COLOUR AND CONSTITUTION 657 



been shown that, in general, salt formation in the auxochrome 

 lessens the colour considerably, whilst a similar combination in 

 the chromophore deepens it. 1 As examples of the former class 

 may be mentioned the nitranilines, aminocoumaric acids, amino- 

 anthraquinone, etc. OrZ/zo-aminobenzophenone is deep yellow, 

 its sulphate and hydrochloride, on the other hand, colourless. 

 The deepening of the colour on chromophoric combination with 

 acids is evidenced by the acridines, the quinoxalines, 2 and other 

 compounds, which though themselves colourless give coloured 

 salts. Phenazine again gives red salts, although the base itself 

 is only yellow-coloured. Most of these substances will be 

 mentioned again later. 



When salt formation is due to combination with a " colour- 

 less " metal, the effects produced are reversed. In the auxo- 

 chrome a most marked deepening of the colour is noticed ; in 

 some cases indeed the colour is first called forth by salt forma- 

 tion. The nitrophenols, the phthaleins, the oxyazobenzenes 

 and many triphenylmethane derivatives are all examples of 

 this class of compounds. Very few examples are known of 

 metallic salts produced directly from chromophoric groups, but 

 those examined all show that the colour tends to disappear. 

 Thus oxytoluquinoxaline carboxylic acid is yellow, but gives 

 colourless ammonium, potassium, and sodium salts : 



/ N \CC00H 

 C 7 H 6 < I 



\ N /C-OH 



The exponents of Witt's chromophore-auxochromic theory 

 have endeavoured to embody these results in several general 

 statements, and regard the influence of salt formation as being 

 merely passive in character, simply changing the state of strain 

 in the compound, and therefore also the colour either positively 

 or negatively, according to the position of the salt-producing 

 groups. 



Any chemical explanation of the chromophore-auxochrome 

 theory, which as generally accepted is really less a theory 

 than a well-chosen statement of facts, has been left to quite 

 recent years. It has been the endeavour of Kauffmann 3 to 

 show that the benzene ring alters its constitution in its different 



1 Kauffmann and Beisswenger, Ber. d. d. Chem. Ges. 36, 561 (1903). 



2 Hinsberg, Ann. d. Chem. 237, 327. 



3 Ber. d. d. Chem. Ges. 33, 1725 (1900) ; 34, 682 (1901) ; 35, 3668 (1902). 



