in Ferric Salts and in Chromic Acid. 315 



of the change in free energy, there would seem to be here a 

 rather strong probability that the chemical reaction in most or 

 all of the cases with which we shall have to deal is accelerated 

 rather than retarded hy an increase in acidity. At all events 

 we shall make this assumption as a working hypothesis, and 

 shall adhere to it consistently throughout. 



Table VIII. 

 Summary of Velocity Constants. 

 Ferric Alum. 

 Cone, of H 2 S0 4 O'Ol 0"25 T25 5 molar. 



Zinc 4-38 



Cadmium...... 4\12 415 3"54 176 



Iron 3-95 (3-82) 3"92 3"37 174 



Nickel 3-80 (3'41) 375 (3"39) 3"27 (299) 171 (1-55) 



Tin 3-96(4-16) .... 172 



Copper 3-74(3-64) 3-30(3 22) 1-71(1-63) 



Silver 1-67(1-36) 1-63(1-36) 124 (I'll) 



Ferric Chloride. 



Cone, of HC1 01 0-5 molar. 



Cadmium 479 4*17 



Iron 414 4"35 



Copper 3-44 (3"22) 4'20 (4"30) 



We must assume then that sulphuric acid tends to accelerate 

 the chemical reaction with the metal, but we find in Table VIII 

 that if the acid is present in more than 0*25 molar concentra- 

 tion it exerts a strong retarding influence on the rate of solu- 

 tion. From the standpoint of the diffusion theory the 

 explanation of this effect is clear. The important factor here 

 is the viscosity of the solution, which is increased in the ratio 

 of about 2 - 5 : 1 when the sulphuric acid concentration rises 

 from 0*25 molar to 5 molar. An increase in the viscosity 

 retards the diffusion process by lowering the rate of diffusion, 

 and perhaps, to a slight extent, by increasing the thickness of 

 the diffusion layer,* and in this way depresses the rate of solu- 

 tion of the metal, this effect here outweighing any acceleration 

 of the chemical part of the reaction resulting from the higher 

 acidity. 



A predominance of the diffusion effect, however, does not 

 necessarily exclude the influence of the chemical reaction, for 

 we find that velocity constants in the same vertical column 

 (that for 0-25 molar sulphuric acid, for instance), though they 

 differ among themselves in a comparatively systematic manner 

 according to the specific nature of the metal, thus showing the 

 * See Van Name and Hill, this Journal, xxxvi, 552-4. 



