

1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 235 



latter, are difficult to identify positively in small quantities. 

 The formation of these 2 easily oxidized, hydroxy acids in such 

 large quantities by plants, instead of the unsubstituted acids, is 

 probably quite significant, and would class these compounds as 

 storage products rather than as ultimate waste products. 



It would be expected, in view of the ease with which citric 

 acid is oxidized to acetonedicarbonic acid, that acetone might 

 readily be a product of metabolism, That this may actually be 

 the case in some instances was indicated by the fact that a dis- 

 tillate giving a profuse iodoform test in the cold was obtained 

 from a culture solution of Diplodia natalensis. 



The general indications are that CH 3 COOH, C*H 6 OH, and 



other alcohols are produced under conditions where the 0» supply 



is insufficient. The case of isopropyl alcohol and butyric acid is 



not well established, but it would seem probable that if one were 



formed the other might be formed also. However, such a sp] itting 



of the citrate molecule would apparently furnish little energy to 

 the organism. 



Miscellaneous Chemical Methods 



TOTAL OR TITRATABLE ACIDITY 



Sodium hydroxide and phenolphthalein were used for the 

 titration of culture media. In part of the work the samples were 

 aerated with CO,-free air before titration, but comparison of 

 aerated and unaerated samples showed so little difference that the 

 procedure was abandoned. Titrations were carried to a strong 

 pink color, since, according to Merck's handbook, both oxalic 

 and citric acids are completely neutralized by this procedure. 

 In so far as this method was applied to culture solutions it must 

 also be noted that any KH,PO< present in the solution would 

 be titrated to KJIPO*. 



In some of the work the amount of citrate was roughly esti- 

 mated by titration with HC1, using thymol blue as indicator. 

 Thymol blue changes from yellow to pinkish orange as the P H 

 changes from 1.8 to 2.0, but the end point is uncertain even 

 when blanks of free citric acid and the indicator are used. This 

 is especially the case when ammonium salts are present in the 

 solution. 



