One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c. of a 25 per cent. caustic soda solution added, together with 50 c.c. of redistilled methylated spirit. The whole is stirred down on the water bath until a pasty soap is obtained, when another 50 c.c. of methylated spirit is added, which redissolves the soap, and the whole again stirred down to a solid soap. This is then dissolved in distilled water, a slight excess of dilute sulphuric acid added to liberate the fatty acids, and the whole warmed until the fatty acids form a clear liquid on the surface. The water beneath the fatty acids is then syphoned off, more distilled water added to wash out any trace of mineral acid remaining, and again syphoned off, this process being repeated until the washings are no longer acid to litmus paper, when the fatty acids are poured on to a dry filter paper, which is inserted in a funnel resting on a beaker, and the latter placed on the water-bath, where it is left until the clear fatty acids have filtered through.
About 10-15 grammes of the pure fatty acids are now transferred to a test tube, 6" × 1", warmed until molten, and the tube introduced through a hole in the cork into a flask or wide-mouthed bottle. A very accurate thermometer, graduated into fifths of a degree Centigrade (previously standardised), is immersed in the fatty acids, so that the bulb is as near the centre as possible, and when the fatty acids just begin to solidify at the bottom of the tube, the thermometer is stirred round slowly. The mercury will descend, and stirring is continued until it ceases to fall further, at which point the thermometer is very carefully observed. It will be found that the temperature will rise rapidly and finally remain stationary for a short time, after which it will again begin to drop until the temperature of the room is reached. The maximum point to which the temperature rises is known as the "titre" of the sample.
Alkalies and Alkali Salts.
Care should be bestowed upon the sampling of solid caustic soda or potash as the impurities during the solidification always accumulate in the centre of the drum, and an excess of that portion must be avoided or the sample will not be sufficiently representative. The sampling should be performed expeditiously to prevent carbonating, and portions placed in a stoppered bottle. The whole should be slightly broken in a mortar, and bright crystalline portions taken for analysis, using a stoppered weighing bottle.
Caustic Soda and Caustic Potash.—These substances are valued according to the alkali present in the form of caustic (hydrate) and carbonate.
About 2 grammes of the sample are dissolved in 50 c.c. distilled water, and titrated with N/1 sulphuric acid, using phenol-phthalein as indicator, the alkalinity so obtained representing all the caustic alkali and one-half the carbonate, which latter is converted into bicarbonate. One c.c. N/1 acid = 0.031 gramme Na2O or 0.040 gramme NaOH and 0.047 gramme K2O, or 0.056 gramme KOH.
After this first titration, the second half of the carbonate may be determined in one of two ways, either:—
(1) By adding from 3-5 c.c. of N/10 acid, and well boiling for five minutes to expel carbonic-acid gas, after which the excess of acid is titrated with N/10 soda solution; or
(2) After adding two drops of methyl orange solution, N/10 acid is run in until the solution acquires a faint pink tint.
In the calculation of the caustic alkali, the number of c.c. of acid required in the second titration, divided by 10, is subtracted from that used in the first, and this difference multiplied by 0.031, or 0.047 gives the amount of Na2O or K2O respectively in the weight of sample taken, whence the percentage may be readily calculated.