Certain Metals in Dissolved Iodine. 217 



Table V. 



Metal : Tin. KI 0-5 normal. Temperature 25°. r = 170. 



HC1 1" normal in experiments 1, 2, 3, and 4. 



1. v = 580 560 540 520 500 480 

 At= 10 10 10 10 10 10 



c = 20'52 18-16 15-92 1367 11-71 993 8"05 



K= 7'08 7-37 8-21 8-04 8-25 (10-06) Av. 779 



2. At= 6 6 5 7 10 7 



e = 20-79 19-22 17-74 1651 1495 12-69 11-36 



K= 7-58 7-47 7-77 7-36 8'18 7-59 Av. 7'66 



3. At= 10 14 11 14 15 21 



c = 20-95 18-60 15-60 13'54 11-04 8"76 5-71 



K= 6-90 7-03 6-94 7-57 7'70 (9-77) Av. 7*27 



4. K= 7-07 6-99 6"99 750 8-11 (9-56) Av. 733 



5. HC1 0-33 normal. 



K= 7-87 6-83 7-30 7"52 7'08 7-39 Av. 733 



6. HC1 0-25 normal. 



K= 7-43 7-49 7-71 8"18 7-69 8-67 Av. 786 



the change from left to right being nearly complete.* At the 

 actual surface of the metal the iodine concentration is practi- 

 cally zero, and the tin will therefore dissolve as stannous iodide. 

 As this diffuses through the "unstirred" layer it will encounter 

 an ever increasing concentration of iodine, and will therefore 

 soon be converted into stannic salt. The diffusion path for 

 the iodine so used will be less than the thickness of the diffu- 

 sion layer, thus causing a more rapid consumption of iodine 

 than in a normal case and giving the high velocity constants 

 actually observed. 



To prevent hydrolysis of the tin salt and the resulting 

 accumulation of hydroxide on the disk, the solutions were 

 made at least 0*25 normal with hydrochloric acid. Oxidation 

 was prevented by the use of carbon dioxide as described above. 



In the experiments with metallic magnesium, hydrogen was 

 always evolved from the disk, thus introducing a disturbing 

 factor of a new kind. None of the metals discussed above 

 had evolved hydrogen in visible amounts even at the highest 

 acidity employed, but with magnesium hydrogen was pro- 

 duced to some extent even in neutral solution, and when 

 enough acid was present to keep the disk free from hydroxide, 

 the gas evolution became very marked. 



There are two obvious ways in which the gas evolution 

 would affect the results; first, the adhering gas bubbles would 



* The use of this reaction as a quantitative method of estimating iodine 

 has heen proposed by Spring, Jour. Am. Chem. Soc, xix, 809. 



Am. Jour. Sci. — Fourth Series, Vol. XXXII, No. 189. — September, 1911. 

 16 



