The animal-fibre component may consist solely of hair (2 and 3) or wool (1) or of any mixture of the two. This method is not applicable to textile mixtures containing non-fibrous matter (dyes, finishes, etc.) with a nitrogen base.
2. PRINCIPLE
The nitrogen content of the mixture is determined, and from this and the known or assumed nitrogen contents of the two components, the proportion of each component is calculated.
3. APPARATUS AND REAGENTS (other than those specified in the general instructions)
3.1. Apparatus
(a) Kjeldahl digestion flask, 200-300 ml capacity.
(b) Kjeldahl distillation apparatus with steam injection.
(c) Titration apparatus, allowing precision of 0,05 ml.
3.2. Reagents
(a) Toluene.
(b) Methanol.
(c) Sulphuric acid, relative density at 20 °C: 1,84(12).
(d) Potassium sulphate(12).
(e) Selenium dioxide(12).
(f) Sodium hydroxide solution (400 g/litre). Dissolve 400 g of sodium hydroxide in 400-500 ml of water and dilute to 1 litre with water.
(g) Mixed indicator. Dissolve 0,1 g of methyl red in 95 ml of ethanol and 5 ml of water, and mix with 0,5 g of bromocresol green dissolved in 475 ml of ethanol and 25 ml of water.
(h) Boric acid solution. Dissolve 20 g of boric acid in 1 litre of water.
(i) Sulphuric acid, 0,02N (standard volumetric solution).
4. PRE-TREATMENT OF TEST SAMPLE
The following pre-treatment is substituted for the pre-treatment described in the general instructions:
Extract the air-dry laboratory test sample in a Soxhlet apparatus with a mixture of 1 volume of toluene and 3 volumes of methanol for 4 hours at a minimum rate of 5 cycles per hour. Allow the solvent to evaporate from the sample in air, and remove the last traces in an oven at 105 ± 3 °C. Then extract the sample in water (50 ml per g of sample) by boiling under reflux for 30 minutes. Filter, return the sample to the flask, and repeat the extraction with an identical volume of water. Filter, remove excess water from the sample by squeezing, suction, or centrifuging and then allow the sample to become air-dry.
Note:
The toxic effects of toluene and methanol shall be borne in mind and full precautions shall be taken in their use.
5. TEST PROCEDURE
5.1. General instructions
Follow the procedure described in the general instructions as regards the selection, drying and weighing of the specimen.
5.2. Detailed procedure
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