Manchester Memoirs, Vol. Hit. (1909), No. 18. 



XVIII. On some Colour Demonstrations of the 

 Dissociating Action of Water. 



By R. L. Taylor, F.I.C. 



(Received and read April 6th, rgog.) 



It is now almost universally recognised that the 

 Theory of Ionic Dissociation affords the most satisfactory 

 explanation of many otherwise inexplicable phenomena 

 which accompany the solution of most acids, bases, and 

 salts in water. For example, there are a considerable 

 number of remarkable facts connected with the colours 

 of various solutions of salts in water, which find the most 

 reasonable explanation in this theory. 



Thus it was long ago shown by Ostwald that the 

 colours of dilute solutions of all permanganates were the 

 same, no matter what the bases were. This is exactly 

 what one would expect from the theory. All permanga- 

 nates when in dilute solution are almost completely disso- 

 ciated, and the negative ion is always Mn04, which is a 

 purple coloured ion. The colour of a dilute solution of a 

 permanganate, then, is always the colour of the Mn04 ion, 

 and is therefore always the same. Similarly, the colour 

 of the negative ion of chromates (Cr04) is yellow, and 

 therefore dilute solutions of chromates all have the same 

 colour, no matter what the bases are, unless they are 

 coloured as well. The blue colour of solutions of copper 

 salts also is recognised as due to the positive copper ion. 

 Copper chloride, however, both solid and in strong solu- 

 tion, is green, and copper bromide is dark brown, — almost 

 black. These are the colours of the coviponnds ; but when 

 they are diluted with water they both turn blue ; — they 



May 2^lh, igog. 



