54 METHODS OF ANALYSIS. 



per cent alcohol and then with a little strong alcohol. Place the residue in a 

 beaker with 50 cc of water, immerse the beaker in a boiling water bath, and 

 stir constantly for fifteen minutes or until all the starch is gelatinized; cool 

 to 55 C., add 20 cc of malt extract, and maintain at this temperature for an 

 hour. Heat again to boiling for a few minutes, cool to 55 C., add 20 cc of 

 malt extract, and maintain at this temperature for one hour or until a micro- 

 scopic examination of the residue with iodin shows no starch. Cool and make 

 up directly to 250 cc ; filter. Place 200 cc of the filtrate in a flask with 20 cc of 

 hydrochloric acid (sp. gr., 1.125) ; connect with a reflux condenser and heat in a 

 boiling water bath for two and one-half hours. Cool, nearly neutralize with 

 sodium hydroxid, and make up to 500 cc. Mix the solution well, pour through a 

 dry filter, and determine the dextrose in an aliquot as directed on page 49. The 

 weight of the dextrose obtained multiplied by 0.90 gives the weight of starch. 



9. Pentosans. Provisional, 

 (a) PREPARATION OF REAGENTS. 



(1) QUALITATIVE TEST OF THE PURITY OF THE PHLOROGLUCOL. 



Dissolve a small quantity of the phloroglucol in a few drops of acetic anhy- 

 drid, heat almost to boiling, and add a few drops of concentrated sulphuric 

 acid. A violet color indicates the presence of diresorcol. A phloroglucol which 

 gives more than a faint coloration may be purified by the following method : 



(2) PURIFICATION OF PHLOROGLUCOL. 



Heat in a beaker about 300 cc of hydrochloric acid (specific gravity, 1.06) and 

 11 grams of commercial phloroglucol, added in small quantities at a time, stirring 

 constantly until it has almost entirely dissolved. Some impurities may resist 

 solution, but it is unnecessary to dissolve them. Pour the hot solution into a 

 sufficient quantity of the same hydrochloric acid (cold) to make the volume 

 1,500 cc. Allow it to stand at least overnight better several days to allow 

 the diresorcol to crystallize out, and filter immediately before using. The 

 solution may turn yellow, but this does not interfere with its usefulness. In 

 using it, add the volume containing the required amount to the distillate. 



(b) DETERMINATION. 



Place a quantity of the material, chosen so that the weight of phloroglucid 

 obtained shall not exceed 0.300 gram, in a flask, together with 100 cc of 12 per 

 cent hydrochloric acid (specific gravity, 1.06), and several pieces of recently 

 heated pumice stone. Place the flask on a wire gauze, connect with a condenser, 

 and heat, rather gently at first, and so regulate as to distil over 30 cc in about 

 ten minutes, the distillate passing through a small filter paper. Replace the 

 30 cc driven over by a like quantity of the dilute acid added by means of a 

 separatory funnel in such a manner as to wash down the particles adhering to 

 the sides of the flask, and continue the process until the distillate amounts to 360 

 cc. To the completed distillate gradually add a quantity of phloroglucol (puri- 

 fied if necessary) dissolved in 12 per cent hydrochloric acid and thoroughly 

 stir the resulting mixture. The amount of phloroglucol used should be about 

 double that of the furfural expected. The solution first turns yellow, then 

 green, and very soon an amorphous greenish precipitate appears, which grows 

 rapidly darker, till it finally becomes almost black. Make the solution up to 

 400 cc with 12 per cent hydrochloric acid, and allow to stand over night 



