FURTHER OBSERVATIONS ON "COLOR STANDARDS" 



FOR THE COLORLMETRIC DETER.AIINATION 



OF H-ION CONCENTRATION 



LEON S. MEDALIA 



From the Research Laboratories Department of Biology and Public Health, 

 Massachusetts Institute of Technology 



Received for publication March 23, 1922 



The following additional observations on color standards for 

 the colorimetric determination of H-ion concentration have 

 been made by the author since the publication of his first article 

 on this subject (INIedaUa, 1920). 



It will be recalled that in the first article just referred to a 

 method was described of preparing "color standards" for the 

 principal indicators of Clark and Lubs. This was done by 

 using varying amounts of indicator in acid and alkaUne solutions 

 choosing such solutions as would bring out the acid and alkaline 

 colors of the double colored indicator. Seven pairs of tubes 

 were used; each pair containing 0.8 cc. of indicator solution in 

 the ratio of 1:7, 2:6, 3:5, etc., increasing by 0.1 cc. in the alkaline 

 solution up to 0.7 cc. and decreasing by 0.1 cc. in the acid solution 

 down to 0.1 cc. These pairs of tubes were viewed by looking 

 through them preferably in a comparator block, when placed 

 one behind the other, i.e., superimposing the acid and alkaline 

 colors of the indicator in the varjnng strengths — similar to the 

 superimposing method used by Salm (1904) to determine the 

 half transformation point, and by Barnett and Chapman (1918) 

 to prepare the color standards for phenol red. The pH repre- 

 sented by the color of each pair of "color standards," was caU- 

 brated by using buffer mixtures of known H. I. C, prepared 

 according to Clark and Lubs (1917). 



The possibility suggested itself of making use, in addition to 

 the seven pairs of "color standards" just referred to, of an 



589 



