118 BACTERIOLOGY 



dilution; pH, therefore, increases as H.I.C. diminishes. Thus: 



Tj-i N acid TTO N acid TTO N acid TTI 



P H1 = ~To^; P H2 = Too~; P H3 = ToTocr et c., or pHl = 



0.1 N acid; pH2 = 0.01 N acid; pH3 = 0.001 N acid, etc. 



Estimation of H.I.C. This may be done by the employ- 

 ment of either electrolytic or colorimetric methods. The 

 former is the more exact, but for a variety of reasons is less 

 well adapted to routine bacteriological work than is the 

 latter. 



By the former, the exact electrical method, the true 

 acidity, i. e., the H.I.C. of a number of substances in known 

 dilutions has been accurately determined. These "stand- 

 ard solutions" are so adjusted that in known dilutions 

 they can be made to represent a fairly regular range of 

 H.I.C. from extreme concentration to almost infinite dilu- 

 tion. When thus prepared they serve as objects on which 

 to establish the value of various indicators for various 

 H.I.C. ranges, for no single indicator will function through- 

 out the entire series of dilutions; that which gives its most 

 trustworthy indication at pH3, for instance, would be 

 valueless when mixed with a solution the H.I.C. of which is 

 symbolyzed by pH7. 



By arranging, therefore, a regular succession of known 

 dilutions of an electrolyte whose pH under dilution is known, 

 from the strongest point down to the weakest, i. e., by 

 having each step in the range only & the strength of the 

 preceding, it is found that several indicators each function- 

 ing best at some fixed point will be required to cover the 

 range embraced between concentration and neutrality. And 

 also that as we pass from the maximum point of efficiency 

 for such indicators to either the acid side of the scale, that 

 is to the lessor pH, or to the alkaline, or greater pH, the 

 color of the indicator is so modified as to serve as a guide for 

 fairly accurate interpolations between the periods. 



