Radiometric Measurements of Constants of Indicators. 75 



If the quantity of disodium phosphate in the solutions investigated 

 is always kept the same, the hydrogen ion concentration can be varied 

 simply by the addition of different amounts of hydrochloric acid. In 

 this case the total salt concentration is constant. This is extremely 

 desirable, as it has been shown by Rosenstein 1 that neutral salts have 

 a great effect upon the fraction of the indicator transformed. The 

 value of the ionization constant in the case of phenolphthalein is 

 doubled by increasing the total salt concentration from 0.03 to 0.40 

 normal. 



PRELIMINARY WORK ON ROSOLIC ACID. 



Three stock solutions were prepared, all solutions being made up 

 at 20 with conductivity water. The stock solution of disodium phos- 

 phate was prepared from a pure sample obtained from Kahlbaum. Its 

 concentration, 0.1036 gram-molecules per liter, was determined gravi- 

 metrically as magnesium pyrophosphate. The concentration of the 

 stock solution of hydrochloric acid was 0.08085 normal. The stock 

 indicator solution was prepared by dissolving about 0.4 gm. of an 

 excellent sample of rosolic acid obtained from Merck in 2 liters of 

 conductivity water. The total quantity of the indicator did not 

 dissolve, but, as has been shown, it is not necessary to know the concen- 

 tration of the indicator employed. 



The incompletely transformed solutions to be tested were prepared 

 from the stock solutions, so that all contained the same amounts of 

 indicator and disodium phosphate, but different amounts of hydro- 

 chloric acid. This procedure was followed to keep the total salt con- 

 centration the same in all solutions. The volume of each solution 

 was 100 c.c. The percentage transmissions (I/Io), were taken with 

 a 20 mm. depth of each solution, and for the same 5 wave-lengths of 

 light. As explained in the original article, the percentage transmissions 

 were determined by a differential method, which avoided the necessity 

 of introducing certain correction factors due to the glass ends with 

 which the cells were provided. 



A consideration of equation 6 will show that the method of calcu- 

 lating the ratio of the red to the yellow component of any incompletely 

 transformed indicator solution, not only depends upon the transmission 

 of the solution under investigation, but also upon the transmission of 

 an indicator solution containing a large excess of acid, in which the 

 indicator is totally transformed into its yellow constituent; and also 

 upon the transmission of an indicator solution having a large excess 

 of alkali in which the indicator is totally transformed into its red com- 

 ponent. Table 47 gives the results of a series of measurements made 

 upon the indicator solution which has an excess of alkali. All solu- 

 tions contain 50 c.c. of the stock solution of rosolic acid plus the 



Abbott and Bray: Journ- Amer. Chem. Soc., 34, 1128 (1912). See also: Ibid., 37, 804 (1915) . 



