60 PHYSIOLOGICAL CHEMISTRY 



i c.c. from tube 2 to tube 3. Continue in this manner until you have transfer 

 solutions of gradually decreasing strength. Now add starch paste in equal 

 amounts to each tube, mix very thoroughly, and place in the incubator or water- 

 bath at 40C. After 10-20 minutes test by both the iodine and Fehling's tests. 

 In how great dilution does your saliva act? 



18. Influence of Acids and Alkalis. (a) Influence of Free Acid. Prepare a 

 series of six tubes in each of which is placed 4 c.c. of one of the following strengths 

 of free HC1: 0.2 per cent, o.i per cent, 0.05 per cent, 0.025 per cent, 0.0125 per 

 cent and 0.006 per cent. Now add 2 c.c. of starch paste to each tube and shake 

 them thoroughly. Complete the solutions by adding 2 c.c. of saliva to each and 

 repeat the shaking. The total acidity of this series would be as follows : o.i per 

 cent, 0.05 per cent, 0.025 per cent, 0.0125 per cent, 0.006 per cent and 0.003 per 

 cent. Place these tubes on the water-bath at 4oC. for 10-20 minutes. Divide 

 the contents of each tube into two parts, testing one part by the iodine test and 

 testing the other, after neutralization, by Fehling's test. What do you find? 



(b) Influence of Combined Acid (Protein Salt). Repeat the first three ex- 

 periments of the above series using combined hydrochloric acid (see page 139) 

 instead of the free acid. How does the action of the combined acid differ from 

 that of the free acid? (For a discussion of combined acid see page 139.) 



(c) Influence of Alkali. Repeat the first four experiments under (a) replac- 

 ing the HC1 by 2 per cent, i per cent, 0.5 per cent and 0.25 per cent NajCO 3 . 

 Neutralize the alkalinity before trying the iodine test (see Starch, 5, page 43). 



(d) Nature of the Action of Acid and Alkali. Place 2 c.c. of saliva and 2 c.c. 

 of 0.2 per cent HC1 in a test-tube and leave for 15 minutes. Neutralize the 

 solution, add 4 c.c. of starch paste and place the tube in the incubator or water- 

 bath at 40C. In ro minutes test by the iodine and Fehling's tests and explain 

 the result. Repeat the experiment, replacing the 0.2 per cent HC1 by 2 per cent 

 Na 2 CO 3 . What do you deduce from these two experiments? 



19. Influence of Metallic Salts, etc. In each of a series of tubes place 4 c.c. 

 of starch paste and 0.5 c.c. of one of the solutions named below. Shake well, add 

 0.5 c.c. of saliva to each tube, thoroughly mix, and place in the incubator or water- 

 bath at 4oC. for 10-20 minutes. Show the progress of digestion by means of the 

 iodine and Fehling tests. Use the following chemicals: Metallic salts, 10 per cent 

 lead acetate, 2 per cent copper sulphate, 5 per cent ferric chloride, 8 per cent mer- 

 curic chloride; Neutral salts, 10 per cent sodium chloride, 10 per cent magnesium 

 sulphate, 3 per cent barium chloride, 10 per cent Rochelle salt. Also try the influ- 

 ence of 2 per cent carbolic acid, 95 per cent alcohol, and ether and chloroform. 

 What are your conclusions? 



Antiseptics do not necessarily inhibit enzyme action. 



20. Excretion of Potassium Iodide. Ingest a small dose of potassium iodide 

 (0.2 gram) contained in a gelatin capsule, quickly rinse out the mouth with 

 water, and then test the saliva at once for iodine. This test should be negative. 

 Make additional tests for iodine at two-minute intervals. The test for iodine is 

 made as follows: Take i c.c. of NaNO 2 and i c.c. of dilute H2SO4 1 in a test- 

 tube, add a little saliva directly from the mouth, and a small amount of starch 

 paste. The formation of a blue color signifies that the potassium iodide is being 

 excreted through the salivary glands. Note the length of time elapsing between 



1 Instead of this mixture a few drops of HN0 8 possessing a yellowish or brownish color 

 due to the presence of HNO 2 may be employed. J 



