CHAPTER 8 

 HYDROGEN-ION CONCENTRATION 



The growth of fungi and bacteria may be inhibited or prevented by 

 media which are too acidic or too alkaline. A completely satisfactory 

 medium may be made useless by the addition of relatively small amounts 

 of strong acids or bases but may have its former usefulness restored if the 

 excess acid or base is neutralized. This suggests that the ions which 

 characterize acids and bases are particularly active in life processes. It is 

 necessary to understand certain fundamental ideas about acidity and 

 ways of measuring concentration of these ions before discussing in detail 

 the effects of acids and bases on the activities of the fungi. 



IONIZATION OF COMPOUNDS 



Since water is the universal solvent for all life processes, our discussion 

 will be confined to aqueous solutions. The chemical compounds which 

 comprise natural and synthetic media may be divided into two classes, 

 those which form ions in solution (acids, bases, and salts), and those 

 which do not ionize (organic compounds in general, except organic acids 

 and bases). Water is a compound of the first class, although it forms ions 

 to a very slight degree. The ionization of water may be represented by 

 the following equation: 



(1) HOH ^ H+ + OH- or HOH + HOH ^ H3O+ + OH" 



For each molecule of water ionized one hydrogen and one hydroxyl ion 

 are formed. 



In any aqueous solution the product of the concentrations of the 

 hydrogen and the hydroxyl ions (in moles) is equal to a constant (K^), 

 Water is a neutral compound, i.e., the concentrations of hydrogen and 

 hydroxyl ions are equal. A solution which contains a greater concentra 

 tion of hydrogen than of hydroxyl ions is acidic ; a solution which contains 

 a greater concentration of hydroxyl ions than of hydrogen ions is basic, or 

 alkaline. Since all aqueous solutions contain hydrogen and hydroxy] 

 ions, the deleterious effects of these ions must be due to their relative 

 concentrations. At room temperature (23 to 25°C.) the concentration of 

 hydrogen and hydroxyl ions in water is 1 X 10"^ mole per liter, or 1 mole 

 each of these ions in 10 million liters. The degree of ionization of water 

 increases with temperature. However, water is a neutral substance at 



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