DETERMINATION OF II-ION CONCENTRATION 591 



With brom thymol blue: Tlic acid tube contains 10 co. of 0.1 

 per cent concentrated HCl and the alkahne tube N/200 NaOH 

 solution. 



The acid and alkaline tubes of methyl red, brom cresol piirpk, 

 brom thymol blue, and cresol red, are made with 0.1 per cent 

 concentrated HCl and N/20 NaOH, respectively. To each, of 

 course is added 0.8 cc. of the respective indicator solution, in 

 dilutions given in the previous article for the preparation of the 

 color standards. The acid and alkaline tubes of phenol red 

 contain 0.1 per cent HCl, and N/100 NaOH respectively,' and 

 those of thymol blue (alkaline range) were made with 0.001 per 

 cent concentrated HCl and N/20 NaOH respectively. 



As to the second question: whether these tubes at the acid 

 and alkaline ends of each set could be shown to represent a 

 definite H-ion exponent, the tubes were matched against buffer 

 mixtures of known H-ion concentration and showed the results 

 indicated in the appropriate column of table 1. 



With methyl red: The "alkaline tube" practically matched a 

 pH 6.0 buffer mixture, the latter showing a very slightly more 

 reddish tinge. With phenol red: The "acid tube" practically 

 matched a pH 6.8 buffer mixture, approaching a pH 6.6 buffer 

 mixture (there seems to be only a very slight difference in color 

 between pH 6.6 and pH 6 8 with this indicator while, pH 6.4 of 

 buffer and pH 6.6 are alike in color). With cresol red: The 

 "acid tube" is very sUghtly clearer or brighter in color than a 

 pH 7.2 buffer mixture, being practically the same otherwise. 

 With thymol blue (alkaline range): The "acid tube" is slightly 

 more yellowish in color than a pH 8 buffer mixture approaching 

 pH 7.8 of the buffer. There is a marked difference in color be- 

 tween this tube and pair no. 1 of the set. The "alkaline tube" 

 matched the pH 9.6 buffer mixture. There is a difference in 

 color between this tube and pair no. 7, of the set. 



The value of these "acid" and "alkaline" tubes, at the end of 

 each set of "color standards" is not so much as to the actual pH 

 which they represent, but rather in the fact that they allow a 



' The N/100 NaOH was found to be the better strength to use for the alkaline 

 tubes for the color standards of this indicator rather than N/20 NaOH as recom- 

 mended in the previous article. 



