Biomedical Engineering Reference
In-Depth Information
11.1.3.13
Research Methods for Tannin Content
Tannin measurement methods include a spectrophotometric method and a volumet-
ric method [
5
]. The spectrophotometric method is based on the property that tannin
is not dissolved in benzene but is dissolved in ethanol. A sample is first extracted
with benzene to remove organic solvent and then is treated with ethanol to dissolve
tannin. A spectrophotometer is used to determine the optical density (OD) at a
wavelength of 500 nm. The content of tannin can be obtained from a standard curve.
11.1.4
Research Methods for the Function Group Content
in Primary Refining [
5
]
11.1.4.1
Methoxy Group Content
The methoxy group mainly exists in lignin and less in cellulose and pectin. The
methoxy group can be studied by chemical methods (gravimetric and bulk density
methods) and instrument analysis methods (chromatography, nuclear magnetic
resonance [NMR] spectroscopy, and electrochemical method). For the chemical
method, the basic principle is to make the sample react with concentrated hydrogen
iodide acid, leading to a methoxy group in lignin pyrolysis. Alkyl iodide is generated
and removed by distillation. Then, different absorbent is used to absorb alkyl iodide
for determination of the content of the generated compound. In this way, the content
of the methoxy group is determined. According to different absorbents, there are the
silver nitrate method, bromide method, nitrogen benzene method, and so on. The
silver nitrate method is a gravimetric method. The pyridine and bromide methods
are volumetric methods. The bromide method measures methoxy group content by
taking bromine as an absorbent. The bromide method is also known as the WeiBoke
method, which has the advantages of higher accuracy, simple operation, rapid rate,
and so on. Therefore, the bromide method is widely applied.
11.1.4.2
Hydroxyl Group Content
There are many groups in lignin, cellulose, and hemicellulose. There are two kinds
of hydroxyl groups in lignin. One is the phenolic hydroxyl groups that exist in
the structural unit of the phenyl ring. Another is aliphatic hydroxyl groups that
exist on the side chain (alcoholic hydroxyl groups). The hydroxyl content can be
determined by chemical and instrumental analysis methods. Instrumental analysis
methods include NMR spectroscopy, GC, UV spectroscopy, or the electrochemical
method. For the H-NMR spectral data, 2.50-2.17 mg/kg represents a phenolic
hydroxy, and 2.17-1.70 mg/kg represents alcoholic hydroxyl groups. If GC is used
