CHAPTER IV. 



RADIOMETRIC MEASUREMENTS OF THE IONIZATION CONSTANTS OF 



INDICATORS. II. 



BY M. G. PAULUS AND J. F. HUTCHINSON. 



An investigation of the ionization constants of methyl orange and 

 phenolphthalein has already been published by Shaeffer, Paulus, and 

 Jones. 1 A new method, based upon the absorption of light by solutions 

 of indicators, was developed for the determination of the constants of 

 indicators, and is now presented. It was shown that this method 

 serves as well for a two-colored as for a one-colored indicator. The 

 work recorded herein is to be regarded as a continuation of the original 

 investigation, and the purpose is to test the applicability of the method 

 to the determination of the ionization constant of rosolic acid. A 

 description of the apparatus used has already been given in detail in 

 the original paper. 



THEORETICAL DISCUSSION. 



Considering first of all that rosolic acid is monobasic, 2 the ionization 

 constant K ( is expressed by the simple equilibrium equation 



(H+) X (IE) 



(HIn) 



= K, (1) 



If, then, the hydrogen ion concentration of the indicator solution is 

 fixed, the ratio (In) /(HIn) at equilibrium can be determined. It 

 has been shown that the percentage transmission of a solution, such 

 as that of rosolic acid, containing two absorbing components is given 

 by the equation 



= -Kc-K / c 1 (2) 



where c and c\ are the concentrations of the two absorbing components, 

 and K and K' are constants depending upon the nature of the absorbing 

 components and the wave-length of light employed. Applying this 

 equation to rosolic acid, let c represent the concentration of the red 

 component, or (In) in equation 1, and let ci represent the concentration 

 of the yellow component or (HIn). Equation 2, then, will represent 

 the percentage transmission for some given depth of an incompletely 

 transformed solution of rosolic acid. In a solution containing a large 

 excess of acid c = 0, and equation 2 becomes 



^-K'c^-K'T (3) 



Mourn. Amer. Chem. Soc., 37, 776 (1915). 



2 The behavior of rosolic acid as a dibasic acid will be discussed later. 



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