GRAVIMETRIC ESTIMATIONS 87 



filter paper next to a drop of aniline acetate solution ; * if no 

 red colour appears when the two liquids come in contact with 

 each other, the solution is free from furfural, and the distillation 

 can be discontinued. 



The furfural contained in the united distillates is then 

 precipitated from solution by means of phloroglucinol which 

 reacts according to the equation : 



C 5 H 4 2 + C 6 H 8 3 = C U H 6 3 + 2H 2 

 go 126 



To this end about the amount of phloroglucinol f likely 

 to be required by the furfural obtained is dissolved in hydro- 

 chloric acid (sp. gr. I -06), and added to the furfural solution, 

 and the total volume is then made up to 400 c.c. with more 

 of the same acid. The solution at once turns yellow, then 

 becomes turbid, and, on the next day, the greenish-black pre- 

 cipitate of the phloroglucide is filtered off on to a tared Gooch 

 crucible; the precipitate is washed with 150 c.c. of water, 

 dried for four hours at 97, then cooled in a desiccator and 

 weighed in a weighing bottle.} From the weight (a) of the 

 precipitate, which under ordinary conditions should lie between 

 0*03 and o - 3 gram, the weight of furfural, pentose or pentosane 

 may be calculated by substituting the value of (a) in one of 

 the following formulae : 



a lies between 0-03 and 0-3 gram. 

 Furfural = (a + '0052) x '5185 

 Pentose = (a + -0052) x 1-0075 

 Pentosane == (a + "0052) x -8866 



in which '0052 is the weight of phloroglucide, which remains 

 in solution under the conditions of the experiment as given 

 above. 



If the precipitate weighs less than 0*03 gram or more 

 than o - 3 gram, one of the following formulae must be em- 

 ployed : 



* This is best prepared, according to Tollens, by shaking up equal volumes 

 of aniline and water in a test tube and adding glacial acetic acid drop by drop 

 until the turbid solution suddenly becomes clear. 



f The phloroglucinol employed must be pure. To ascertain this, test as 

 follows : Dissolve a small quantity in a few drops of acetic anhydride, heat al- 

 most to boiling and add a few drops of concentrated sulphuric acid ; a violet 

 colour indicates the presence of diresorcinol ; if more than a faint coloration 

 appears, the sample should be rejected. 



J This is necessary to prevent the phloroglucide, which is hygroscopic, from 

 absorbing moisture. 



