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5.8 UNSAPONIFIABLE MATTER

 ​           The term “unsaponifiable matter” refers to those substances present in oils or fats that are not saponified by alkali hydroxides and are extractable into ether. The unsaponifiable matter is the percentage content, w/w, of the extracted material not volatile at 100º to 105º.

 ​           Use Method I unless otherwise indicated in the monograph. Use ungreased ground-glass glassware for each method.

Method I

 ​           To 2.0 to 2.5 g of the substance being examined in a 250-ml flask add 25 ml of 0.5 M ethanolic potassium hydroxide and boil under a reflux condenser on a waterbath for 1 hour, swirling frequently. Cool to below 25º, transfer the contents of the flask into a separator with the aid of 50 ml of water and extract carefully by shaking vigorously with three 50-ml portions of peroxide-free ether, rinsing the flask with the first portion of ether. Combine the ether extracts in another separator containing 20 ml of water. (If the ether solutions contain solid suspended matter, filter them into the separator through a fat-free filter paper and wash the filter paper with peroxide-free ether.) Gently rotate the separator for a few minutes without violent shaking, allow the liquids to separate and discard the aqueous layer. Wash the ether solution by shaking vigorously with two 20-ml portions of water and then treat with three 20-ml portions of 0.5 M potassium hydroxide, shaking vigorously on each occasion, each treatment being followed by washing with 20 ml of water. Finally wash with successive 20-ml portions of water until the aqueous layer is no longer alkaline to phenolphthalein TS. Transfer the ether extract to a weighed flask, rinsing the separator with peroxidefree ether, distil off the ether and add 3 ml of acetone to the flask. With the aid of a gentle current of air, remove the solvent completely from the flask, which is almost entirely immersed in boiling water and preferably held obliquely and rotated. Dry to constant weight at a temperature not exceeding 80º. Allow to cool in a desiccator and weigh.

 ​           Dissolve the residue in 10 ml of freshly boiled ethanol, previously neutralized to phenolphthalein TS, and titrate with 0.1 M ethanolic sodium hydroxide VS.

 ​           If the volume of 0.1 M ethanolic sodium hydroxide required does not exceed 0.1 ml, the amount of residue weighed is to be taken as the unsaponifiable matter. Calculate the unsaponifiable matter as a percentage of the substance being examined.

 ​           If the volume of 0.1 M ethanolic sodium hydroxide required exceeds 0.1 ml, the amount of residue weighed cannot be taken as the unsaponifiable matter and the test must be repeated.

Method II

 ​           Add 50 ml of 2 M ethanolic potassium hydroxide to the prescribed portion of the substance being examined in a 250-ml flask and boil under a reflux condenser on a water-bath for 1 hour, swirling frequently. Cool to below 25º, transfer the contents of the flask into a separator with the aid of 100 ml of water and extract carefully with three 100-ml portions of peroxide-free ether. Combine the ether extracts in another separator containing 40 ml of water, shake gently for a few minutes, allow to separate and discard the aqueous layer. Wash the ether extract with two 40-ml portions of water and then with three 40-ml portions of a 3 per cent w/v solution of potassium hydroxide, each treatment being followed by washing with 40 ml of water. Finally wash the ether extract with successive 40-ml portions of water until the aqueous layer is no longer alkaline to phenolphthalein TS. Transfer the ether extract to a weighed flask, washing out the separator with peroxide-free ether. Distil off the ether with suitable precautions and add 6 ml of acetone to the residue. Remove the solvent in a current of air and dry to constant weight at 100º to 105º. Allow to cool in a desiccator and weigh.

 ​           Dissolve the residue in 20 ml of ethanol, previously neutralized to phenolphthalein TS, and titrate with 0.1 M ethanolic sodium hydroxide VS.

 ​           If the volume of 0.1 M ethanolic sodium hydroxide required exceeds 0.2 ml, the separation of the layers has been incomplete; the amount of residue weighed cannot be taken as the unsaponifiable matter and the test must be repeated.