You can not determine contents of hemicelluloses in wood and related materials directly.
1. First of all isolate and determine content of holocellulose in the material analysed Use the method of Clauditz or of Wisse /the former is more convenient/. Extractives free wood meal /0.5 - 1.0 mm/ should be used for isolation and determination of holocellulose.
2. Determine content of residual lignin in holocellulose preparation /use acetyl-bromide spectroscopic method/, not gravimetric one!/.
3. Detemine alpha- cellulose content in your holocellulose preparation /submit holocellulose to extractions with 17.5 % NaOH solution/. The washed solid residuum after exrations is alpha-cellulose.
4.In the filtrate are beta- and gamma cellulose. You can obtain beta-cellulose as precipitate after filtrate acidification. Gamma-cellulose can be detrmined via oxidimetric titration of the filtrate with K2Cr2O7.
Determination of alpha-, beta- and gamma- cell. is described in TAPPI or ASTM standards.
5. Contents of hemicelluloses in holocellulose is determined as /mass of holocellulose corrected for residual lignin/ - mass of /alpha- plus beta- and gamma cellulose/.
The portion of hemicelluloses in holocellulose should be recalculated and expressed on the initial mass of extractives free wood or the initial mass of wood.
Best regards,
R.S.
I am sorry, the first answer concerns more determination of hemi. in various pulps. than hemi. in wood and plants. The following method is similar, simpler and more suitable for erstimation of hemi. contents.
1. Prepare holocllusose and determine contens of ash and res. lignin in the preparation.
2. Extract holocellulose with 5% KOH for 2h at temp. of 20oC /hydromodul:1g of holo. to 60 ml of hydroxide H2O solution. The procedure is carried out in N2 atmosphere
with tiny stream of gas passing through the suspension.You will obtain solution of hemicelluoses A fraction /first filtrate/
3. Separate solid particles of holo. using a S3 glas funnel and vacuum. Transfer the solid into the extraction flask and add 60 ml of 24 % KOH solution in water. Let the extration proceed for 24 h in N2 atmosphere again. Separate solid residuum /alpha-cellulose/, wash it with H2O, than with acetic acid acidified water , with hot water and acetone. Dry the solid at 102 oC and weigh.The filtrate contains fraction of hemi. B.
Then you may determine hemicelluloses in two ways:
a/ Directly. Neutralise both extracts with acetic acid and precipitate hemi. /fractions A and B/ purring them into 3 triple volume of ethanol containing 0.1 % of HCl, centrifuge precipitates. Wash precipitates with diethyl ether, dry at 102oC and weigh.
b/ Indirectly from weight of dry residuum after extractions.
In both ways /A and B/ express the contents of hemi in % of the initial weight of holocellulose preparation reduced for ash and res. lignin contents.
Regards,
R. S.
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