=155. Method Of Krug.=—In conducting the determination of furfurol, according to the method of Chalmont and Tollens just noticed, Krug observed that the filtrate, after standing for some time, yielded a second precipitate of furfurol hydrazone. Great difficulty was also experienced in collecting the precipitate upon the filter on account of the persistency with which it stuck to the sides of the vessel in which the precipitation took place.[129] In order to avoid these two objections, Krug modified the method as described below and this modified method is now exclusively used in this laboratory. After the precipitation of the furfurol hydrazone, it is stirred vigorously, by means of an appropriate mechanical stirrer, for at least half an hour and then allowed to rest for twenty-four hours. On filtering after that length of time the filtrate remains perfectly clear and no further precipitation takes place. After the filtration is complete and the beaker and filtering tube well washed, no attempt is made to detach the part of the filtrate adhering to the beaker but the whole of the precipitate, both that upon the filter and that adhering to the sides of the beaker, is dissolved in strong alcohol, from thirty to forty cubic centimeters being used. The alcoholic solution is collected in a small weighed flask, the alcohol evaporated at a gentle heat and the last traces of water removed by heating to 60° and blowing a current of dry air through the flask. After weighing the precipitate of furfurol hydrazone, obtained as above, the calculation of the weight of pentose bodies is accomplished by means of the usual factors. =156. Precipitation of Furfurol with Pyrogalol.=—Furfurol is thrown out of solution in combination with certain phenol bodies by heating together in an acid solution. Hotter has proposed a method for the determination of furfurol based on the above fact.[130] The furfurol is obtained by distillation in the manner already described and hydrochloric acid is added if necessary to secure twelve per cent of that body in a given volume. The furfurol is thrown out of an aliquot portion by heating with an excess of pyrogalol in closed tubes for about two hours at 110°. The reaction takes place in two stages, represented by the following equations: C₅H₄O₂ + C₆H₆O₃ = C₁₁H₁₀O₅ and 2C₁₁H₁₀O₅ = C₂₂H₁₈O₉ + H₂O. The aliquot part of the distillate used should not contain more than one-tenth of a gram of furfurol. The precipitate formed in this way is collected on an asbestos felt, dried at 103° and weighed. The weight obtained divided by 1.974 gives the corresponding amount of furfurol. There is some difficulty experienced in loosening the precipitate from the sides of the tubes in which the heating takes place, but this defect can be overcome by heating in covered beakers in an autoclave.