HYDROGEN ION CONCENTRATION AND PROTEOLYSIS. 145 



GENERAL METHODS OF PROCEDURE. 



Choice op Methods. 



Determination of the Hydrogen Ion Concentratio7i 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 hj^drogen 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 1^/io 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 h3^dioxyl ions are dissociated. The "true alkalinity" of 

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

 but exclusively by the concentration of dissociated hydrox}^ ions. For 

 example, in a one-tenth normal solution of the strong base, sodium hj^drox- 

 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 hydroxide, approximately 

 l4io per cent, of the total amount of the base. The " true alkalinity " of 

 these solutions, therefore, is eightj'-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 alkalinitj^ " 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 : — 



+ - 

 H2O ^ H -l-OH 



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 



1 Sorensen, S. P. L. Ergebnisse d. Physiologie, 12, 399, 1912. 



