160 PHYSIOLOGY OF MICROORGANISMS 



When an electric current is introduced into a solution of sodium 

 chloride (NaCl), the ions carry the electricity, sodium (Na) carrying 

 the positive charge and chlorine (Cl) the negative charge. The positive 

 or sodium ions gather at the electrode or pole or cathode, the pole by 

 which the current leaves the solution; the chlorine ions which have the 

 negative charge pass against the current to the anode, the pole by which 

 the current enters the solution. Those gathered at the cathode are 

 the cations, those at the anode, the anions. 



The electric relations existing between sodium (Na) and chlorine 

 (Cl) in sodium chloride (NaCl) also exist in hydrochloric acid (HC1) 

 in which hydrogen and chlorine bear the sa-me relationships as sodium 

 (Na) and chlorine (Cl) in sodium chloride (NaCl). 



The strength of the acid is measured by displaceable hydrogen atoms. 

 In nitric acid (HNOs) one hydrogen atom (H) is displaceable, in acetic 

 acid (CH 3 COOH) only one hydrogen atom (H) is displaceable, for the 

 hydrogen atoms of the methyl group (CH 3 ) are not displaceable. 



If an acid is neutralized it is the hydrogen which has been substi- 

 tuted by some other cation. 



Since it is the hydrogen (H) which determines the strength of an acid 

 and this hydrogen (H) has definite values in every acid then it must be 

 possible to establish a standard of measurement by taking a gram- 

 molecular solution as of hydrochloric acid (HC1) which is called a normal 

 (N) solution. On the other hand, alkali solutions may be established 

 of standard values against the acid standards. In these the hydroxyl 

 (OH) ions act as the neutralizing agents against hydrogen (H) ions 

 of the acids in such proportion as to form water. 



"True Reaction" ("True Acidity," "True Alkalinity," "True 

 Neutrality").* 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 concentration, of this solution is only 91 per cent, of the 

 one-tenth normal hydrochloric acid, or ninety-one thousandths normal. 

 The dissociation of weak acid is still less. For instance, in a solution of 



* Bull. 167, Mass. Agr. Exp. Sta. by Arao Itano. 



