322 Van Name and UUl — Solution of Metals 



the nickel disks to become passive. In Experiments 6, 7, and 

 8, to insure activity of the disk at the start, after cleaning in 

 the usual manner it was immersed in dilute hydrochloric acid 

 and thoroughly rubbed under the acid with a zinc rod. This 

 treatment, did not make the constants regular. Since low con- 

 stants are found following much higher ones (notably in Experi- 

 ment 8) passivity must have been produced in the chromic acid 

 solution. That the reaction velocity also increases at times 

 during the course of the experiment is perhaps explained by 

 the fact that a partly passive disk in sulphuric acid would con- 

 stitute a short-circuited element of which the active areas 

 would be the auodes, and the adjacent passive areas would con- 

 sequently be subject to cathodic reducing effects which would 

 tend to destroy their passivity in so far as it was not re-estab- 

 lished by the chromic acid. No explanation is offered for the 

 fact that these two opposing tendencies produce fluctuations in 

 the degree of passivity. It may only be suggested that the 

 variations are in some way connected with the gradual uncover- 

 ing of impurities in the metal. 



In Experiments 9 and 10 the attempt was made to restrict so 

 far as possible the formation, on the disk, of the blackish 

 deposits above mentioned, by removing the disk from the solu- 

 tion at the end of each reaction period and cleaning with nitric 

 acid. The treatment with a zinc rod was omitted. Neither 

 hot concentrated hydrochloric acid nor iodine in potassium 

 iodide solution would remove the deposit completely. Con- 

 centrated nitric acid removed it easily and gave the metal a 

 perfectly clean surface, but a disk so cleaned was invariably 

 wholly passive and was no longer attacked by the chromic acid 

 solution. Nitric acid of two-thirds strength was accordingly 

 selected for trial in Experiment 9 but proved too strong, the 

 results showing practically complete passivity during two of 

 the reaction periods, and a very low activity during the rest. 

 The more dilute nitric acid used in Experiment 10 produced 

 high passivity during the first reaction period, but the later 

 constants showed no more passivity than might be expected, 

 according to the results of Experiments 6, 7, and 8, from the 

 action of the chromic acid alone. 



For Experiments 11 and 12 the sulphuric acid concentration 

 was increased to 5 molar with very beneficial results. The 

 absence of any indication of passivity in these experiments 

 seems to prove that this concentration of sulphuric acid was 

 sufficient to entirely overcome the tendency of chromic acid 

 to produce passivity in the nickel disks. The rate of the 

 reaction shows a satisfactory constancy, and the mean value of 

 the velocity constant agrees with the values obtained with 

 several other metals under like conditions. 



