38 MEDICAL MYCOLOGY 



Assume now that we have in a given solution a mixture of solutions of HAc and NaAc 

 in such proportions that the resulting reaction of the mixture is acid. We have, then, present 

 large quantities of Na*, 0H-, H* and Ac- ions and of un-ionized HAc, practically no un- 

 ionized NaOH. Let us now visualize what would happen if to such a solution we add a 

 solution of a strong acid, such as HCl, almost completely ionized to H+ and C1-. The reaction 

 does not, as one might hastily assume, turn sharply acid. Before the pH can be materially 

 lowered, the free H+ added must first adjust its equilibrium with the free Ac- and OH- in the 

 solution. This will be accomplished by association into almost un-ionized HAc and HOH 

 and until this is accomplished, addition of HCl will not materially affect the reaction of the 

 solution. 



By suitably selecting the acid and its salt, we may secure satisfactory buffers for almost 

 any region of the whole pH scale. Boric acid, H3BO3, and phosphoric acid, HjPO^, are among 

 those most commonly used. These have the double advantage of being weak acids and of 

 ionizing in three stages: e.g. 



H3PO, ^ H* + H„PO,- 



H,PO,- ^ H+ -1- HPO,- 



HPO,= ;^ H+ -f-PO/ 



The dissociation constants of these three stages are successively smaller, the possible use in 

 buffering correspondingly greater. 



The actual composition of the buffer solutions we will not go into here, save to state 

 briefly that boric acid is usually used in conjunction with borax, while, instead of using 

 phosphoric acid itself, there is used a mixture of the two salts KH.PO, and Na.HPO, (or, 

 rather, its hydrate). Clark (1922) gives a series of possible buffers, with directions for their 

 preparation and tables showing their pH range. 



For a comprehensive discussion of the subject one should consult such works as those of 

 Clark (1922) and Michaelis (1926). 



This matter of buffers is very important in the preparation of media and 

 in interpreting the results of older authors, who measured the total acidity, 

 usually using phenolphthalein as an indicator. It was customary to titrate 

 a sample medium and add the calculated amount of acid or alkali necessary 

 for a given total acidity. If the medium was made up of phosphates, etc., as 

 some of the early media were, this added acid had little real effect, while if 

 few buffers were present it might change the reaction very much. 



While generalizations are unwarranted, a large number of observations 

 indicate that most bacteria grow best betwen pH 7 and 9 while most fungi 

 grow best between pH 5 and 7 with some, especially members of the Asper- 

 gillaeeae, able to grow slowly at much higher acidities. This fact is sometimes 

 utilized in restraining the growth of bacteria in isolations by using media to 

 which a small quantity of an organic acid, such as acetic or lactic acid, has 

 been added (see p. 59). Klebs' dicta hold in this case as well as in other 

 factors of the environment, since growth is possible at a much wider range 

 than is reproduction. Talice (1930) records optima, maxima, and minima of 

 about thirty common human pathogens on three usual media. (See also notes 

 of Mallinckrodt-Haupt 1929, 1932; Kadisch 1929.) 



Oxygen Requirements. — Since the fundamental processes of respiration 

 are the same for plants and animals and are already known as to conditions 

 and end products, no attempt will be made to discuss aerobic and anaerobic 



