82 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



this scale, the higher the hydrogen ion concentration, that 

 is, the greater the acidity ; and the larger the number, the 

 greater the alkalinity. 



Indicators are chemical substances which are of one color 

 in a certain range of pH values or hydrogen ion concen- 

 trations, and of another color in other ranges. The indi- 

 cator most commonly used is litmus, which has a lilac color 

 at true neutrality, that is, at a pH value of 7. At a pH 

 value of 8, that is, in a more alkaline solution, it is blue. 

 At a pH value of 5, that is, in a stronger concentration of 

 hydrogen ions, it is red. Other indicators change color at 

 other pH values. For example, phenolphthalein is color- 

 less in all hydrogen ion concentrations heaving a pH value 

 less than 8.2 ; in more alkaline solutions, it is red. Many 

 other indicators are known which change color at other 

 points in the hydrogen ion scale. This change of color is 

 not instantaneous. In adding alkali, for example, to an 

 acid solution containing litmus, there is not an instantane- 

 ous transformation of red into blue. By the use of stand- 

 ards whose hydrogen ion concentration is known it is possi- 

 ble to determine approximately the hydrogen ion concen- 

 tration of any material which it is desired to test, by the 

 use of the intensity of color of appropriate indicators. 



Most bacteria grow in a hydrogen ion concentration of 

 10~ 7 to 10~ 8 , that is, in solutions having a pH value between 

 7 and 8. The pH value of blood is usually about 7.35 

 This indicates, therefore, the hydrogen ion concentration 

 most useful in cultivating many species of pathogenic bac- 

 teria. Inasmuch as most media are on the acid side of true 

 neutrality it is necessary to add alkali, usually potassium 

 or sodium hydrate, until test shows that the hydrogen ion 

 concentration has become satisfactory. 



The hydrogen ion concentration of medium or solution 

 depends not only upon the actual concentration and kind 

 of the acid but upon the concentration of substances which 



