The titration is conducted as follows: Twenty-five cubic centimeters of the distillate obtained from a pentose body, by the method described above, are diluted with an equal volume of water, a certain quantity of the phenylhydrazin solution added to the mixture from a burette and the whole heated quickly to boiling. The flask is rapidly cooled and a portion of its contents poured on a filter. The filtrate should have a pale yellow color and be perfectly clear. If it become turbid on standing, it should be refiltered. Two cubic centimeters of the clear filtrate are boiled with double the quantity of the fehling reagent. If phenylhydrazin be present, the color of the mixture will change from blue to green. By repeating the work, with varying quantities of phenylhydrazin, a point will soon be reached showing the end of the reaction in a manner entirely analogous to that observed in volumetric sugar analysis. In practice the volumetric methods have given place to the more exact gravimetric methods described below. =154. Gravimetric Methods.=—The distillation is carried on and the volume of the distillate completed to half a liter as described above. Chalmot and Tollens then proceed as follows:[128] Ten cubic centimeters of a solution of phenylhydrazin acetate, containing in 100 cubic centimeters twelve grams of the phenylhydrazin and seven and a half grams of glacial acetic acid dissolved and filtered, are added to the distillate and the mixture stirred with an appropriate mechanism for half an hour. The furfurolhydrazone at the end of this time will have separated as small reddish-brown crystals. The mixture is then thrown onto an asbestos filter and the liquid separated with suction. The suction should be very gradually applied so as not to clog the felt. The precipitate adhering to the beaker is washed into the filter with 100 cubic centimeters of water. The precipitate is dried at about 60° and weighed. As a check the hydrazone may be dissolved in hot alcohol, the filter well washed, dried and again weighed. To obtain the weight of furfurol the weight of hydrazone found is multiplied by 0.516 and 0.025 added to compensate for the amount which was held in solution or removed by washing. Less than one per cent of pentose can not be determined by this method since that amount is equalled by the known losses during the manipulation. _Factor._—To convert the furfurol found into pentoses, the following factors are used: Per cent furfurol Multiply for obtained from Multiply for Multiply for penta-glucoses five grams of arabinose by. xylose by. by. pentoses. 2.5 per cent or less 1.90 1.70 1.67 5.0 ” ” ” more 2.04 1.90 1.92