HYDROGEN ION CONCENTRATION AND PROTEOLYSIS. 145 



GENERAL METHODS OF PROCEDURE. 

 Choice of Methods. 



Determination of the Hydrogen Ion Concentration in Culture Media. 



A theoretical discussion of "true reaction'' will be pertinent at this 

 stage. 



"True Reaction'^ {"True Acidity" "True Alkalinity,''^ "True Neutral- 

 ity"). ^ — The "true acidity" of an acid solution is brought about by the 

 dissociated (hydrogen) ions; therefore the acidity is proportional to the 

 concentration of the dissociated hydrogen ions, and not to the total gram 

 molecules of acid present. For example, if one-tenth normal hydrochloric 

 acid is taken, approximately only 91 per cent, of the total amount of acid 

 becomes dissociated. The " true acidity," i.e., the hydrogen ion concentra- 

 tion, of this solution is only 91 per cent, of the one-tenth normal hydro- 

 chloric acid, or ninety-one thousandths normal. The dissociation of 

 weak acid is still less. For instance, in a solution of one-tenth normal 

 acetic acid only I'jio per cent, approximately of the total acid is dissociated, 

 and the hydrogen ion concentration of this solution is therefore thirteen 

 ten-thousandths normal. The " true acidity " of one-tenth normal hydro- 

 chloric acid is also about seventy times greater than that of one-tenth 

 normal acetic acid, although both solutions contain the same amount of 

 acid. 



The same holds true with the electrically dissociated base in which the 

 metallic and hydioxyl ions are dissociated. The "true alkalinity" of 

 such a solution is not determined by the total amount of base present, 

 but exclusivelj^ by the concentration of dissociated hydroxyl ions. For 

 example, in a one-tenth normal solution of the strong base, sodium hydrox- 

 ide, about 84 per cent, of the total amount of the base is dissociated, and 

 in the case of a weak base, such as ammonium hj'^droxide, approximately 

 l%o per cent, of the total amount of the base. The " true alkalinity " of 

 these solutions, therefore, is eighty-four thousandths normal and fourteen 

 thousandths normal, respectively. Thus, regarding the alkalinity as in 

 the case of acidity, we may say in conclusion that " true alkalinity " of a 

 solution is proportional to the concentration of hydroxyl ions. 



From the above discussion, "true neutrality" of a solution may be 

 stated as follows : it is a solution in which the same amount of H and OH 

 ions are present. For example, a "true neutral solution," viz., pure water, 

 contains as many hydrogen ions as hydroxyl ions. It can be expressed as 

 follows : — 



+ - 

 H20#=H+0H 



in which Ch~^OH' ^ indicating the concentration. 



Again, a solution may not necessarily be neutral, although it contains 

 equivalent quantities of acid and alkali. For example, if a solution which 



» Sorensen, S. P. L. Ergebnisse d. Physiologic, 12, 399, 1912. 



