24 



Scientific Proceedings (69). 



16 (1080) 



On a colorimetric method of adjusting bacteriological culture 

 media to any optimum hydrogen ion concentration. 



By S. H. Hurwitz, K. F. Meyer and Z. Ostenberg. 



[From the George Williams Hooper Foundation for Medical Research, 

 University of California.] 



In most bacteriological laboratories of this country adjustment 

 in the reaction of culture media by titration has largely replaced 

 all other methods. The indicator most commonly employed is 

 phenolphthalein, and the results are expressed in terms of the 

 amount of normal alkali necessary to bring one liter of the medium 

 to the desired reaction (Fuller Scale). 



Recent studies have shown that the titrimetric method in its 

 present form is inaccurate. The results of titrations done in 

 this laboratory support the observations of Clark 1 that media 

 titrated to the end point of phenolphthalein and corrected to 

 definite degrees on the Fuller scale have different hydrogen ion 

 concentrations. 



An exact knowledge of the reaction of a medium can be gained 

 only from a determination of its hydrogen ion concentration. It 

 is our purpose to present a simple colorimetric method which 

 makes possible the accurate and rapid determination of the hydro- 

 gen ion concentration of culture media and their adjustment to 

 any optimum concentration of ionized hydrogen. 



For our work we have made use of a set of standard solutions 

 recommended by Levy, Rowntree, and Marriott 2 for determining 

 the hydrogen ion concentration of the blood. These consist of 

 standard phosphate mixtures containing phenolsulphonephthalein. 

 The advantages of this indicator have been set forth by these 

 workers. 



The medium is tested first to ascertain what its ionization is 

 before adjustment. This preliminary test can be carried out 

 quickly: to 3 c.c. of fluid is added 0.3 c.c. of a O.oi per cent. 



1 Clark, W. M., Jour. Infect. Dis., 1915, XVII, 109. 



! Levy, Rowntree, and Marriott, Arch. Int. Med., 1915, XVI, 389. 



