5. _Determination of Acidity._--Take 10 grams of the sample, dilute with 50 cc. distilled water free from carbon dioxide, and titrate with _N_/1 NaOH and phenolphthalein. Express in terms of Na_{2}O required to neutralize 100 grams. 6. _Determination of Total Residue at 160deg. C._--For this determination the crude glycerine should be slightly alkaline with Na_{2}CO_{3} not exceeding 0.2 per cent. Na_{2}O, in order to prevent loss of organic acids. To avoid the formation of polyglycerols this alkalinity must not be exceeded. Ten grams of the sample are put into a 100 cc. flask, diluted with water and the calculated quantity of _N_/1 HCl or Na_{2}CO_{3} added to give the required degree of alkalinity. The flask is filled to 100 cc., the contents mixed, and 10 cc. measured into a weighed Petrie or similar dish 2.5 in. in diameter and 0.5 in. deep, which should have a flat bottom. In the case of crude glycerine abnormally high in organic residue a smaller amount should be taken, so that the weight of the organic residue does not materially exceed 30 to 40 milligrams. The dish is placed on a water bath (the top of the 160deg. oven acts equally well) until most of the water has evaporated. From this point the evaporation is effected in the oven. Satisfactory results are obtained in an oven[20] measuring 12 ins. cube, having an iron plate 0.75 in. thick lying on the bottom to distribute the heat. Strips of asbestos millboard are placed on a shelf half way up the oven. On these strips the dish containing the glycerine is placed. If the temperature of the oven has been adjusted to 160deg. C. with the door closed, a temperature of 130deg. to 140deg. can be readily maintained with the door partially open, and the glycerine, or most of it, should be evaporated off at this temperature. When only a slight vapor is seen to come off, the dish is removed and allowed to cool. An addition of 0.5 to 1.0 cc. of water is made, and by a rotary motion the residue brought wholly or nearly into solution. The dish is then allowed to remain on a water bath or top of the oven until the excess water has evaporated and the residue is in such a condition that on returning to the oven at 160deg. C. it will not spurt. The time taken up to this point cannot be given definitely, nor is it important. Usually two or three hours are required. From this point, however, the schedule of time must be strictly adhered to. The dish is allowed to remain in the oven, the temperature of which is carefully maintained at 160deg. C. for one hour, when it is removed, cooled, the residue treated with water, and the water evaporated as before. The residue is then subjected to a second baking of one hour, after which the dish is allowed to cool in a desiccator over sulphuric acid and weighed. The treatment with water, etc., is repeated until a constant loss of 1 to 1.5 mg. per hour is obtained.