166 PLANT SOCIOLOGY 



particles, move separately from one another within the solution. 

 When an electric current flows through such a solution, the positive 

 ions migrate to the cathode (hence the name cations for these par- 

 ticles), while the negative ions collect at the anode (hence the name 

 anions) . 



The degree of acidity of any solution depends upon its content of 

 positively charged hydrogen ions (H+), The preponderance of these 

 H''' ions gives acids their common characteristic properties, while in 

 bases the negatively charged hydroxyl ions (0H~) always predominate. 

 In every solution, however, an enduring equilibrium exists between the 

 H+ and 0H~ ions in such a manner that an increase of the H+ ions is 

 always accompanied by a corresponding decrease of the 0H~ ions, or 

 vice versa. The more H+ ions there are present the more strongly acid, 

 the more 0H~ ions, the more basic, is the solution. When H+ and 

 0H~ are present in equal numbers the solution is neutral. This is the 

 case in chemically pure H2O. 



In one liter of pure H2O, at IS'^C, there are 0.0000001 gram-ion 

 of H or, briefly expressed, 10~'^ gram-ion of H+ and 0.0000001 gram-ion 

 of 0H~ or 10"'^ gram-ion of 0H~. Since the total number of H+ and 

 0H~ ions always remains the same, the total mass of ions in pure 

 water is always 10~^^. If the solution is acid, the H+ ions predominate 

 (H+ = 10-1 to 10-0 ; if it is basic, the OR- ions are in excess (H+ = 10"^ 

 to 10-^0 . Thus one can state the degree of acidity of an acid as well 

 as of a basic solution by the quantity of H+ ions per liter of solution. 



Expression of Hydrogen Ion Concentration. — The inconvenience of 

 writing the actual H ion concentration of a solution that is exactly 



neutral as 1 n r>nn non gram-ion of H or as 0,0000001 gram-ion of H led 



Sorensen (1909) to propose the use of the negative logarithm of the 

 H ion concentration as the expression of the degree of acidity, preceded 

 by the sign "pH." When the H ion concentration of a solution is 

 10-^, by the use of the reciprocal the negative exponent is avoided and 

 it is expressed, according to Sorensen, as pH 6. The pH value of a 

 solution is therefore the logarithm of the reciprocal of the H ion con- 

 centration. For the sake of brevity, the pH value is called the hydro- 

 gen number. 



It must always be remembered that large hydrogen numbers (pH 

 7 to 14) correspond to a low H ion concentration; small hydrogen 

 numbers, to a high concentration of H ions. 



In the language of the plant ecologist, the hydrogen numbers with 

 reference to the behavior of individual species and communities have 

 the following significance : 



