Radiometric Measurements of Constants of Indicators. 



77 



considerably higher than those recorded in columns 8 and 9, for a solu- 

 tion containing 0.5 c.c. N/lNaOH. In both cases the solutions have 

 come to equilibrium, since it has been shown that equilibrium is estab- 

 lished after the solutions have stood between 1 and 5 hours. When 

 very much larger amounts of alkali are added (say 10 c.c.), a very 

 perceptible bleaching takes place. In solutions, then, containing an 

 excess of sodium hydroxide, two opposing reactions take place; first, a 

 gradual intensification of the red color brought about very probably 

 by a time reaction between the metallic phenolate and the quinoid 

 complex; and second, a bleaching of the red color which is greater 

 the larger the amount of the alkali added. Since it is necessary to 

 know the true percentage transmission (I/I )" of a solution completely 

 transformed into the red component, in order to determine the 

 ratio G/CI, it follows that the bleaching must be avoided. This result 

 can be obtained by keeping the concentration of the alkali as small 

 as possible, only adding sufficient to transform completely the indi- 

 cator. The obvious method was to increase gradually the hydroxyl 

 ion concentration until the solutions were shown to be completely 

 transformed. Table 48 gives the results of such a determination. All 

 solutions contain 50 c.c. of the stock solution of indicator plus the 

 amount of disodium phosphate and hydrochloric acid indicated in 

 the table. All solutions were allowed to stand a sufficient length of 

 time for equilibrium to be established. 



TABLE 48. 

 f(I/Io)" -depth of solution = 20 mm.] 



Solution 1, table 48, in which the hydrogen ion concentration is 

 0.8266 X10~ 8 , gives higher values for the percentage transmissions than 

 solutions 2 and 3, in which the hydrogen ion concentrations are, respec- 

 tively, 0.4157 X10~ 8 and 0.043 XlO" 8 . This shows that solution 1 is 

 not completely transformed, since an increase in the red component 

 and a corresponding decrease in the yellow component make the solu- 

 tion more opaque to yellow fight. Solution 2 must, however, be com- 

 pletely transformed, since, when the hydrogen ion concentration is still 

 further decreased, as is the case in solution 3, the transmission values 

 remain the same. The values of the percentage transmissions recorded 

 in columns 2, 3, and 4 are the values to be subsituted for (I/Io)" in 

 equation 6. 



