Adjusting Bacteriological Culture Media. 25 



solution of phenolsulphonephthalein, the fluid being read directly 



in the comparator. 1 In most instances the culture fluid has been 



roughly adjusted by the usual methods so that its reaction falls 



+ + 

 within the limits of the scale (pH = 6.4 to pH = 8.4). 



If the medium has not received a preliminary adjustment of 

 reaction, it may be too acid or too alkaline to be read directly. 

 In that event a specimen of the medium is titrated as follows: 

 to a 3 c.c. sample is added rc/20 acid or alkali solution 2 depending 

 upon the initial reaction of the medium, until a color is obtained 

 which corresponds to the hydrogen ion concentration desired. 

 The conversion of the amounts of w/20 solutions read on the 

 pipette into w/l solutions is made by referring to a curve plotted 

 for an average medium in which the amounts of the n/20 solutions 

 required are plotted as abscissae and the corresponding amounts 

 of n/i solutions as ordinates. Final adjustment in reaction must 

 be made with sterile acid or alkali in order to avoid the change in 

 ionization caused by sterilization. 



To illustrate the accuracy of the method the results of the 

 titration of five media are given in the table. 



table. 



No. of 

 Exp. 



Date. 



Medium. 



Titra- 

 tion by 

 Fuller 

 Scale. 



Preliminary 

 Test. 



Stan- 

 dard 

 Desired. 



«/20 



Alkali 

 in c.c. 



Alkali 

 Added 



per 

 25 c.c. 



Value of 

 pH Ob- 

 tained. 



I 



Aug. 6 



Veal infusion 



1.0 



Below 6.4 



7-5 



O.46 



0.225 



7-45 



a 



Aug. 7 



Plain broth 



0.8 



6.9 



7.6 



0.17 



O.O73 



7-55 



3 



Aug. 12 



Liebig's broth 



0.8 



6.9 



7-6 



0-195 



O.087 



7-55 













7-7 



0.20 



O.O95 



7-7 



4 



Aug. 16 



Plain broth 



0.8 



7-15 



7-6 



0.13 



0.06 



7.6 













7-9 



0.I9S 



0.088 



7-9 



















8.15 



5 





Extract 



1.0 



6.9 



7-5 



0.08 



0.04 



7-55 













7-9 



0.21 



O.O96 



7-9 



1 In order to make direct comparisons possible even in the presence of the natural 

 color of the fluid tested, we have constructed a simple device whereby the medium 

 tested serves as a background for the standard test color to which it imparts its own 

 characteristic quality of color. 



2 In order to keep the concentration of indicator during titration the same as its 

 concentration in the standard comparison tubes (0.3 c.c. to 3 c.c. or 1 to 11) the 

 solutions of n/20 acid and alkali are so made up that one eleventh of their volume is 

 indicator solution. The solutions are kept in vessels protected against light, air, 

 and moisture, and the apparatus so arranged that the solution can be delivered 

 directly into a graduated one cubic centimeter pipette provided with a ground glass 

 stopcock on the principle of a burette. 



