104 HYDROGEN ION CONCENTRATION 



stock solutions which can be easily reproduced, and the pH of such a 

 mixture is once for all established by standardization in a concentra- 

 tion (gas) chain. The other method which was developed by the 

 author'' depends upon the colorimetric estimation of the degree of 

 color of one-color indicators. This is achieved by determining the 

 relative amounts of indicator required to impart to the unknown 

 solution and to an alkali solution the same shade of color. Since the 

 shade of color depends upon the degree of color transformation and 

 dissociation constant of the indicator (cf., page 97), the pH of the 

 solution is obtained directly. No buffer solutions are required for 

 this method. In its principle the method is also applicable to two- 

 color indicators, and in this form it was developed by Bjerrum'' 

 and shortly before the author's work by Gillespie,^ And yet the 

 procedure with one-color indicators is simpler, and the necessary 

 one-color indicators covering the entire pH range were first given by 

 the author. Both indicator methods depend upon the gas chain 

 method; the buffer method insofar as the buffer solutions must be 

 standardized by the gas chain, and the method without buffers 

 insofar as the determination of the dissociation constants of the 

 indicators is best carried out by means of the gas chain method, 

 although this can also be accomplished in other ways, such as by con- 

 ductivity measurements. The details of the indicator methods will 

 be fully described in the experimental part of this work. 



A third, and historically the oldest, method is based upon the prin- 

 ciple that the catalytic effect of a solution upon the velocity of cer- 

 tain chemical reactions depends upon its [H+], as in the case of the 

 hydrolysis of cane sugar, esters etc., or upon its [0H~] as in the case 

 of the saponification of esters. This method will also be dwelt upon 

 in greater detail in the part on methods. 



29. The resistance of solutions to change in reaction and 



buffer value^ 



The values for equivalent acid content, neutralizing capacity and the 

 hydrion concentration do not exhaust the definition of the acidic or 



« L. Michaelis und A. Gyemant, Biochem. Zeitschr. 109, 166 (1920).— L. 

 Michaelis und R. Kruger, Biochem. Zeitschr. 119, 307 (1921). 



'' N. Bjerrum, Die acidimetrische und alkalimetrische Titration. Stutt- 

 gart. 



« L. J. Gillespie, Journ. of the Amer. Chem. Soc. 42, 742 (1920). 



9 Cf. Koppel und K. Spire, Biochem. Zeitschr. 65, 409 (19141 



