RATE OF DIFFUSION OF IODINE IN KI 479 



where is the surface of the reacting solid. It would seem natural to 

 suppose that k had the same significance as in the first case, but according 

 to the "Diffusion Theory" this is not true. A; is a constant depending on 

 the rate of diffusion of the reacting, or dissolving, substance and is inde- 

 pendent of the "affinity" of the reacting substances, {k is also independent 

 of the order of the reaction). 



Numerous investigations have been made which seem to support this 

 hypothesis, and it has been accepted as a working basis by many writers, 

 but the criticism of it has been vigorous by others. In testing its vahdity 

 we should choose such reagents as will react to produce only soluble 

 bodies under the conditions of the experiment. Thus we should not expect 

 the theory to hold for metals reacting with acids to liberate hydrogen, 

 or for metals reacting with soluble salts to jaeld insoluble salts. These 

 facts should be remembered in evaluating the criticisms advanced by 

 Marc, Wildermann, and others (4) (5) (6) (7) (8) (9). 



Let us now briefly survey the chief of these objections that have been 

 advanced agamst the diffusion theory of reaction velocity. 



I. That it is unnecessary, as the same equation for reaction velocity 

 in heterogeneous systems can be, and have been (9) deduced from other 

 and better established premises. 



II. That the temperature coefficient of reaction velocity is found by 

 experiment to be very different from the temperature coefficient for 

 diffusion in many cases (4). 



III. That if the diffusion theory were correct metals having widely 

 different solution pressures should dissolve in a given solution at the same 

 rate; likewise, that such substances as marble, dolomite, and magnesite 

 should dissolve in acids at the same rate; which conclusions, it is urged, 

 are manifestly not in accord with the well known facts. 



The above list of objections, while not by any means exhaustive, 

 embody the most important arguments against the theory. 



A type of reaction that, seemed to lend itself well to this investigation 

 was the rate of solution of certain metals in a solution of iodine in potas- 

 sium iodide. The advantages were: (1) that no gaseous product was 

 formed which might disturb the "unstirred" layer; (2) the ease with which 

 the materials might be obtained pure and in the desired form; (3) the 

 ease and accuracy ■nath which iodine can be estimated in solutions of all 

 concentrations. A little work along this line had been done as early as 

 1891 by Schiikarew, who worked with various metals in solutions of 

 halogens (10). (This work was done about six years before the diffusion 

 theory of reaction velocitj'- had been advanced by Noyes and Whitney.) 



