CH. V.j STARCH. 121 



NOTE. Starch paste is rapidly converted into " soluble starch " on boiling 

 with dilute mineral acids. Soluble starch differs from starch paste in that it is 

 not completely precipitated by half-saturation with (NH 4 ) 2 SO 4 in the course of 

 five minutes. If it be allowed to stand for twenty-four hours, however, it is 

 completely precipitated. 



139. Measure 2 cc. of the paste into a test-tube, add 6 drops 

 of concentrated hydrochloric acid by means of a dropping pipette 

 (see fig. 5). Heat to J boiling and maintain the boiling for one 

 minute by the watch. Cool thoroughly under the tap. Add 

 first one drop, and then another drop, of the iodine solution. A red 

 or violet colour is produced, indicating the conversion of the 

 starch into erythro-dextrin by acid hydrolysis. 



140. Boil 2 cc. of the paste with 6 drops of concentrated 

 hydrochloric acid as before. Cool. Add 3 drops of glycerol and 8 

 drops of 20 per cent, copper sulphate. Add 20 per cent, sodium 

 hydroxide until a grey precipitate is produced. Now add another 

 2 cc. of the sodium hydroxide and boil for a minute. A slight 

 reduction is obtained. 



141. Repeat the previous exercise, using 20 drops of the 

 acid, and boiling for two minutes. Cool. Add 3 drops of 

 glycerol and 16 drops of the copper sulphate. Neutralise with soda 

 and then add 2 cc. in excess. Boil for one minute. Complete 

 reduction of the copper is obtained, indicating that the starch has 

 been hydrolysed to glucose (see Ex. 102). 



NOTE. If 12 drops of hydrocloric acid be added and the mixture boiled 

 for one minute, it will generally be found that only a yellow cokmr is produced 

 with iodine, and that the amount of glucose formed is not sufficient to reduce 

 9 drops of copper sulphate. At this stage a considerable proportion of the 

 carbohydrate is in the form of achroo-dextrin. It is important to note that 

 the complete hydrolysis of starch by acids is relatively slow compared to that 

 of sucrose and the other disaccharides (see Ex. 130). The addition of a 

 couple of glass beads makes it easier to obtain smooth boilmg in the above 

 exercises. 



142. Shake a little commercial dextrin with some cold water. 

 An opalescent solution is formed. Boil the solution. It becomes 

 perfectly translucent. (Distinction from glycogen.) 



Use a 3 per cent, solution of commercial dextrin for the following re- 

 actions : 



143. To about 5 cc. of the dextrin solution add iodine solution, 

 drop by drop, noting the colour at every addition. The colour is at 



