CULTURAL METHODS 83 



are termed buffers. A buffer is any substance in a solution 

 which tends to prevent rapid changes in hydrogen ion con- 

 centration upon the additions of alkalies or acids. Certain 

 salts, particularly the phosphates, and many organic sub- 

 stances, particularly the amino acids and peptones, act in 

 this manner. For example, the addition of a small amount 

 of an acid to distilled water will give a marked change in 

 the hydrogen ion concentration, but the same amount of 

 acid added to solutions containing considerable amounts of 

 buffers may result in very slight differences in hydrogen ion 

 concentration. It is evident that since microorganisms are 

 affected far more by differences in hydrogen ion concentra- 

 tion than they are by the total amount of acid present, 

 heavily buffered media are preferred for their growth. 



It is noted above that media are sometimes standardized 

 by determining the amount of acid or alkali required to 

 bring a hundred cubic centimeters to the neutral point of 

 some indicator. The one usually chosen in bacteriology is 

 phenolphthalein. A solution is said to be — 1, for example, 

 when it will require one cubic centimeter of a normal solu- 

 tion of acid to bring it to the neutral point of phenolph- 

 thalein. A heavily buffered medium, such as ordinary pep- 

 tone broth, having a reaction of +1 or -j-l.S, usually has a 

 pH value between 7 and 8. Direct determination of hydro- 

 gen ion concentration is preferable to titration in adjusting 

 the reaction of the medium.^ 



When a medium has been finally adjusted in its reaction, 

 that is, when the right amount of alkali or acid has been 

 added to give the desired hydrogen ion concentration, it is 

 frequently necessary to boil or heat the medium to precipi- 

 tate any materials not soluble in boiling water at the new 

 hydrogen ion concentration. This is followed by filtration. 

 If this step is omitted the medium will ordinarily be cloudy 

 or full of sediment. 



1 For methods of adjusting reactions see laboratory manual. 



