148 MASS. EXPERIMENT STATION BULLETIN 167. 



Later, other metliods for the estimation of dissociation were estabUshed, 

 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: — 



+ - 

 H20:^H+0H 



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

 accordance with the foUomng equation : — 



(HKOH) = K (1) 



(H2O) ^ ' 



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.HgO, or ionization constant of water. 



Equation (2) is another form of equation (1). 



Note. — (H) and (OH) express the concentration. 



This ionization constant of water has been determined by several noted 

 physical chemists, and found to be 10"^^ at 22° C; that is, 



(H) . (OH) =Kw or 



Kw = 10-i^ (3) 



Since pure water is a neutral solution it contains the same number of 

 dissociated hydrogen and hydroxyl ions. Therefore equation (3) can be 

 , expressed as follows : — 



lO-'XlO-' =10-14 (4) 



That is, a pure water contains of each 10"^ dissociated hydrogen and 



hydroxyl ions, or .0000001 gram ions per litre, which is, in a general term, 



N 



one ten-millionth normal The acidity, alkalinity and neutral- 



10,000,000 



ity, therefore, are expressed in terms of hydrogen ion concentration in 



the following manner: — 



Acid reaction (H)>-10-' 

 Alkaline reaction (H)-<10-' 

 Neutral reaction (H)=10-' 



Note. — (X)=notation of the concentration of ions. 



That is, in an acid solution there are more than gram mole- 



10,000,000 



cule of dissociated hydrogen; in an alkaline solution, less; and in a neutral 



