METHODS OF STUDYING PHYSIOIjOGIC CHARACTERS 93 



the kind of acid produced, tliird, the amount of buffer 

 present in the nutrient medium, fourth, the amount of 

 alkali (that is, the concentration of hydroxyl ions) pro- 

 duced at the same time. Alkali may be developed either 

 by the production of ammonia or by the transformation 

 of the salt of a strong acid to the salt of a relatively weak 

 or little dissociated acid. For example, certain bacteria 

 may transform sodium citrate into sodium carbonate, the 

 latter being decidedly alkaline in its reaction. 



Two methods are in common use for detecting changes in 

 hydrogen ion concentration. The first is by a determina- 

 tion of the electric conductivity. The second is a colori- 

 metric method. For usual laboratory routine the colori- 

 metric method is the simpler and is the only one which will 

 be discussed. In this method it is customary to add a 

 suitable indicator either to the solution to be tested or 

 to a portion of this solution diluted somewhat with dis- 

 tilled water. The color secured is then compared with the 

 color produced by similar addition of indicator to standard 

 solutions whose hydrogen ion concentration is known. The 

 indicators most used in the bacteriology laboratory for this 

 purpose are those developed by Clark and Lubs. In Table 

 I the name of each of these indicators is given followed by 

 the color in its acid range, next the color in its alkaline 

 range, and finally the range of pH values through which it 

 changes color. 



It is apparent that an approximate idea can be secured 

 of the change in hydrogen ion concentration by using dif- 

 ferent indicators and determining which gives an acid 

 color and which an alkaline color. For example, if it is 

 found that phenol red gives a yellow reaction the medium 

 is acid and must be below the pH value of 6.8. If methyl 

 red, on the other hand, gives an alkaline color it must have 

 a pH value above 6. The exact pH value can be deter- 

 mined then by the use of brom thymol blue, comparing the 



