ISOLATION AND PURE-CULTURE METHODS 33 



Neutralization of Culture Media 



Neutralization, or properly titration, of most culture media is 

 required. It is required in order that definite standards may be 

 maintained. In fact, titration is superfluous only in rough work. 

 In general, the degree of alkalinity or acidity of the medium may 

 affect some of the characteristics of organisms, and, therefore, a 

 description of any organism should be made either at a fixed 

 standard of alkalinity or acidity, or it should at least be possible 

 to reproduce exactly the reaction of the medium employed. The 

 colonies of Bacillus prodigiosus and other pigment-forming bac- 

 teria are less brilliantly colored when the media are distinctly acid. 

 To slight differences of reaction in the substratum fungi ordinarily 

 show no marked cultural variations ; yet to greater differences they 

 may respond by variations in color, modifications of colony form, 

 amount or character of fruiting, etc. 



Most culture media are acid, and sodium carbonate was formerly 

 employed in neutralization. From this, however, carbonic acid is 

 liberated, and litmus is temporarily reddened, so that potassium 

 hydrate or sodium hydrate is preferable. Moreover, in this titration 

 work phenolphthalein, a reliable indicator, has been adopted. It is 

 more desirable than litmus, rosolic acid, or other indicators. Litmus 

 may be used for rough work, but it is less sensitive to certain acids 

 and .too variable. In peptone, gelatin, and other organic substances 

 there are bodies which are amphoteric, that is, which possess 

 both basic and acid properties, the latter predominating. Phenol- 

 phthalein is particularly serviceable with respect to those substances. 

 Litmus fails to detect such weak acids ; again, litmus reacts alkaline 

 to the dibasic phosphates, while phenolphthalein reacts neutral. 



In titration, the following solutions are desirable : -i per cent 

 phenolphthalein in 50 per cent alcohol, as indicator ; ^ T normal 

 caustic alkali (preferably sodium hydrate) for the titration ; and 

 a normal solution of the caustic alkali for actual neutralization of 

 the medium. 1 



1 For practical purposes, a normal solution of sodium hydrate may be prepared 

 by dissolving 4.5 grams of c.p., fresh NaHO in somewhat less than 100 cc. of 

 distilled water, and after it is dissolved make up with water to exactly 100 cc. 

 (roughly, this amount makes due allowance for the water and impurities in fresh 

 NaHO). 



