56 president's address— section b. 



Only three solvents have been examined in any detail from this 

 point of view, namely — water, ammonia and sulphur dioxide, and 

 we will consider very briefly the behaviour of solutions in each of 

 these solvents. 



Taking first the case of a solution in water, it is well known 

 that the following changes occur during electrolysis : — 

 {a) At the Cathode 



(1) If the solute is a salt of an alkali, as, for example, 



potassium iodide, hydrogen is liberated and an 

 alkahne hydroxide is formed. 



(2) If a salt of a heavy metal, the metal is deposited on 



the cathode. 

 (b) At the Anode : 



(1) If the solute is a halogen salt the halogen may be 



liberated, or the anode may be dissolved ; or, 



(2) If the solute is an alkali or any one of certain oxygen 



salts, oxygen may be liberated. 

 ~ The liberation of the hydrogen and oxygen is interpreted by 

 chemists as affording evidence of the occurrence of these two 

 substances in the ionic condition in the solution, and their actual 

 concentration has been measured with a considerable degree of 

 accuracy. 



The formation of a caustic potash at the cathode is explained 

 as follows : — Water is ionised to form H- and OH . The H- are 

 discharged and escape and the OH accumulate and partner theK- 

 which are constantly arriving in the neighbourhood of the cathode. 



Compare this with the behaviour of a solution of potassium 

 iodide in ammonia when such a solution is ionised. In this case it 

 is found that — 



(a) at the cathode 



(1) hydrogen is liberated and 



(2) potassamide (K NHo) is formed ; 



(b) at the anode, iodine derivatives of ammonia are formed. 

 If these results are interpreted analogously to the corresponding 



results in water, we must conclude that ammonia is ionised to form 

 H- and NH2 , and that the latter accumulate in the neighbourhood 

 of the cathode and partner the K- which are constantly arriving 

 under the directive influence of the current. 



This would lead to the conclusion that a metallic amide in 

 ammonia solution would correspond to a metallic hydroxide in 

 aqueous solution, a conclusion which has been verified in many 

 ways. Thus a solution of phenolphthalein in dry ammonia is 

 colourless, and is not turned red by an alkah, but is so turned by 

 an amide of one of the alkali metals. 



Similarly an ammonium salt is found to behave as an acid — 

 a fact which is explained at once if we consider the relation between 

 the ammonium salt NH4 Br, and the hypothetical oxonium salt 

 OH3 Br, which is probably formed when hydrogen bromide is 

 dissolved in water. 



