Section III., 1904 [ 41 "I Tbans. R. S. C 



VI. — Phenol- pliihalein and the Theory of Indicators} 

 By D. McIntosh. 



(Presented by Prof. B. J. Harrington, and read June 22, 1904). 



When an alkali is added to a solution of phenol-phthalein, the pro- 

 duction of the deep red colour may be explained in two ways. 



The first, due to Ostwald,- accepts phenol-phthalein as an extremely 

 weak acid, and consequently but little dissociated. On the addition of 

 an alkali a salt is 'formed which is largely dissociated, and the red 

 colour is due to the negatively charged ion. 



The second explanation may be called a chemical one, and is based 

 on the change of constitution when phenol-phthalein is made alkaline. 

 The phenol-phthalein is changed from a lactone. 



CgH, < ^ (^^^^^)2 ^ O, to the salt of a carboyl acid, 



(MOOC CgH,) (HOUgHJ C: CJI, : O; and to the quinoid complex 

 C rCgH^ :0 the red colour is ascribed. 



This view seems the more likely one from the standpoint of organic 

 chemistry. In the organic dyes, colour is associated with a certain 

 chromophoric group, and disappearance of this group means disappear- 

 ance of colour. Indeed Nietski and Burckhart ^ have prepared coloured 

 ethers of tetra-bromphenol-phthalein which are necessarily non-ionized 

 and which contain the quinoid group, and colourless isomeric lactoid 

 ethers, without this chromophoric group. 



Strongly supporting the " chromophoric " theory is the fact that 

 the addition of a large amount of salt to an alkaline solution of phenol- 

 phthalein causes the colour to gradually fade. While this may 

 be due to a change in dissociation of the salt of phenol-phthalein, i.e., 

 to the disappearance of the negative ion, the slow colour change in- 

 dicates rather a chemical change. 



I have at various times carried out some experiments with phenol- 

 phthalein, with the object of testing the above theories: and wliile these 

 experiments have led to no definite results, they are, I believe, new and 

 may be of interest. 



If solutions of phenol-phthalein and sodium hydroxide in alcohol 

 are mixed, a red colour is produced. On warming, the colour becomes 



' Stieglitz. Amer. Chem. Soc., 25, 1112 (1903). 

 * Ostwald. Lehrbuch der allgemeinen Chemie. 

 » Ber. der Chem. Ge.s. 30, 175 (1897). 



