42 Conductivities and Viscosities in Pure and in Mixed Solvents. 



has applied the theory of indicators to volumetric analysis, making use 

 of the ionization constants of the various indicators. He expresses 

 the opinion that our knowledge of the ionization constants of the 

 indicators is for the most part inexact and needs to be supplemented 

 by further careful investigation. Stieglitz, 1 McCoy, 2 and Salm have 

 also recognized the importance of such a study. McCoy, Salm, and 

 in particular Noyes and Bjerrum, 3 have already applied the indicator 

 constants to some of the more important problems which the chemist 

 must face in titrating w r eak acids and bases; and they have shown in 

 certain cases the extent of the error involved in such titrations. It will 

 then be readily seen that it is important to know the ionization con- 

 stants of the more important indicators, especially of methyl orange 

 and phenolphthalein. In view of these facts, and the value of such 

 ionization constants in many other lines of work involving the use of 

 indicators, it was decided to make a radiometric investigation of this 

 problem. Methyl orange was first studied, and a method was devel- 

 oped through the application of Beer's law, w r hich readily gave the 

 concentration of the yellow azo-base and that of the red quinoid ion. 

 Knowing these values and the hydrogen ion concentration, the hydroly- 

 sis constant of methyl orange can be readily calculated. The method 

 employed is different from, and was worked out independently of any 

 other method thus far used in investigating this problem. 



HISTORICAL. 



The results of a number of preliminary investigations on the ioni- 

 zation constants of various indicators were published in 1904. McCoy, 

 like most of the other investigators who studied this problem, worked 

 calorimetrically, the intensity of the color being judged by the eye. 

 McCoy employed Nessler tubes for this purpose, while others made use 

 of special calorimeters to determine the amount of indicator trans- 

 formed into its salt by varying amounts of hydrogen or hydroxyl ions. 

 Tables showing roughly the hydrogen ion concentration at which a 

 large number of indicators undergo change in color, have been pub- 

 lished by Friedenthal, 4 Salessky, 5 Fels, 6 and Salm. 7 Nearly all of the 

 early investigators utilized the principle that the ionization constant 

 of an indicator is equal to the hydrogen ion concentration at which it 

 is one-half transformed into its salt. (See Salm and A. A. Noyes.) 

 Salessky, and later Salm, determined the concentration of the hydrogen 

 ions at this point by means of the hydrogen electrode. Criticisms 

 showing how inexact most of these methods are will be found in the 

 separate articles. Salm, 8 referring to the work previous to 1907, says 



'Journ. Amer. Chem. Soc., 25, 1126 (1903). b lbid., 10, 204 (1904). 



2 Amer. Chem. Journ., 31, 503 (1904). Ibid., 10, 208 (1904). 



8 Ahr. Versamm., 21, 1 (1914). Ubid., 10, 344 (1904). 



4 Zeit. Elektrochem., 10, 113 (1904). "Zeit. phyg . Chem., 57, 490 (1907). 



