DETERMINATION OF ACIDITY AND ALKALINITY 97 



has long ago established the theory, which in its essentials is still 

 valid, and according to which this change is explained by the assump- 

 tion that the ions of a dye-stuff possess a color different from that of 

 its undissociated molecule. An apparent objection to this theory 

 arose from the work of Hantzsch' who showed indicators to be pseudo- 

 acids and bases, and according to whom the change of color is not 

 due to ionization but to tautomeric rearrangement. But since these 

 two processes are inseparably related to each other, no real contra- 

 diction can be found in these two theories. The theory of Hantzsch 

 serves only to elucidate further the mechanism by which the color 

 changes of indicators occur. We may even go further in the same 

 direction. In accord with the conception of the atomic model the 

 configuration of the atoms in the molecule is determined by electro- 

 static forces. It is, therefore, not surprising that the appearance of a 

 free charge, as it occurs in the process of ion formation, may recast 

 the configuration of the atoms in the molecule. Thus we can still 

 retain the older conception and state that an indicator is a substance 

 which is characterized by a difference in color between its unionized 

 and its ionized states. 



For practical purposes two groups of indicators are recognized: 

 the one-color indicators which change from a colorless to a colored 

 state, as phenolphthalein, and the two-color indicators, such as lit- 

 mus, w^hich change from one color to another. It is more convenient 

 in the development of the theory to begin with the one-color indica- 

 tors. Let us choose, as an example, p-nitrophenol. This is an acid 

 whose dissociation constant, kg = 6.5 X 10~^, or, pk = 7.19. The 

 free acid is colorless, while its ions are yellow. Therefore at any 

 given [H+J a part of this dye is present as the colorless free acid and 

 another part in the fonn of yellow colored ions. The proportion of 

 the ions to the total amount of indicator in solution is its degree of 

 dissociation, or a. This latter value can be quite easily determined 

 experimentally. Thus we may place the same amount of indicator 

 into a second vessel and render it strongly alkaline with sodium 

 hydroxide in which case the indicator will be completely dissociated 

 and show its maximum color intensity. If now we compare colori- 

 metrically the color intensity of the first solution with that of the 

 second, and if this numerical relation of the concentration of the 

 indicator be designated as the "degree of color," or, as F, then F = a. 



2 H. Hantzsch, Ber. deut. chem. Ges. 46, 1537 (1913); ibid. 48, 158 (1915). 



