TOXICITY OF SODIUM, POTASSIUM AND LITHIUM 



311 



Tests already reported 4 showed that the optimum initial pH 

 for E. parasitica is about 5.0. Besides possessing the proper 

 initial pH the medium should have a fairly strong buffer effect 

 in the region between pH = 3 and pH = 7 in order that it may 

 be stable and resist any marked changes in pH which might be 

 brought about through the organic acids generated by the fun- 

 gus, accidental impurities, carbon dioxide of the air, solubility 

 of glass, etc. Accordingly 2.5 per cent malt extract medium buf- 



TABLE 1* 



To test the toxicity of NaCl toward E, -parasitica; 2.5 per cent malt extract + M/50 

 K2HPO4 + M/50 acetic acid + 1.5 per cent agar; Petri dish cultures 



* Data taken from notes made by Mr. Hopfield. 

 t Indicates dish completelj covered. 



fered with M/50 K 2 HP0 4 and M/50 acetic acid was chosen. 

 The acetic acid acts as a buffer to smooth out the marked change 

 in pH found in passing from the neutralization of the first hydro- 

 gen ion to the second of the phosphoric acid, as discussed in 

 other articles. 5 This medium was brought to a pH of about 5.0 

 by the addition of 0.5 cc. of normal hydrochloric acid to each 

 100 cc. To this medium, containing 1.5 per cent agar, was added 

 pure sodium chloride in varying concentrations ranging from 

 to 0.63 normal. Three dishes of each salt concentration were 



4 This Journal, March, 1920. See reference 5, this article. 



5 Address before Amer. Chem. Soc, Philadelphia, September, 1919. Science 

 48: 449-450, November 15, 1918. This Journal, March 1920, and unpublished 

 articles. 



