Radiometric Measurements of Constants of Indicators. 43 



that the investigations have been for the most part qualitative, and that 

 with few exceptions the dissociation constants of the indicators are 

 still unknown. 



Salm made a more careful study, employing a satisfactory calori- 

 meter to determine when the indicator was one-half transformed into 

 its salt, and like Salessky obtained the hydrogen ion concentration 

 at this point by means of the hydrogen electrode. His results with 

 phenolphthalein illustrate the uncertainty of the values found by his 

 method. Wegscheider 1 proceeded in much the same manner as the 

 others who have used the colorimetric method, and obtained con- 

 stants for the ionization of phenolphthalein which seem to be in fair 

 agreement with those found by Hildebrand. 2 Hildebrand's method, 

 involving the use of the spectro-photometer for the estimation of color 

 intensities, is the most exact of all the methods previously employed. 

 His photometric method somewhat resembles the radiometric method 

 which we employed. Rosenstein, 3 following essentially the procedure 

 of McCoy, has carried out very carefully a colorimetric investigation of 

 the ionization constant of phenolphthalein, and the effect upon it of 

 neutral salts. He employed the Duboscq type of colorimeter to deter- 

 mine the fraction of the indicator transformed by a known hydrogen 

 ion concentration. As he shows, the ionization constant is equal to 

 the hydrogen ion concentration multiplied by the fraction of the indi- 

 cator transformed into its salt. When the indicator is a fairly strong 

 electrolyte, e. g., p-nitrophenol, its dissociation constant was deter- 

 mined by the conductivity method. 



Before we can judge of the absolute value of the results obtained in 

 any radiometric investigation, it is necessary that the radiometric 

 instrument and the other parts of the apparatus used in connection 

 with it, which fulfill several requirements, be taken up in the discus- 

 sion of the apparatus which follows. In view of the fact that radio- 

 metric apparatus has not thus far been extensively used by chemists, 

 it seems desirable that this be discussed in some detail. 



Photographs of the assembled apparatus and also of the various 

 parts are given in plate 1, figure 2, and plates 2 and 3 show the assem- 

 bled apparatus. The long box to the left (plate 1, fig. 2) incloses the 

 spectroscope. The standard which holds the Nernst glower, lenses, 

 sliding carriage, etc., occupies the center, and the tall box to the right 

 incloses the radiomicrometer. Below the standard is shown the rheostat 

 ammeter, etc., for controlling the current supplied to the Nernst glower. 

 Plate 2, figure 1, is a photograph of the apparatus with the covers 

 removed from the spectroscope and the radiomicrometer. To the 

 extreme left is shown the grating and a 4-inch Brashear lens, and at 

 the extreme right is the tube containing the radiomicrometer with the 



eit. Elektrochem., 14, 510 (1908). z lbid., 14, 351 (1908). 



3 Journ. Amer. Chem. Soc., 34, 1117 (1912). 



