244 Van Name and Edgar — Velocities of Certain Reactions 



age thickness was estimated, and the necessary small correction 

 calculated and applied to the mean value of the velocity con- 

 stant. Where " corrected " values of the constant are given 

 in this and the following tables the corrections have been made 

 necessary by slight deviations of this kind from the normal 

 dimensions. In cases where no correction is mentioned the 

 disk was of the standard diameter and thickness. 



A relatively high concentration of potassium iodide was 

 found to be necessary in the work with copper. An experi- 

 ment carried out with a concentration of 200 grams of the 

 iodide per liter was a failure owing to the formation of a layer 

 of insoluble cuprous iodide on the disk, which caused a rapid 

 fall in the value of the constant. Even in the presence of 400 

 grams of potassium iodide per liter traces of such a film were 

 noticed at the close of the experiment, although, as the table 

 shows, fairly good constants were obtained. With 800 grams 

 per liter no visible film was formed. 



With silver the same difficulty was encountered. A concen- 

 tration of 200 grams per liter of potassium iodide was not 

 enough, and 400 grams per liter barely sufficient, to prevent a 

 decided interference with the reaction by a coating of silver 

 iodide on the metal. The disks were cut from a very pure 

 sheet silver and were a little below the standard thickness, so 

 that a correction had to be applied to the values of the veloc- 

 ity constant as in the case of copper. The results are given in 

 Table III. 



For the experiments with cadmium, recorded in Table IY, 

 Kahlbaum's metallic cadmium was employed. This was cast 

 into thick disks which were either rolled out to the proper 

 thinness or ground down with emery. Experiment 1 was 

 made with a disk of rolled metal, experiments 2 and 3 with 

 cast and ground disks. The more crystalline nature of the latter 

 was apparent after the action of the iodine ; in fact, these two 

 disks showed more roughening of the surface during the experi- 

 ment than was the case with any of the other metals investi- 

 gated. Even here, however, the roughening was comparatively 

 slight, and seems to have affected the value of the constant but 

 little, although experiment 2 shows a rise in the constant which 

 may be due to this cause. 



In order that the constants might be comparable with those 

 obtained with mercury, the concentration of the potassium 

 iodide in the work both with cadmium and with zinc was fixed 

 at either 100 or 200 grams per liter, a much larger amount, in 

 view of the ready solubility of the iodides of these two metals, 

 than would otherwise have been needed. 



The zinc disks used in the experiments of Table Y were 

 prepared by casting and grinding into shape from a sample of 



