1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 



263 



growth. The determining factor in the latter precipitation is 

 apparently the hydrogen-ion concentration of the solution. The 

 use of potassium would seem to unbalance the solution by in- 

 creasing disproportionately the amount of this cation, since 

 KNOi and KHjPO* were being used as inorganic nutrients. How- 

 ever, potassium had already been used successfully, and as the 

 citric acid was to be only partially neutralized it seemed prefer- 

 able to use potassium rather than a cation of unknown physiologi- 

 cal reaction. In adding a large amount of citrate radical to the 

 solution it was apparent that one solution was to have a somewhat 

 higher osmotic pressure than the other. To compensate for such 

 a disparity it would have been necessary either to cut down the 

 dextrose in the solution to a small amount and to substitute 

 sufficient citrate mixture to make up for the dextrose or to add 

 to the dextrose solution enough of an inert buffer substance to be 

 equivalent to the osmotic pressure of the citrate mixture. Both 

 of these methods would involve numerous difficulties. If the first 

 method were used the amounts of dextrose in the 2 solutions 

 would be so widely different as to make difficult an accurate 

 comparison of the growth in the 2 solutions even if the amount 

 of the citric acid-potassium citrate mixture to be added could be 

 accurately determined, and the second method is at present 

 impossible owing to the fact that no absolutely inert (physiologi- 

 cally) buffer mixture for culture media is known. 



Using the usual concentrations of mineral nutrients, one solu- 

 tion contained M/4 dextrose as a source of carbon and was desig- 



on 1. This solution, where necessary, was adjusted 



i second solution contained M/4 dextrose and M/4 



fa mixture of citric acid and potassium citrate) 



nated 



ro 



citrate radical (a 

 and was designated 



9 



The adjustment 



plished by varying the ratio of citric acid to potassium 



citrate. When only a small amount of potassium 

 needed, neutralization with 2 N KOH sufficed, but w 



amount was needed solid 



added 



The 



basic 



ition probably contained free citric acid and a mix- 

 assium salts of citric acid, that is, mono-, di-, and 

 ites. In so far as possible the solutions were made 



bulk and distributed to the 300-cc. flasks by means of 



