306 MEACHAM, HOPFIELD AND ACREE 



in the pH range which changes very greatly between the straight 

 parts of the neutralization curves for the first and second hydro- 

 gen ions of phosphoric acid. Our central idea in using these 

 buffer mixtures 4 is to straighten out these H ion inflection curves 

 and hence to keep the pH near the optimum by permitting only 

 small changes in acidity by the organic acids generated by the 

 fungi or bacteria. Acetic, phthalic, formic, malic, asparaginic, 

 aminoacetic and other acids with ionization constants between 

 the three constants for phosphoric acid, i.e., between 1.2 X 

 10- 2 and 1.1 X 10- 7 and 1.1 X 10~ 7 and lO" 12 , accomplish the 

 desired end excellently. 



Since the co-workers of one of us 5 have shown that the activity 

 of both ions and molecules of acids, bases and salts must be meas- 

 ured in all cases, and that in many reactions the nonionized mole- 

 cules are even more active than the hydrogen or other ions, we 

 shall in all cases consider that the growth curves of every organ- 

 ism are influenced by every ionized and nonionized substance 

 present. In the present case, then, the better growth in the 

 phosphate-phthalate mixture shows that the phthalate or acid 

 phthalate anion, or the nonionized phthalic acid compounds 

 present, must be considered better for E. parasitica than the 

 acetate ion or molecular acetic acid compounds. Since, in gen- 

 eral, the molecular forms of stronger acids have greater activities 

 than the molecular forms of weaker acids, it will be interesting 

 to see how stronger acids like phthalic increase (or depress) the 

 rate of growth, or other biological factors, more than does the 

 weaker acetic acid, for example, as in the present study. In 

 such comparative studies the factors known to be of great im- 



4 General formulae for calculating the form of the titration curves and data 

 for mixtures of a large number of acids and bases have been developed. These 

 and data on the growth of organisms on a number of such regulated media will 

 be published soon. 



6 Researches by Nirdlinger, Rogers, Shadinger, Loy, Desha, Chandler, Mar- 

 shall, Johnson, Harrison, Robertson, Myers, Gruse, Shrader, Taylor, and Brown 

 in cooperation with one of us. See Am. Chem. Jour. 39:, 275 (1908) ; 49: 116 (1913) ; 

 43: 519; 49: 474; 49: 177, 369; 49: 122, 132, 485 (1913) ; 48: 374; 49: 350, 378, 396, 403; 

 41: 466. Jour. Am. Chem. Soc. 37: 1902. Jour. Phys. Chem. 19: 589 (1915); 20: 

 365 (1916). 



