l62 PHYSIOLOGY OF MICROORGANISMS 



It is understood, therefore, that the "true acidity, alkalinity and 

 neutrality' are not determined by the amount of such substances 

 present, but entirely by the H and OH ion concentration. 



Theory of H Ion Concentration-^^ announcement of the theory of 

 electric dissociation by Svante Arrhenius, in 1887, marked a new era in 

 physical chemistry. It was F. Kohlrausch and A. Heydweiller who 

 demonstrated that even the purest water is a conductor of electricity, and 

 accordingly prepared a distilled water of the least specific conductance. 

 They measured the specific conductance by means of electric conduc- 

 tivity. Later, other methods for the estimation of dissociation were 

 established, and the results obtained by Kohlrausch were confirmed. 

 Now it is proved that a very small portion of the water molecule is 

 dissociated into two electrically charged parts (or ions), as follows: 



H 2 O * H + OH 



Its dissociation takes place according to the law of mass action in 

 accordance with the following equation:- 



(H)(OH) _ 



in which K denotes the ionization constant; that is to say, the product of 

 the hydrogen and hydroxyl ion concentration, divided by the concentra- 

 tion of the undissociated water molecule, should be constant. 



The concentration of water is generally constant. Therefore it may 

 be expressed as follows :- 



(H).(OH) = Kw (2) 



in which Kw denoted K.H 2 O, or ionization constant of water. 



Equation (2) is another form of equation (i). 



This ionization constant of water has been determined by several 

 noted physical chemists, and found to be io~ 14 at 22; that is, 



NOTE. (H) and (OH) express the concentration. 



(H).(OH) = Kw or 



Kw = io~ 14 (3) 



