Method 78a Determination of oil in feeding stuffs by extraction with petroleum spirit

Reagents.

• Petroleum spirit, boiling range 40-60°C

Procedure. The sample should not be oven dried, but may be dried over concentrated sulphuric acid or dried at 95-100°C at ^100 mmHg pressure. A separate sample may be taken for moisture determination. Weigh accurately to ±0.001 g approximately 3-5 g of the sample, ground to 1 mm, into a double thickness extraction thimble, size 22 x 80 mm, and plug the thimble with a wad of oil-free cotton wool. Place a dried (102°C for 1 h) and weighed 250-ml flat-bottom short-neck flask with ground glass 34/35 socket into a recess on the Soxhlet heating unit, and attach the 60 ml size Soxhlet extractor with size 34/35 cone and socket. Place the thimble plus sample into the extractor and pour in sufficient petroleum spirit (40-60°C) so that it just siphons into the flask; then half fill with more solvent and attach the Graham coil reflux condenser with cone size 34/35 (if it is to be left overnight, plug the condenser vents with oil-free cotton wool). Ensure a steady flow of cold water from the condenser, turn on the heater and adjust the control to give a reflux rate of approximately 10 changes per hour; continue refluxing for at least 6 h. Turn the heater off just before the extractor is full, and when it has siphoned out, remove the thimble. Replace the extractor and resume distillation until about 10 ml petroleum spirit-oil solution remains in the flask. The solvent may need to be emptied a couple of times from the reservoir into a waste bottle for redistillation. Turn off the heater, remove the flask and place on a boiling water bath in a fume cupboard to remove the remaining solvent. When no smell of solvent can be detected, place the flask in an oven at 102°C for 2 h, cool and weigh.

Calculation. Subtract the weight of the empty flask from the weight of the flask plus residue of oil, multiply by 1000 and divide by the sample weight to obtain the g kg-1 of oil in the sample. Multiply by 100 instead of 1000 to obtain the percentage oil. If required, use the moisture determination to correct to g kg-1 oil in DM. Explanation: weight of oil divided by sample weight is the number of grams of oil per gram of sample. Therefore, multiplying by 1000 gives the g kg-1 oil in the sample.

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