24 



PHYSIOLOGICAL CHEMISTRY 



termines its so-called "strength". Strong acids are much dis- 

 sociated, giving off much hydrogen ion; weak acids are but 

 slightly dissociated, giving off little hydrogen ion. Even water 

 is a weak acid (and a weak base) since it dissociates thus 



+ + 



H 2 = H + OH. In water the numbers of H and OH ions are 

 equal, and such a solution is called neutral. If the hydrogen 

 ions exceed the hydroxyl ions in number, the solution is called 

 acid. If the hydroxyl ions exceed, it is called alkaline. 



The extent to which a substance dissociates -into its ions can 



be expressed by a figure called the dissociation constant. For 



i _ 



TT v/ 



water this is expressed as follows: 



= k. 



But for water or an aqueous solution the concentration of 



undissociated H 2 is so great that it can be regarded as a con- 



I _ 



stant, and the equation thus becomes H X OH = Kw. Kw is 

 called the water constant. It has been found that there are 

 0.000,000,1 gram ions of hydrogen ions and of hydroxyl ions in 

 1 liter of water. Expressed with logarithms this figure is 10' 7 ". 

 The above equation then becomes 10' 7 X 10' 7 = Kw = 10' 14 . If 

 we measure the concentration either of hydrogen ions or of 

 hydroxyl ions it is thus possible to calculate the other from this 

 equation. 



The symbol used to express hydrogen-ion concentration is CH. 

 It indicates the amount of hydrogen ion in 1 liter of the liquid. 

 Thus the C H of water is 0.000,000,1 or 10' 7 . It is obviously in- 

 convenient to use such an expression, so a shorter symbol has 

 been devised, the pH. The pH is the logarithm of the CH, with 

 the minus sign omitted. Thus the pH of water is 7. If the 

 solution is acid, the amount of hydrogen ion will be greater 

 than 10- 7 , say 10' 6 , 10' 5 , etc., and the pH will be 6,5, etc. A pH 

 smaller than 7 indicates thus that the liquid is acid in reaction, 

 whereas in an alkaline solution the C H is less than 10' 7 , say 10" 8 , 

 10' 9 , etc., and the pH will be 8, 9, etc., that is, greater than 7. 



