PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 163 



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: 



io~ 7 X io~ 7 = io~ 14 (4) 



That is, a pure water contains of each io~ 7 dissociated hydrogen and 

 hydroxyl ions, or .000000 1 gram ions per litre, which is, in a general 



N 



term, one ten-millionth normal The acidity, alkalinity 



10,000,000 



and neutrality, therefore, are expressed in terms of hydrogen ion 

 concentration in the following manner : 



Acid reaction (H) > io~ 7 

 Alkaline reaction (H) < io~ 7 

 Neutral reaction (H) = io~ 7 



That is, in an acid solution there are more than pram 



10,000,000 



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



neutral solution, just - gram molecule. Thus the reaction is 



10,000,000 { 



usually expressed in terms of hydrogen ion concentration unless it is 

 indicated otherwise. 



From the above discussions it is readily seen that if the ionization 

 constant is known, and the hydrogen ion concentration is determined 

 experimentally, then the hydroxyl ion concentration can be calculated. 

 The determination of hydrogen ion concentration is accomplished by 

 the use of the gas cell, of which the principle is based upon the potential 

 of the chain. This chain as described in physical chemistry, consists 

 of 



Hg-HgC] | n/ioKCl | cone. KC1 | solution | Pt H 2 



calomel electrode concentrate (unknown) platinum elec- 



potassium trode saturated 



chloride with hydrogen 



in a dish. gas. 



The potential of such a chain can be determined by the usual physical 

 method. Then the relation between the measurement of potential and 

 hydrogen ion concentration can be calculated by the following equation :- 



P ~ Q-3377 _ 



-577 + 0.0002 (t 1 8) 

 NOTE. (X) = notation of the concentration of ions. 



