262 PHYSIOLOGICAL CHEMISTRY 



36. 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 c.c. of the starch solution prepared in 35. No- 

 tice 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. 



To another portion of the blue liquid add alcohol. 



37. Perform Benedict's Test. 



38. To starch solution add 2-3 c.c. taunic acid solution. The 

 tannic acid solution should be fresh, as tannic acid solutions 

 decompose on standing. 



39. Starch will not dialyze through a parchment membrane. 



40. 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). When iodine no longer gives a color, make the hydro- 

 lyzed starch solution alkaline with sodium hydrate and perform 

 a Benedict's test. Reduction indicates the maltose or glucose 

 stage. 



Dextrines. 



41. Test the solubility of dextrin in cold water, hot water, 

 alcohol and ether. 



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

 of diluted iodine solution, as under "starch." Refer 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. 



43. Make a Benedict's Test on dextrin solution. Remember 

 that if reduction is observed it may oe due to the presence of 

 maltose or dextrose as impurities in the dextrin. In the manu- 

 facture of dextrin, small amounts of maltose and dextrose are 

 likely to be formed by complete hydrolysis of a portion of the 



