'116 BACTERIOLOGY 



Of prime importance to the bacteriologist is the fact that 

 while the degree of dissociation undergone by an electro- 

 lyte when dissolved in pure water may be accurately deter- 

 mined and is constant under fixed conditions, if such electro- 

 lyte be dissolved in organic fluids the estimation of the 

 amount of dissociation is interfered with by a number of 

 organic bodies present. Such interfering matters are known 

 as "buffers" and their influence must always be taken into 

 account in this method of estimating reaction of a fluid, 

 i. e., its hydrogen-ion concentration. 



The meaning of the term "hydrogen-ion concentration" 

 can best be understood after the statement of several funda- 

 mental facts: If an acid, such as HC1, for instance, be dis- 

 solved in water, some of it retains its characteristic mole- 

 cular form HC1, but a much larger proportion is broken 

 down, "dissociated," into its component elements H and Cl, 

 which are conceived as atoms or groups of atoms denomi- 

 nated electrically charged "ions;" those carry ing the positive 

 charge being the hydrogen (H) ions, those carrying the 

 negative charge being the chlorine (Cl) ions, or "cations" 

 and "anions" respectively. According to this theory the 

 acidity of a fluid (the strength of the acid dissolved) is pro- 

 portionate to the amount of dissociated hydrogen ions 

 present, and not to the amount of alkali required to neutra- 

 lize the acid. "Thus, the percentage dissociation or the 

 amount of H ions set free in YT HC1 solution was found 

 (Talbot, 1908) to be 90 per cent., while that of A acetic 

 acid is 1.4 per cent. Hydrochloric acid according to these 

 findings is therefore sixty-four times stronger in acidity 

 that acetic acid, although 10 c.c. of yV of either acid will 

 require a similar 10 c.c. portion of YTT NaOH to neutralize 

 it. Accordingly the only correct method of measuring the 



