in Ferric Salts and in Chromic Acid. 319 



molar with respect to sulphuric acid. Both metals under these 

 conditions behaved normally, giving satisfactorily constant 

 reaction velocities, and the values obtained for the two metals 

 are, moreover, in excellent agreement with one another 

 throughout. 



On the other hand a few experiments conducted in solutions 

 only 0'05 molar with respect to sulphuric acid gave distinctly 

 abnormal results, the reaction velocities showing progressive 

 variation and no approach to agreement between cadmium and 

 copper. Since these experiments were plainly affected by 

 specific disturbances arising from insufficient acidity, the results, 

 though included in the table, are of little significance. 



Iron. — The experiments on the rate of solution of iron in 

 chromic acid are complicated by the fact that the oxidation 

 takes place in two stages. Two examples of such two-stage 

 reactions have already been considered, tin in ferric sulphate 

 and copper in ferric chloride, but in both cases the method of 

 analysis was such that the second stage of the oxidation had no 

 direct effect upon the titer of the solution. Consequently, the 

 concentration of the oxidizing agent, as calculated from the 

 titrations and used in calculating the velocity constant, was in 

 reality the combined concentrations of two oxidizing agents 

 present in unknown proportions in the solution, namely, the 

 ferric salt, and the higher oxidation product of the dissolving 

 metal. (See p. 308.) In the present case it is the concentra- 

 tion of the chromic acid alone which is given by the titrations, 

 and therefore both stages of the oxidation change the titer of 

 the solution. This difference must be borne in mind in com- 

 paring the results. 



Now in general, in a two-stage reaction of this type between 

 a metal and a dissolved oxidizer, if the second stage is suf- 

 ficiently rapid the lower oxidation product will be oxidized 

 where it is formed, that is, at the surface of the metal. Thus 

 it may happen that a metal in passing through two stages of 

 oxidation gives the same velocity constant as a metal undergo- 

 ing only one stage, the observed velocity being that of the dif- 

 fusion process. Such an instance is apparently offered by the 

 case of tin in chromic acid, to be discussed later. On the other 

 hand if the second stage of the oxidation is not quite rapid 

 enough to produce the result just mentioned, some of the 

 molecules of the lower oxidation stage will not be oxidized 

 until they have diffused part way through the diffusion layer, 

 or perhaps have passed through it into the solution. The 

 effect in either case is to shorten the average length of the dif- 

 fusion path for the molecules of the oxidizing agent and there- 

 fore to raise the observed reaction velocity. The reaction 

 between tin and iodine dissolved in potassium iodide solution 



