COLORIMETRIC METHOD OF STANDARDIZATION 171 



mination, has been tested for the two indicators above 

 mentioned by a Committee of the Society of American 

 Bacteriologists, and is '' highly recommended" by them. 



If the proper color pairs are furnished the student by 

 the instructor, the titration and standardization will take 

 even less time than the older phenolphthalein titration, 

 though each student should be taught to make up his own 

 pairs. 



It must be remembered that media change in reaction 

 toivard the acid on sterilization, and allowance should be 

 made for this. For exact work a portion of the sterilized 

 media should be restandardized. For ordinary work and 

 the common media the change will be about 0.2 pH, i. e., 

 a medium standardized ^H7.2 will change to pWl after 

 sterilization. 



The question is frequently asked why the phenolphthalein 

 method so often gives good results. One reason is that so 

 many of the bacteria usually worked with in the laboratory 

 can grow throughout a fairly wide 2^11 range. x\nother 

 is that it accidentally hapjjens that standardizing to 1 fer 

 cent acid to j^henolphthalein so frequently brings the jjH 

 within the range of these common organisms. 



It is hoped that the above discussion has explained why 

 this method cannot give an accurate pYi. Many organisms 

 require a very narrow pH range. For accurate work, 

 research work and for work which must be duplicated by 

 others the p¥i must be as concisely defined as any of the 

 other environmental conditions. 



The " colorimetric method" affords the best means yet 

 available for such determination on a laboratory scale. 

 The ultimate standard must be potentiometer determination 

 and every laboratory should possess one as above indicated. 



COLORIMETRIC METHOD OF STANDARDIZATION. 



The following outline directions will illustrate how students 

 in the laboratory may use this method. 



Preparation of Standard Colors.— Select 19 small culture 

 tubes of clear white glass and as nearly the same size as 



