312 A Suggested Theory of the Aluminium Anode. 



being- used. The intervals of time required for the evolution 

 of 50 c.e. of hydrogen were 



(1) GO c.c. acid. 180 minutes, diminishing to 150 minutes. 



(2) 60 c.c. acid -1-1 c.c. KC1. 70 minutes, diminishing to 



50 minutes. 



(3) 60 c.c. acid + 2 c.c. KC1. 2 minutes. 



(4) 60 c.c. acid + 4 c.c. KC1. 2\5 minutes, diminishing to 



1*7 minute. 



These figures show that chloride has a very decided 

 influence on the velocity of the reaction, but it does not seem 

 to play the part of a simple catalysator. One noticeable 

 feature is that when 2 c.c. of the solution have been added, 

 its maximum effect has been almost reached. 



The action may be explained as follows — when a piece of 

 aluminium is put into dilute sulphuric acid, it is covered with 

 a film of hydroxide ; this film, being impermeable to SO/ 

 ions, is impermeable to H* ions also, for the one ion cannot 

 go anywhere without the other. Thus there is no action 

 between the metal and the acid. The aluminium, however, 

 acts slowly on the water in the film, forming hydrogen and 

 aluminium hydroxide, which maintains the continuity of the 

 film. In this way a slow continuous action takes place. If 

 potassium chloride be added to the acid, H ■ ions can permeate 

 the membrane, for the CI' ions can go with them, and the 

 metal thus comes in contact with H* ions. This action 

 breaks up the film and so admits the sulphuric acid to the 

 surface of the metal. 



If the concentration of CV ions is too small to cause violent 

 action and so destroy the film, they will still have an accele- 

 rating influence, but the maximum effect will not be attained. 



Bromide. — Addition of bromide appears to have very little 

 influence on the velocity of reaction. Four experiments 

 were made, a 4-molar solution of potassium bromide being- 

 used. The times of evolution of 50 c.c. of hydrogen were 



(1) 60 c.c. acid 150 minutes. 



(2) 60 c.c. acid + 2 c.c. KBr. 160 „ 



(3) 60 c.c. acid + 4 c.c. KBr. 155 



(4) 60 c.c. acid (no. 1) + 1 c.c. KBr. 140 „ 



The fourth experiment was made with the aluminium and 

 acid already used in experiment (1), in order to remove 

 uncertainty as to the uniformity of the surface. Hence the 

 influence of bromide is very slight and the experiments are 

 not sufficient to show whether it accelerates or retards the 

 reaction. 



