CARBOHYDRATES 43 



EXPERIMENTS ON STARCH 



1. Preparation of Potato Starch. Pare a raw potato, comminute it upon a fine 

 grater, mi* with water, and "whip up" the pulped material vigorously before 

 straining it through cheese cloth or gauze to remove the coarse particles. The 

 starch rapidly settles to the bottom and can be washed by repeated decantation. 

 Allow the compact mass of starch to drain thoroughly and spread it out on a watch 

 glass to dry in the air. If so desired this preparation may be used in the experi- 

 ments which follow. 



2. Microscopical Examination. Examine microscopically the granules of the 

 various starches submitted and compare them with those shown in Figs. 9-19, 

 page 44. The suspension of the granules in a drop of water will facilitate the 

 microscopical examination. 



3. Solubility. Try the solubility of one form of starch hi each of the ordinary 

 solvents (see page 22). If uncertain regarding the solubility in any reagent, 

 filter and test the filtrate with iodine solution as given under 5 below. The pro- 

 duction of a blue color would indicate that the starch had been dissolved by the 

 solvent. 



4. Iodine Test. Place a few granules of starch in one of the depressions of a 

 porcelain test-tablet and treat with a drop of a dilute solution of iodine hi potas- 

 sium iodide. The granules are colored blue due to the formation of so-called 

 iodide of starch. The amylo-cellulose of the granule is not stained as may be 

 seen by examining microscopically. 



5. Iodine Test on Starch Paste. 1 Repeat the iodine test using the starch 

 paste. Place 2-3 c.c. of starch paste 2 hi a test-tube, add a drop of the dilute 

 iodine solution and observe the production of a blue color. Heat the tube and 

 note the disappearance of the color. It reappears on cooling. 



In similar tests note the influence of alcohol and of alkali upon the so-called 

 iodide of starch. 



The composition of the iodide of starch is not definitely known. In per- 

 forming this test the solution must always be neutral or acid in reaction. 



6. Fehling's Test. On starch paste (see page 25). 



7. Hydrolysis of Starch. Place about 25 c.c. of starch paste hi a small 

 beaker, add 10 drops of concentrated HC1, and boil. By means of a small pipette, 

 at the end of each minute, remove a cfrop of the solution to the test-tablet and 

 make the regular iodine test. As the testing proceeds the blue color should 

 gradually fade and finally disappear. At this point, after cooling and neutraliz- 

 ing with solid KOH, Fehling's test (see page 25) should give a positive result 

 due to the formation of a reducing sugar from the starch. Make the phenyl- 

 hydrazine test upon some of the hydrolyzed starch. What sugar has been 

 formed? 



8. Influence of Tannic Acid. Add an excess of tannic acid solution to a small 

 amount of starch paste in a test-tube. The liquid will become strongly opaque 

 and ordinarily a yellowish-white precipitate is produced. Compare this result with 

 the result of the similar experiment on dextrin (page 47). 



1 Preparation of Starch Paste.-^ Grind 2 grams of starch powder in a mortar with a small 

 amount of cold water. Bring 200 c.c. of water to the boiling-point and add the starch mix- 

 ture from the mortar with continuous stirring. Bring again to the boiling-point and allow 

 it to cool. This makes an approximate i per cent starch paste which is a very satisfactory 

 strength for general use. 



2 For this particular test a starch paste of very satisfactory strength may be made by 

 mixing i c.c. of a i per cent starch paste with 100 c.c. of water. 



