80 AGRICtJLTUKAL AND INDUSTRIAL BACTERIOLOGY 



neutral point of some particular indicator, or second, by 

 the designation of the true hydrogen ion concentration or, 

 conversely, of the true hydroxyl ion concentration of the 

 medium. While the first method is still the more commonly 

 used, the second method is in most instances preferable. 



Bacteria, yeasts, and molds are usually quite sensitive to 

 the presence of an excess of acid or alkali. Some grow best 

 in a medium which is strictly neutral, others prefer one 

 which is somewhat on the acid side of neutrality, still others 

 on the alkaline side of neutrality. Careful adjustment of 

 reactions is, therefore, necessary in many cases. Some 

 organisms, for example, will not grow unless the medium 

 has almost exactly the same reaction as does the blood or 

 the tissues of the body in which they are accustomed to grow. 



The chemist tells us that acidity is determined by the 

 presence of free hydrogen ions, alkalinity is determined by 

 the presence of free hydroxyl ions, and that a solution is 

 truly neutral when equal numbers of hydrogen and 

 hydroxyl ions are present in a given volume. Pure dis- 

 tilled water is neutral. This does not mean that it has no 

 free hydrogen ions and no free hydroxyl ions, but that when 

 molecules of water dissociate they break up into equal 

 numbers of each ion. The chemist has discovered, further- 

 more, that pure water (and therefore any neutral solution 

 of materials in water) contains approximately one ten- 

 millionth of a gram of hydrogen ions to the liter. This may 

 also be written 10""'' grams hydrogen ions per liter, or still 

 more conveniently expressed in teims of normality of hy- 

 drogen ions. A normal solution of hydrogen ions is one 

 which contains one gram of hydrogen ions per liter. A 

 neutral solution, therefore, is one which has a concentration 

 of hydrogen ions of 10"^ normal. It was noted above that 

 in a neutral solution there is the same number of hydroxyl 

 ions as hydrogen ions. The hydroxyl ion concentration 

 must, therefore, be also 10~' normal. 



