410 M. C. Lea — Color Relations of 



tion still fully remaining in the dry product. Now the potash 

 compound of phenol phthaleine is known to be anhydrous. 

 Anhydrous substances thoroughly desiccated cannot' be in a 

 dissociated state and, therefore, the rose red-color of the potash 

 compound is not due to dissociation. 



Further, if to the potash solution a solution of a silver salt 

 is added there is formed a silver compound having the same 

 constitution as that of the potassium compound. This silver 

 compound is anhydrous and insoluble, nevertheless it has the 

 same color as the potassium compound under conditions which 

 make dissociation to be out of the question. 



Paranitrophenol. — This is another acid indicator. In a solid 

 state it is nearly colorless ; it dissolves in warm water to a 

 nearly colorless solution which on the addition of an alkali 

 becomes gold-yellow. A portion of this solution evaporated 

 so as to leave a thin film exhibits the same intensity of color as 

 the solution, which color it retains after thorough desiccation. 

 The color of the solution is, therefore, not due to dissociation. 

 The solution'of the substance itself when largely diluted gives 

 no indication of the dissociation of a colored ion. The be- 

 havior of phenacetoline, another acid indicator, is analogous. 



It seems, therefore, that dissociation has no essential connec- 

 tion with the reactions of acid indicators. It is simply that 

 these substances by combining with alkalies either have their 

 color much intensified or in some cases change it altogether. 



Such changes are not in any way confined to acid indicators 

 so-called. Picric acid in combining with alkalies has its color 

 much intensified ; the acid itself has a pale yellow color : the 

 sodium salt (dry) has a pure deep yellow color. Chrysammic 

 acid affords an example of a complete change of color ; the 

 acid itself is yellow, the potash salt a deep blood red, not only 

 in solution but in the dry state so that its color cannot be due 

 to dissociation. It may be remarked also that the potash salt 

 in its dry state has remarkable optical properties which were 

 investigated by Brewster, properties not possessed by the acid. 



To the reactions which have been here examined might be 

 added those of methyl orange, rosolic acid, and other acid 

 indicators. I have found no exception to the rule that the' 

 characteristic colors exhibited by these substances when placed 

 in contact with alkalies, are retained after a thorough desicca- 

 tion. Also, no acid indicator when dissociated by a large dilu- 

 tion shows any tendency to exhibit an anion similar in color to 

 that of its alkaline compound. 



