From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 9

Precautions in acid-base titration are as follows: (1) To prevent the error caused by hydro-
chloric acid degrading the main chain of chitosan, the sample should be dissolved at room
temperature and not at high temperature. (2) The higher the DD, the larger the solubility of
the sample, and vice versa. Hence some samples need to be treated overnight. (3) The
deacetylation is uneven, usually resulting in incomplete dissolution. If data from three
measurements are very different, the sample should be measured again. (4) The sample
must be neutral, or the result may be incorrect. The sample that is not neutral should be
washed to neutral or the data should be corrected. (5) The influence of oxygen in the atmo-
sphere can be ignored, and hence nitrogen protection is not necessary. (6) Obvious agglu-
tination lowering the measured values should be prevented. (7) The color of the colloid
solution with large viscosity changes slowly when the titration end point is near, and hence
the operator should pay attention to the rate of titration.
DD can be measured by the absorption peak of characteristic groups in the infrared
spectrum of chitosan [47]. In this method, it is not necessary to dissolve chitosan by solvent
or solution, which means that the infrared spectrum can be directly obtained by using dry
powder. By using a series of samples with known DDs, a standard curve can be plotted by
using the absorption peak ratio of special bands such as amide I or amide II to a certain
band. The DD of the tested sample can be determined by using the curve. Compared with
acid-base titration, the error is larger in this method. But the infrared spectrum is more
convenient and samples used in the method can be recycled. Dryness is significant to the
repeatability of experimental data. Generally, the amide I peak is hardly affected by water
while the amide II peak is not; hence the sample must be dried carefully. Furthermore, the
sample must be ground into very fine powder, or the absorption peak will not be sharp
enough, making the determination of peak height inaccurate.
Electrolytic titration has the same mechanism as acid-base titration, but is different from
the end point determination method, which means that electrolytic titration uses a poten-
tial curve while acid-base titration uses a single indicator or indicator mixture [34].
Electrolytic titration comprises the following steps: dissolving chitosan in a standard
hydrochloric acid solution in a small beaker, measuring the standard NaOH titration pro-
cess by a potentiometric titrimeter, recording pH when 0.25-0.5 mL of NaOH is used,
recording pH more frequently when the end point is near, plotting a pH- V curve by using
pH as the vertical coordinate and the volume of NaOH as the horizontal ordinate, finding
the volume of NaOH corresponding to the equivalent point, and calculating DD by the
given formula.
There are still some disadvantages: (1) Chitosan that separates out before and after the
end point may cover the electrode film, influencing the measurement of pH. (2) It is differ-
ent to find out the equivalent point on the titration curve, which is usually shaped as S,
causing personal error in the determination of the end point.
1.3.2 Molecular Weight
Relative molecular weights of chitin and chitosan can be measured by gel permeation
chromatography [48], steam osmotic pressure method, membrane osmometry, end group
method [49], viscosity measurement, light scattering method, and coupled light scattering-
gel permeation chromatography. Gel permeation chromatography is applied for measur-
ing weight-average relative molecular weight and number-average relative molecular
weight. The light scattering method is applied for measuring weight-average relative
molecular weight. The viscosity measurement is applied for measuring viscosity-average
relative molecular weight.