198 Biological Stains 



or rather of phthalic anhydride: 



CO 



\ 

 O 



/ 

 CO 



with phenol or a phenol derivative. If phthalic acid is heated 

 with phenol and sulfuric acid it combines with two molecules of 

 the phenol and forms phenolphthalein. In the same way, a sul- 

 fonphthalein is a compound of ortho-sulfo-benzoic acid: 



/\_S03-H 



"COOH 



and phenol or a phenol derivative. These compounds, although 

 sometimes behaving as dyes, are not used as dyes or stains, but as 

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

 valuable. 



Phenolphthalein, although 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 



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: 



HO /\ /\—0 



COONa 



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. 



