120 THE CARBOHYDRATES. [cH. V. 



of iodine solution, a blue colour is not obtained. Treat with strong 

 acetic acid. A deep blue colour appears. 



NOTE. Free iodine is necessary to give the blue adsorption compound 

 with starch. Sodium hydroxide removes free iodine, converting it into iodide 

 and iodate. The action of the acid on the latter causes the appearance of free 

 iodine and the blue colour. Always neutralise an alkaline solution before testing 

 for the polysaccharides. The following equations show the effect of sodium 

 hydroxide on iodine, and of acid on a mixture of iodide and iodate : 



(i.) 3l 2 + 6NaOH = 5NaI + NaIO 3 + 3H 2 O. 

 (ii.) 5NaI + NaIO 3 + 6HC1 = 3l 2 + 6NaCl 



135. Preparation of starch paste. Boil about 75 cc. of 

 distilled water in a beaker. Weigh out I gram, of dry potato starch 

 in another small beaker, add about 10 cc. of cold water, and stir to 

 get a uniform suspension. Pour this into the boiling water and stir 

 well. Wash the small beaker out with another 10 cc. of cold water, 

 adding this to the boiling fluid. Stir again, and keep boiling for i 

 minute. Cool, and make the volume up to TOO cc. Note that the 

 " solution " is distinctly opalescent. It should be quite uniform and 

 free from lumps. 



136. To a small amount of the paste add a drop or two of 

 dilute iodine. A deep blue colour is produced. 



NOTE. The iodine solution should be about o-oi N. (See appendix, 

 p. 390.) 



137. Treat 5 cc. of the cold starch paste with an equal bulk of 

 saturated ammonium sulphate. Shake the test-tube and allow it to 

 stand for five minutes. The starch is precipitated. Filter through 

 a dry paper, and add a drop of iodine solution to the nitrate. No 

 blue colour, or only the very slightest tint is obtained, showing that 

 the whole of the starch paste is precipitated by half-saturation with 

 ammonium sulphate. 



138. Boil 5 cc. of the starch paste with two drops of concen- 

 trated sulphuric acid for about 15 seconds. Note that the solution 

 becomes perfectly clear and translucent. Add two drops of strong 

 ammonia to neutralise the acid, cool under the tap, add an exactly 

 equal bulk of saturated ammonium sulphate, shake the tube vigor- 

 ously, and allow it to stand for five minutes. Filter through a dry 

 filter-paper and add two drops of iodine solution to the filtrate. A 

 deep blue colour is obtained. 



