HYDROGEN-ION CONCENTRATION OF BROTH MEDIA 145 



this time a number of modified procedures have been suggested 

 by Levy, Rowntree, and Marriott (1915), Hurwitz, Meyer and 

 Ostenberg (1915, 1916}; McLendon (1916); Barnett and Chap- 

 man (1918); Clark and Lubs (1916a, 1917a, 1917b, 1917c) ;Ha^s 

 (1919); SO that at the present time it is a relatively simple mat- 

 ter to prepare and have on hand in the ordinary bacteriological 

 laboratory a suitable set of colorimetric standards for the meas- 

 urement of the hydrogen-ion concentration of media and cultures. 

 Tt is to Clark and Lubs (1917b, 1916b), Lubs and Clark (1915, 

 1916) that we are especially indebted for several new and valu- 

 able indicator substances as well as for a careful study of the 

 ranges and usefulness of an entire set of indicators for the exam- 

 ination of biological fluids. 



Deeleman (1897) in 1897, using the titration procedure, noted 

 that media underwent certain changes in reaction during steriliza- 

 tion and sought to avoid such variation through the addition of 

 proper amounts of sterile acid or alkali to the autoclaved material. 

 Hesse (1904) used the same procedure in the adjustment of his 

 media and further emphasized* the fact that only that type of 

 glassware vv^hich yields no alkali should be employed for contain- 

 ers, to prevent the increase in alkalinity that otherwise might 

 occur. According to Sorensen (1909a) however, such factors as 

 alkalinity from glassware and CO2 from the atmosphere exert 

 only slight effects if the medium in question is properly buffered. 

 Using the titration method, Anthony and Ekroth (1916) at- 

 tempted to bring media to a stable reaction by repeatedly alka- 

 linizing and autoclaving, but were unable to produce such a 

 stabilized condition even after many additions of alkali, supple- 

 mented by a total of fourteen hours autoclaving. They explain 

 the change as due to the formation of acid principles through hy- 

 drolysis. In one case five times the quantity of base needed was 

 added through an error, with the result that after several steriliza- 

 tions the reaction of the broth fell to the required level. Wright 

 (1917) has suggested that the amount of alkali indicated by titra- 

 tion is never sufficient to bring about a complete neutralization 

 of the medium, it being always necessary to add a considerable 

 excess over the amount indicated. On the other hand, Noyes 



