36 



HYDROGEN ION CONCENTRATION 



is inversely proportional to the hydrion concentration.-^ Since the 

 concentration of the sugar ions must be also inversely proportional 

 to the [H+], it may then be surmised that the transformation of the 

 sugar must occur through its ions. Furthermore, since, as was shown 

 above, this transformation must occur by means of the enolic form, 

 it follows that the ions must be in an enolic form, and that there 

 cannot be any enolic sugar present in this process beside the ions. 

 In other words: Sugar is a dissociable acid only in its enolic form, 

 and in this form it is a quite strong acid. This is entirely plausible, 

 since all enols are strong acids, while their corresponding aldehydes 

 or ketones are not. And the sugar is a weak acid only because its 

 only modification which possesses acid properties is present in such 

 small amounts in comparison with its other chemical forms. 



Many analogous cases are met with among the indicators. Phenol- 

 phthalein has an apparently very low dissociation constant. In 

 the free state it has the constitution shown in I below. This form is 

 tautomeric with the form shown in II. 



OH 



HO 



/^ 



A^ 



o 



I 



C .OH 



II 



The form I is colorless, while the tautomer II because of its 

 quinonoid double bond is colored, and because of its carboxyl group 

 strongly acidic. The latter form cannot exist in appreciable amounts 

 together with the colorless isomer and only its ions may so occur, 

 and the constitution of these ions may only be the one shown by 

 formula II. Thus the colorless phenolphthalein I is considered as a 

 very weak acid whose ions are colored red, only because in its solu- 

 tions modification II, which is a strong acid, is present in minute 

 concentrations. All color changes of the indicators depend upon 



28 L. Michaelis and P. Roaa, Biochem. Zeitschr. 47, 447 (1912). 



