310 MEACHAM, HOPFIELD AND ACREE 



(1) total activity = cation activity X cation concen- 

 tration + anion activity X anion concentration -f 

 molecular activity X molecular concentration + . . . . 



developed 3 for studying the activities of various ions and mole- 

 cules in pure chemical reactions. When a given series of salts 

 with a common anion in the same concentration is used, this 

 equation is naturally simplified and gives the values for the 

 specific effects of the cations and molecules. 



There are two good ways to vary the cation and molecular 

 species. The first is to use buffer mixtures having common 

 anions but different cations. For example, the mixture of M/50 

 phosphoric acid and M/50 acetic acid employed as buffer mate- 

 rial in the present study can be titrated or neutralized with so- 

 dium, potassium, lithium, etc. hydroxides to keep the hydrogen 

 ion concentration at desired values. In such a case the ioniza- 

 tion values of these bases and their salts are already nearly iden- 

 tical and the chemical aspects of the problem are much simplified. 

 The second method is simpler in technique though slightly more 

 complicated electrochemically, and involves adding sodium, 

 potassium, lithium, etc., salts with common anion to the medium 

 buffered with the same substances throughout. If the pH of the 

 medium is to be kept absolutely constant while the concentration 

 of one of the salts is to be varied, it is obvious that the salts used 

 must be neutral compounds from strong acids and bases. The 

 chlorides of the metals to be used fulfill this condition as well as 

 any salts. Besides, the chlorides of all these metals except cal- 

 cium are about equally ionized — another condition to be consid- 

 ered. Even this procedure involves changes in the pH or acidity, 

 because of changing ionization of the buffer salts, but such devia- 

 tions in pH are known to be small and will be corrected in our 

 future more accurate investigations. 



3 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). 



