Biomedical Engineering Reference
In-Depth Information
to measure the phenolic hydroxyl content, a sample is treated by dimethyl sulfate
for ethylation first, then the ethane content is measured, which could be converted
into phenolic hydroxyl content. In addition, the lignin phenolic hydroxyl content
can also be measured using UV spectroscopy at a wavelength of 300 or 250 nm.
Phenolic hydroxyl groups and alcoholic hydroxyl groups in the lignin are called the
total hydroxyl content. Usually, the total hydroxyl content and phenolic hydroxyl
content in the lignin are measured by a chemical method, and then the alcoholic
hydroxyl content is recalculated. For determination of the phenolic hydroxyl group,
the periodate oxidation method and nonhydroderivative titration method are widely
used. The nonaqueous conductometric titration method can measure the carboxyl
content at the same time.
11.1.4.3
Carboxyl Group Content
The carboxyl group mainly exists in the hemicellulose. The carboxyl group can
be measured by a variety of methods, such as the direct alkali titration method,
exchange reaction with phosphorus-nitrophenol silver salt, reaction with methylene
blue, sodium bicarbonate-sodium chloride method, conductometric titration, and
dynamic ion exchange method. The dynamic calcium acetate ion exchange method
(GB/T 10338-1989 of China national standard method) and sodium bicarbonate-
sodium chloride method are widely applied.
11.1.4.4
Simultaneous Measurement Methods for Carboxyl Group
and Phenolic Hydroxyl Group
The nonaqueous titration method could determine the carboxyl group and phenolic
hydroxyl group contents in lignocellulosic material simultaneously and includes
the nonaqueous conductometric titration method and nonhydropower titration
method. The nonaqueous titration method uses the principle of the nonaqueous
component to titrate weak organic acids. Water is a strongly polar solvent, and
weak organic acids express low intensity in water. Because of homogenization
of water, the intensity difference between water and weak organic acid is small.
Therefore, it is difficult to analyze organic weak acid in water. However, in
organic solvent, the acidic property of some weak acid is greatly strengthened.
So, the difference of acid property between week acid and organic solvent is
significantly increased. Therefore, it could be titrated by strongly organic alkali.
The lignin carboxyl group and phenolic hydroxyl are acidic groups with weak
acidity. So, the electrical conduction titration or potentiometer titration method
could be used in solvent of organic medium with weak catalyzing ability. According
to system conductivity or potential change before and after titration, the titra-
tion end point can be obtained, thereby calculating the content of these acidic
groups.
