SALT EFFECTS OF PHOSPHATES 173 



the ratio of the two intensities can be expressed by — = 10" . The 



io 



term N we shall call the absorption index. If the absorption 



index for the middle of the band, when the indicator is partly 



ionized, is N a , and the index when the indicator is completely 



ionized is N, and we assume the absorption to be proportional 



to the ionization, then (1 - a) /a = (N - N a )/N a and if the 



H X N 

 hydrogen ion concentration (H) were known then K = — — -• 



It is well known 9 that the presence of neutral salts affects the 

 color of some indicators. It is then impossible to determine K 

 directly if disturbing neutral salts are present. It is possible, 

 however, to make solutions of approximately the same hydrogen 

 ion concentration and yet vary the concentration of the salt. 

 By determining the value of (1 - a)/ a due to the successive 

 concentrations of the salt we can extrapolate and find the value 

 that (1 - a) /a would have were there no salt present, and, from 

 that and the known hydrogen ion concentration, we can find the 

 value of the ionization constant of the indicator and its variation 

 with changing concentration of salt. 



The method adopted was as follows: Three solutions were 

 made up of sodium phosphate and alkali in such a manner that 

 the amount of phosphate salt in each was the same (N/2). The 

 concentration of the phenolsulfonphthalein was N/5000, but the 

 values of the hydrogen ion concentration were different in each 

 solution. These solutions were called A, B and C. To 100 cc. 

 of these solutions 50 cc. of N/2500 phenolsulfonphthalein were 

 added and the whole made up to 200 cc. with conductivity water. 

 These solutions were denoted by A 2 , B 2 , and C 2 , and are N/4 

 in concentration. To 100 cc. of these solutions 25 cc. of N/2500 

 phenolsulfonphthalein were added and the whole made up to 150 

 cc. with conductivity water. These were denoted by A z , B 3 , 

 and C 3 , and are N/6 in concentration. 



The absorption index at the middle of the green band for each 

 of these solutions was then measured by means of a Brace spec- 



• For literature see reference 7. 



JOURNAL OF BACTERIOLOGY, VOL. V, NO. 2 



