446 LEON S. MEDALIA 



taining various chemicals in different dilutions. Such solutions 

 when accurately duplicated are supposed to be of the same H-ion 

 content, and should therefore show similar variations in color, 

 in the indicators used. These solutions are referred to as " stand- 

 ard solutions" of known H. I. C. 



The difficulties met with in preparing the "standard solutions" 

 of known H. I. C. in the average bacteriological laboratory con- 

 stitute one reason why this method has not come into general 

 use, and furnished the principal stimulus for this research. 



Experimental work 



In looking over the range of pH of the indicators developed by 

 Clark and Lubs (1917) I found that the sensitive range between 

 the extreme acid color and the extreme alkaline color of each 

 indicator, is pH 1.6. It occurred to me, that by dividing this 

 sensitive range of color, pH 1.6, into eight equal parts, one should 

 obtain a range for each indicator of pH 0.2 intervals. This 

 should be accomplished by adding 0.1 cc. of the indicator solu- 

 tion to one test tube containing 10 cc. of an alkaline solution, 

 and 0.7 cc. of the same indicator to another test tube containing 

 10 cc. of an acid solution — (solutions that will bring out the alka- 

 line and acid colors of the particular indicator) — then looking 

 through the two test tubes placed one behind the other, i.e., 

 superimposing the alkaline and acid colors of the indicator in 

 different strengths 1 the color should be pH 0.2 higher (more alka- 

 line) than the extreme acid color with 0.8 cc. of the same indicator. 

 Increasing by 0.1 cc. in the alkaline solution, and decreasing by 

 0.1 cc. in the acid solution, in a set of seven pairs of tubes should 

 give us the full range of the indicator at an interval of pH 0.2. 



This was tested out with brom thymol blue and it succeeded 

 perfectly, i.e., the green color was found at (pair no. 4) pH 7; 

 or slightly yellowish green at (pah* no. 3) pH 6.8, according to this 

 range. (The change of color of this indicator was found by the 



1 Following the procedure made use of by Salm (1904) to determine the half 

 transformation point and that devised by Barnett and Chapman to prepare the 

 color standards for phenol red. 



