquor.
4.3.1.2 Colloidal Titration Method
The colloidal titration technique was developed by Hirai et al. [
26
] as a method for
analyzing polyelectrolyte. Ch was dissolved in an aqueous solution of acetic acid
and titrated with polyvinyl potassium sulfate solution using toluidine blue as an
indicator. The end point is determined by the color change from blue to reddish
purple.
4.4 Chemical Modifications of Chitosan
Ch is a versatile molecule, which contains, besides the primary and secondary
hydroxyl groups, the highly active amino group at C
2
. This in addition to a slight
ratio of the remaining non-hydrolyzed acetamido group. Due to the high activity of
functional groups present in Ch, it can be easily subjected to chemical modifications
without disturbing its degree of polymerization [
27
]. Figure
4.2
represents the
various functional groups in Ch.
The chemical modifications are designed to change solid viscosity gelling
tendency, hydrophilic character, affinity toward metals and dyes, and moisture
content and to introduce ionic character. The modifications may involve controlled
degradation of Ch chains or the introduction of chemical groups not normally
present.
