38 BIOLOGICAL CHEMISTRY 



A were necessary, the equation would contain C A -C A or 

 C A . 



Applying these equations to the ionic dissociation of elec- 

 trolytes we find that the dissociation of weak electrolytes 

 conforms to the law of mass action : the dissociation is an 

 equilibrium condition. Thus the weak acid carbonic acid 

 (H 2 CO 3 ) breaks up into a hydrogen ion (H + ) and a bicar- 

 bonate ion (HCO" 3 ). Hence 



_ nr 



- 1Y U1 -. 



We note that a further dissociation is possible, namely, that of 

 HCO 3 ~ into H + and CO 3 ~ "which gives the equilibrium equation 

 C H - *C CO ~~ = K'CHCO^ K' is 1-291 X io~ n , * and therefore 

 CO 3 can occur in appreciable quantity only when the con- 

 centration of HCO 3 ~ is very high, or when the concentration 

 of H + is very low, which as we shall see below is in alkaline 

 solution. 



Water can split into two ions, hydrogen (H + ) and hydroxyl 

 (OH ~) which gives the equilibrium condition, C H .C OH - = KC H2 o 

 The concentration of the water is relatively so great and 

 uniform that we can simplify the equation to C H .C OH - = a 

 constant. This constant has been determined in several 

 different ways and is equal to 0*72 X io~ 14 at 18 Centigrade. 



HYDROGEN ION CONCENTRATION 



The result mentioned in the preceding paragraph is 

 extremely important as the hydrogen ion is the characteristic 

 of an acid and the hydroxyl ion that of a base. An increase 

 in acidity must be accompanied by a reciprocal decrease in 

 alkalinity, therefore it is possible to express the various 

 acidities and alkalinities in terms of the concentration of 

 hydrogen ions. A truly neutral solution, such as pure water, 

 is one in which the concentration of hydrogen ions equals that 

 of the hydroxyl ions. 



The hydrogen ion concentration at the neutral point is 

 calculated as follows : 



CH-COH = K = 072 X I0- 14 C H = C OB . 



. . C 2 H = 072 X io~ 14 and (^=Vo72Xio~ 14 ==0-85 Xio~ 7 . 



S0rensen has suggested the use of the symbol p R to represent 



the negative exponent of the hydrogen ion concentration.! 



* J. Walker and W. Cormak, Journ. Chem. Soc., 1900, Trans., p. 5. 

 f S. P. Sorensen, Ergeb. d. Physiol., 1912, vol. 12, p. 401. 



