58 



Conductivities and Viscosities in Pure and in Mixed Solvents. 



Table 30 shows that in a pure aqueous solution of methyl orange 

 there is no appreciable hydrolysis, and that practically all of the indi- 

 cator exists as the azo-base. It is, therefore, not necessary to add alkali 

 in determining the value of (I/Io)' in equation 19. The volume of all 

 solutions used in table 30 was 100 c.c. and each solution contained the 

 same amount of methyl orange. 



TABLE 30. 

 [I/Io for depth of solution = 20 mm.] 



Table 31 gives results for three solutions of methyl orange, each con- 

 taining an excess of acid and the same amount of indicator diluted to 

 100 c.c. It shows that 0.2 c.c. of the acid is sufficient to convert all of 

 the azo-base into the quinoid salt and completely to suppress hydrolysis. 



TABLE 31. 

 [I/Io for depth of solution = 20 mm.] 



Stable solutions of methyl orange are given only when the concen- 

 tration is less than 2 X 10~ 4 gram-molecules per liter. Considerable 

 difficulty was encountered during the preliminary work, owing to the 

 fact that the indicator solutions were more concentrated than the above- 

 named limit. Such solutions of methyl orange, especially those con- 

 taining a large excess of acid and also those with enough acid to give 

 an intense red color, gradually become more transparent on standing. 

 The action of light very greatly accelerates this bleaching process. 

 The solutions most susceptible to the action of light and of time are 

 those containing the greatest amounts of acid. It was found that these 

 solutions could be kept in the dark for 4 or 5 hours unchanged, and that 

 bleaching again took place as soon as they were exposed to the light. 

 Experiments were made which showed that light of the shorter wave- 

 lengths was more actinic in increasing the transparency of the solutions 



