in Ferric Salts and in Chromic Acid. 321 



probably belongs in this category, as has been shown else- 

 where.* There is, moreover, reason to expect similar behavior 

 from iron in chromic acid, for Bensonf has shown that the 

 reaction between chromic acid and ferrous sulphate progresses 

 in dilute solutions at a rate sufficiently slow to be easily 

 measurable. 



The results obtained, shown in Table X, Experiments 1-5, 

 are in accordance with this view. Not only are the constants 

 higher throughout than with the other metals under like con- 

 ditions, but they also show the continuous rise which would be 

 predicted from the accumulation of ferric salt, which itself 

 reacts with the metal, producing ferrous salt in constantly 

 increasing proportion compared with the chromic acid, and 

 thus causing a progressive shortening in the average length of 

 the diffusion path. 



In Experiments 1 to 4 the titrations were carried out during 

 the progress of the expsriment, and with very little delay. 

 Now Benson has shown, in the article just cited, that ferric 

 salts have a retarding effect upon the oxidation of ferrous salts 

 by chromic acid. If in our analyses the time allowed for the 

 completion of this reaction had been too short this would have 

 produced high and rising velocity constants. That the high 

 and rising constants actually observed were not due to this is 

 proved by Experiment 5, in which the solution samples, after 

 mixing as usual with a known amount of ferrous sulphate, were 

 allowed to stand for 2£ hours before titrating back with per- 

 manganate. It is evident that this modification made no 

 appreciable difference in the results. 



Nickel. — The experiments with nickel in chromic acid were 

 carried out in solutions either - 25 or 5 molar with respect to 

 sulphuric acid. In the former solution the velocity constants 

 were in all cases irregular and abnormally low ; in the latter 

 the results were apparently normal and agreed well with the 

 values obtained with cadmium and with copper under like con- 

 ditions. 



The nickel disks after use always showed traces of a blackish 

 deposit or, in the stronger acid, of a brownish discoloration, 

 these surface coatings resembling closely in appearance and 

 amount those observed with nickel in ferric sulphate solutions 

 of like sulphuric acid concentration (see page 306). The 

 effect of these coatings upon the reaction velocity was appar- 

 ently negligible in 5 molar sulphuric acid, and even in the 

 weaker acid they were probably responsible for only a small 

 part of the abnormality observed. 



The chief cause of the low and variable results obtained in 

 the presence of 0'25 molar sulphuric acid was the tendency of 



*TMs Journal (4), xxxii, 216, 1911. f Jour. Phys. Chern., vii, 1, 1903. 



