56 Van Name and Hill — Solution of Silver in Chromic Acid. 



samples of silver were used, designated below as Samples 

 A and B, respectively, both obtained from the firm which 

 had furnished the sample used in the former investigation. 

 The rates of solution of these three specimens were all 

 very different, Sample A giving an average velocity of 

 about 1-95, and Sample B of 1-60, under conditions where 

 1-22 had been obtained in the former work. Moreover, 

 the same sample gave different results according to the 

 amount of metal which had been removed in the pre- 

 liminary treatment with nitric acid, for the outer layer, 

 owing probably to differences in physical structure 

 produced by the greater stresses undergone in the pro- 

 cess of rolling, generally showed a much higher rate of 

 solution than the metal beneath. The following experi- 

 ment illustrates the behavior of a silver disk which had 

 had only a very light etching with nitric acid before the 

 experiment. After the fourth reaction period (each of 

 10 minutes) this disk was removed and another disk sub- 

 stituted which had been cleaned as usual with boiling 

 caustic soda, but had undergone no subsequent etching 

 with acid. 



Silver, Sample B. Sulphur acid 5 molar. 



k = 2-23 1-62 1-68 1-64; (new disk) 2-24 2-02 1-87 



The high initial value of k, followed by a more or less 

 rapid fall to a comparatively constant final rate, is char- 

 acteristic. It was only this final rate which was in any 

 way definite and reproducible, and even this often varied 

 considerably for samples cut from different parts of the 

 same sheet. The more serious initial irregularities could 

 of course be avoided by strong preliminary treatment 

 with nitric acid. 



For these reasons, in attempting to compare rates of 

 solution of silver in the 0-25 and 5 molar acid we have 

 found it best to use the same disk first in one solution 

 for three or four reaction periods and then in the other. 

 The results of a series of experiments so conducted, in 

 each of which the solution was changed either once or 

 twice, are recorded in Table II. 



According to these figures the reaction is about 2-9 

 times more rapid in the weak than in the strong acid, 

 while the former investigation gave a value of about 3-5 

 for this ratio. The new result is undoubtedly more cor- 

 rect as the older one was obtained before the sources of 



