CHAPTER III. 



RADIOMETRIC MEASUREMENTS OF THE IONIZATION CONSTANTS OF 



INDICATORS. I. 



BY E. J. SHAEFFER AND M. G. PAULTJS. 



The radiometric measurements recorded in this paper were made 

 with a very sensitive radiomicrometer and a grating spectroscope. 

 The spectroscope is well known to the chemist, but its use in connection 

 with the radiomicrometer has not, as yet, found an extensive applica- 

 tion to the study of chemical problems. This is perhaps due in part to 

 the difficulty in constructing a sensitive and easily controlled radio- 

 micrometer. By means of the grating spectroscope and radiomicrom- 

 eter, it is possible to study quantitatively chemical reactions involving 

 color changes, and even those not involving such changes, if there are 

 absorption bands in the invisible regions of the spectrum. It will 

 be seen that accurate determinations of very small concentrations of 

 colored components in solutions can be made very rapidly even when 

 two or more such components are present. The structure of the solvent 

 bands and time reactions are among the many other important chem- 

 ical problems which may be investigated by means of this radiometric 

 apparatus. 



PURPOSE OF THIS INVESTIGATION. 



In connection with some problems now under investigation in this 

 laboratory, it was desired to secure accurate measurements of the 

 hydrolysis constants of certain salts. The spectroscope and radio- 

 micrometer suggested the indicator method, which is based on the 

 changes in the transmission of light by solutions of indicators when 

 varying amounts of hydrogen or hydroxyl ions are present. It will be 

 shown that by means of these changes the concentrations of very small 

 amounts of hydrogen and hydroxyl ions can be determined. The 

 method obviously involves a knowledge of the ionization constant of 

 the indicator as an acid or as a base. A comparison of the ionization 

 constants of the indicators obtained by several investigators, showed 

 that the recorded values varied widely, not only with different methods, 

 but with the same method. The results for phenolphthalein, in partic- 

 ular, indicate how inaccurate is our knowledge as to the ionization 

 constant of that important indicator. Salm 1 gives the value SX 10~ 10 ; 

 McCoy, 2 0.8X10" 10 . In many cases the results of an individual 

 investigation vary as much as 300 to 400 per cent. A. A. Noyes 3 



it. phys. Chem., 57, 492 (1907). 3 Journ. Amer. Chem. Soc., 32, 858 (1910). 



2 Amer. Chem. Journ., 31, 503 (1904). 



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