CHAPTER IV 



HYDROGEN AND HYDROXYL ION CONCENTRATION 

 AND C0 2 PRESSURE 



Water is an electrolyte and dissociates into H and OH 

 ions, which are therefore equal in number. At 22° or 23° 

 H* = OH' = 10- 7 , or H" X OH' = io"". If we increase 

 H" by adding acid, OH' decreases, because H' X OH' remains 

 equal to io~ 14 . Conversely, if we add alkali, H* decreases, and 

 hence we may estimate the acidity or alkalinity of a solution by 

 determining the H" concentration. The product of the H and 

 OH ion concentrations is io~ 14 and the — logarithm is 14 (and 

 is designated — log K w ). The following table gives the values 

 of — logK w for various temperatures, calculated thermo- 

 dynamically from the results of Lorenz and Boehi (1909) at 

 18 , and which agree with those of Sorensen, Michaelis and 

 some others within about one degree. 



In order to save space it is better to express H" concentration 

 as the minus logarithm (PH). Thus if H" concentration = io~ T , 

 PH = 7 (Sorensen 1909). 



In order to understand the method by which the H ion con- 

 centration is determined we must first consider the theory of 

 concentration cells and electrode potentials. The fact that an 

 electric potential difference is produced at the surface of con- 

 tact of the metal with a solution is explained as follows : We 

 know that the ions of metals are soluble because they are pro- 

 duced by the solution of metallic salts. There is also a store 



