144 LAUEENCE F. FOSTER AND SAMUEL B. RANDALL 



Hydrogen-ion concentration influences the condition in solution of 

 every substance with acidic or basic properties — native proteins and 

 their hydrolytic products, amines and amides, carboxyl, sulphonic, 

 and phenohc compounds, even alcohoHc compounds, as well as many 

 inorganic compounds. It has a large effect on the effective solubilities 

 and dispersion of colloids, upon determining tautomeric equilibria, and 

 in one way or another in governing the activity of catalysts such as 

 hydrolytic enzymes and oxidases. One or the other of these effects, 

 induced directly or perhaps indirectly by the hydrogen-ion concen- 

 tration must impress bacterial life. 



That the expression of reaction in terms of titratable acid or 

 alkali does not adequately define the true reaction of a solution 

 has perhaps best been brought out by W. M. Clark (1915a) in 

 his admirable paper, ''The 'reaction' of bacteriologic culture 

 media." The objections to the older procedure may be sum- 

 marized in a quotation from Clark and Lubs (1917a) : 



The titrimetric method, designed originally for the quantitative 

 estimation of strong acids and bases, cannot be applied to complex 

 mixtures of very weak acidic and basic groups such as are found in the 

 constituents of most culture media. In so far as the method is used to 

 determine the "free acid" or to adjust to a certain degree of "free acid" 

 it is an absolute failure when applied to culture media. There is how- 

 ever, an even more fundamental reason why the titrimetric method is 

 inappropriate. Two media adjusted to the same degree of acidity may 

 have widely divergent hydrogen-ion concentrations as shown by Clark 

 (1915a). 



With the development of the hydrogen electrode, making pos- 

 sible a direct measurement of hydrogen-ion concentration, some 

 of the experimental and mathematical difficulties involved in the 

 older methods were obviated, but there still remained to be elab- 

 orated some simpler and more rapid procedure that would be 

 adapted to the adjustment of culture media and to the study of 

 reaction changes in bacterial cultures. Guided by the earlier 

 work of Friedenthal (1904), Sahn (1904), Friedenthal and 

 Salm (1907) who were the first to give a well worked-out series of 

 indicators, Sorensen (1909a) in 1909 published his colorimetric 

 method for determining hydrogen-ion concentrations. Since 



