the Elementary Law of Hydrodiffuslon. 525 



iv f upper 1 i j t • ( withdrawn from *) ,t 



tne i i r > boundary layer as is { . , -, , > the 



(. lower J j j ^ communicated to J 



1 i > boundary layer by the galvanic current. 



When the constant galvanic current has passed through the 

 combination a suitably long time, and if an exactly specifiable 

 state of concentration-distribution between the two electrodes 

 has in consequence arisen, the galvanic current is suddenly 

 interrupted. From this moment the solution is exposed to 

 the action of diffusion only. The further course of the diffu- 

 sion is now investigated with the aid of the electromotive force 

 which is generated by the variable concentrations of the boun- 

 dary layers at the two zinc plates. From the found temporal 

 course of this electromotive force both the entire course of 

 the diffusion can be known and the diffusion-constant calcu- 

 lated. 



An eminent advantage of this method consists in the 

 circumstance that, the distance between the electrodes, and 

 consequently the time occupied by the entire course of the 

 diffusion may be made as short as we please, without introdu- 

 cing any uncertainty into the definition of the initial state of 

 the distribution of concentration from which the observed 

 diffusion is developed ; at whatever moment the galvanic 

 current may be interrupted, the distribution of the concentra- 

 tion from layer to layer corresponding to that moment can 

 always be assigned. 



Theory. — When a galvanic current passes through a zinc- 

 sulphate solution between two zinc electrodes, the quantity of 

 salt remains unchanged, in spite of the electrolysis that takes 

 place ; just so the concentration of every single inner layer 

 of liquid remains constant ; only the boundary layers of the 

 solution, situated at the electrodes, undergo an alteration of 

 their concentration: the salt-content of the boundary layer 

 adjacent to the anode is augmented ; the salt-content of that 

 at the cathode is diminished. The quantity of salt which a 

 constant current of intensity I, through the processes of electro- 

 lysis and the migration of the ions, brings during the time t 

 to the boundary of the anode, and carries away from the 

 boundary at the cathode, is, according to Hittorf s comprehen- 

 sive investigations, 



2-48 (l-n)el*, 



where n denotes the transport-number and e the electrochemical 

 equivalent of zinc. It follows from Hittorf s measurements 

 that n for high concentrations, and within a not too great 



