MEASUREMENT OF H-ION CONCENTRATION 405 



The variation of the constant a with wave-length, which leads 

 to the dichromatism of the two-colored indicators, does not lead 

 to any difficulty in the case of the double colorimeter. Dichro- 

 matism leads to the detection of errors made when turbidity of 

 the solution to be measured is balanced optically in the usual 

 manner. With a one-colored indicator the error made is no less 

 because of the absence of dichromatism, but the error is not dis- 

 closed. In routine work, white light is advisable as a source for 

 comparisons, v/hen it can be used, so that such error may be made 

 evident by dichromatism. \ATien the subjective difficulties be- 

 come too great for the use of white light, a screened light (Clark 

 and Lubs, 1917) may be a valuable means of obtaining an 

 approximate result. 



SUMMARY 



The recently published tables of Medalia are in disagreement 

 with other published data and are not correct. 



A colorimeter for tw^o-colored indicators is described for use in 

 accurate study of the indicators and for the measurement of 

 hydrogen-ion exponent. The optical assumptions underlying its 

 use are practically the same as those upon which ordinary col- 

 orimetry is based. 



REFERENCES 



Barnett, George D., and Chapman, Herbert S. 1918 Colorimetric deter- 

 mination of reaction of bacteriologic mediums and other fluids. Jour. 

 Amer. Med. Assn., 70, 1062. 



Clark, W. Mansfield 1920 The Determination of Hydrogen Ions, 318 pp., 

 Baltimore. 



Clark, William Mansfield, and Lubs, Herbert A. 1917 The colorimetric 

 determination of hydrogen-ion concentration and its applications in 

 bacteriology. Jour. Bact., 2, 1-34, 109-136, 191-236. 



Gillespie, Louis J. 1920 Colorimetric determination of hydrogen-ion con- 

 centration without buffer mixtures, with especial reference to soils. 

 Soil Science, 9, 115-136. 



Medalia, Leon S. 1920 "Color standards" for the colorimetric measurement 

 of H-ion concentration pH 1.2 to pH 9.8. Jour. Bact., 5, 441-468. 



TizARD, Henry Thomas 1910 The color changes of methyl orange and methyl 

 red in acid solution. Jour. Chem. Soc. (London) Trans., 97, pt. 2, 

 2477-2490. 



Wegscheider, Rud. 1915 Theorie der azidimetrischen Indikatoren. Ztschr. 

 physik. Chem., 90, 641-680. Especial reference to page 673. 



