122 THE CARBOHYDRATES. [cH. V. 



first almost a pure blue, but it changes through a rich purple-red to 

 a red-brown as the iodine is added. 



NOTE. Some samples of commercial dextrin contain a considerable 

 amount of soluble starch. 



144. Repeat the above experiment, but boil and then cool the 

 tube after each addition. The colour disappears on boiling, but 

 does not reappear on cooling until several drops of iodine have been 

 added, unless much soluble starch is present. 



145. Add a drop or two of the starch paste prepared in Ex. 135 

 to about 5 cc. of the dextrin solution. To this mixture add diluted 

 iodine solution, drop by drop. The first additions produce a pure 

 blue colour, and it is not till a considerable amount of iodine has 

 been added that the solution acquires a purplish tint. 



NOTE. The affinity of starch for iodine is so much greater than that of 

 dextrin that the characteristic colour reactions of erythro-dextrin are not 

 obtained until all the starch has been saturated with iodine. Even then it is 

 sometimes difficult to detect, owing to the deep blue starch reaction. 



146. Treat 5 cc. of the dextrin solution with about 10 drops 

 of the starch paste : to the mixture add an equal bulk of saturated 

 ammonium sulphate, shake vigorously, and allow to stand for five 

 minutes. The starch is precipitated. Filter through a dry paper, 

 and to a portion of the filtrate add a drop or two of iodine solution. 

 The purple red reaction of erythro-dextrin is obtained. 



147. Saturate 5 cc. of the dextrin solution by boiling with an 

 excess of finely powdered ammonium sulphate. Note the precipitate 

 of erythro-dextrin produced. Cool under the tap and filter. To 

 the filtrate add a drop of iodine. A red-brown colour is produced. 



NOTE. This colour is due to the fact that erythro-dextrin III. is not 

 precipitated by ammonium sulphate. This is the method employed for the 

 identification of erythro-dextrin in the presence of glycogen, which is com- 

 pletely precipitated by saturation with ammonium sulphate. 



148. Boil a few cc. of the dextrin solution with a small amount 

 of Fehling's fluid. A well-marked reduction is usually obtained. 



NOTE. Commercial dextrin is generally prepared by the action of dilute 

 acids on starch (see Ex. 139), the action being stopped as soon as a portion fails 

 to give a blue colour with iodine, and the products then being precipitated by 

 alcohol. Such preparations contain some glucose, and often a little soluble 

 starch. At the same time it must be noted that the achroo-dextrins have a 

 reducing action themselves even when thoroughly separated from the glucose. 



