and phenol or a phenol derivative. These compounds, altho some- 

 times behaving as dyes, are not used as dyes or stains, but as in- 

 dicators. For this purpose the members of the group are very 

 valuable. 



Phenolphthalein, altho not used as a dye, is colored and is ap- 

 parently capable of salt formation. In acid solutions it is colorless, 

 and is assumed to have the formula : 



HO OH 



o 



I I c = o 



\/ 



Upon neutralization the alkali is believed to attach itself to the 

 CO-group, which breaks the five-sided ring (the lactone ring) and 

 causes one of the benzene rings to take on quinoid form, thus: 



H0_/\ /\_0 



I l_ =1 I 

 \/ C \/ 



_ COOXa 



\/ 



With this change, the red color of the compound appears, but dis- 

 appears again if the solution is made acid so as to destroy the quin- 

 oid structure. This makes the compound a very valuable indi- 

 cator. 



The sulphonphthaleins are not commercial dyes, altho capable 

 of acting as acid dyes. Their real value is as indicators. Quite a 

 long series of them has been prepared, which in general show their 

 deepest color in alkaline solutions and turn yellow on the addition 

 of acid. Fortunatelv nearlv everv one of them has a different 



*/ t t-' 



point in the hydrogen-ion scale at which its color change begins, so 

 that between them they indicate very accurately the hydrogen- 

 ion concentration of solutions of any reaction ordinarily encoun- 

 tered. Some of them, such as thymol-sulphonphthalein (thymol 

 blue), show two colors besides yellow, one in strong acid solutions 

 and the other in strong basic solutions, while in solutions near the 

 neutral point they are yellow. That these color changes are due to 

 alterations in the structure of the molecule, such as the disap- 

 pearance and reappearance of the quinoid ring, is generally as- 

 sumed; but in the case of these compounds the relation of structure 

 to color is complicated and has not yet been worked out to general 

 satisfaction. 



84 



