( 167 ) 



hydrolic phenomenon, which plays its part. If we liave a sohition 

 containing acid colours, whose anions possess a different colonr from the 

 nndissociated acid, the solution will exhibit a mixed colour composed 

 of the colour of the anions and that of the undivided acid. This 

 phenomenon may be readily demonstrated by means of the acids of 

 the following colouring matters : Methylorange, metanilyellow and hen- 

 zoimrpurin 4 B. 



That the change in colour, whicli these acids undergo wlien their 

 solution is diluted, must really be explained in this manner, is proved 

 in the second part of this paper, where an application is made of 

 the fact that such dilute solutions may be restored to their original 

 colour by addition of dilute acids ^). 



This explanation disposes of the theory of Küster ^) and of that 

 of Glaser ') as to the indicator methylorange and it appears indeed 

 that the raethylorange-acid is, for an indicator, a fairly strong acid. 



This phenomenon also occurs with salts of acid colours, therefore 

 when testing the so-called acid and directly-dyeing technical colours. 



Such a case has been mentioned by C. H. Sluiter ■*), who noticed 

 it when testing solutions of isonitrosoacetoplienonsodium. He found 

 that these solutions assumed an increasing yellow colour on increased 

 dilution and he rightly attributes this to the more powerful electro- 

 lytic dissociation caused by the dilution. In this case the ionisation 

 in iV/10 solutions had proceeded so far that a further dilution caused 

 no further visible change in colour. 



If however we want to measure so\y\{\o\\?>oïhenzo-pure-blue,benzo- 

 azurin or allied colouring matters, it will be noticed that in solutions 

 containing from 0.1 — 0.05 gram in a Liter (approximately ^/3000 — 

 iV76000)the phenomenon is still of such an interfering nature, owing 

 to the great difference in shade of colour, that a direct measurement 

 is impossible. Sluiter's dissertation only reached me when my researches 

 had already been brought to a close. 



In theory, analogous phenomena are possible with basis colours 

 and their salts, but I have not as yet met with any sucli instances 

 and in fact, have not searched for them. 



When salts of very weak acid colours are tested, the hydrolysis proves 

 very troublesome, if the colour of the anions and that of the acids 



1) Compare A. A. Noyes and A. A. Blanghard Journ. Americ. Ghem. Soc. 22 

 p. 726 and Central Blatt 1901. I. p. 11 n» 15. 



2) Küster, Zeitschrift fur Anorganische Chemie 8 p. 127. 



3) Glaser, die Indicatoren. 



*) G. H. Sluiter. Het mechanisme van eenige organische reacties. Academisch 

 Proefschrift. Scheltema en Holkeraa. 1905. 



