CARBOHYDRATES 225 



water lost by evaporation. Use the resulting opalescent solution 

 for the following tests. 



iii. Iodine Test. To about half a test tube of distilled water 

 add 3-4. drops of iodine solution. Pour 3-4 drops of this diluted 

 solution into 4-5 e.c. of the starch solution prepared in ii. Notice 

 the dark blue color. This is due to a starch-iodine compound 

 formed. Iodine gives no color with mono- or disaccharides. To 

 a starch solution colored blue by iodine add 2-3 drops of dilute 

 sodium hydroxide. The color is destroyed. 



To another portion of the blue liquid add alcohol. The blue 

 color disappears. 



iv. Perform Feliling's Test. Starch does not reduce Fehling's 

 solution. 



v. To starch solution add 2-3 c.c. tannic acid solution. The 

 starch is precipitated. The tannic acid solution should be fresh, 

 as tannic acid solutions decompose on standing. 



vi. Starch will not dialyze through a parchment membrane. 



vii. Hydrolysis of Starch. Place about 25 c.c. starch solu- 

 tion in an evaporating dish, add 10 drops of concentrated hydro- 

 chloric acid and boil gently. At intervals of about a minute re- 

 move a drop or two of the liquid with a glass rod and test with a 

 drop of iodine solution. The starch is broken down into dex- 

 trins and finally into simpler carbohydrates (maltoses and glu- 

 cose). The iodine test changes from blue to red, and finally to 

 no color as the hydrolysis proceeds. When iodine no longer gives 

 a color, make the hydrolyzed starch solution alkaline with sodium 

 hydrate and perform a Fehling's test. Reduction indicates the 

 maltose or glucose stage. 



2. Dextrines. 



i. Test the solubility of dextrin in cold water, hot water, alco- 

 hol and ether. 



ii. Iodine Test. Test a solution of dextrin with a few drops 

 of diluted iodine solution, as under "starch." Eefer to the re- 

 sults obtained on the hydrolysis of starch. Try the effect of 

 heat, of alkali and of alcohol on the color produced with iodine. 



iii. Make a Fehling's Test on dextrin solution. Remember 



