Microbes Must Eat 67 



Several rather simple technics have been devised for measuring 

 pH in the laboratory. One of these methods, and the more 

 accurate of the two we shall mention, depends upon electrometric 

 devices, called potentiometers. The other technic, and the more 

 commonly employed method, depends upon indicator dye solutions, 

 usually weak organic acids or bases, which change color as the pH 

 is altered. When attempting to determine the pH of colored 

 solutions, or of solutions containing high concentrations of proteins 

 which mav absorb dye from the indicator, the electrometric 

 devices must be used, since the color changes of pH indicators 

 may be masked under these conditions. However, for most pH 

 determinations, the colorometric technics lend themselves well for 

 use in bacteriological laboratories, in spite of the sacrifice of some 

 accuracy. The series of color standards can readily be checked 

 electrometrically before labeling the tubes with the determined pH. 



The effective range (the useful range) of many indicators 

 commonly employed in pH determinations covers 1.6 points on 

 the pH scale, but again we find exceptions to the rule, some cover- 

 ing a greater spread on the scale. On page 68 is a partial list of 

 some of the satisfactory indicator dyes, with their color changes 

 and their pH ranges. 



Notice in Table 2 that thymol blue has both an acid range 

 (1.2-2.8) and an alkaline range (8.0-9.6). This characteristic 

 is true of a few pH indicators. 



The question may arise as to which indicator one should choose, 

 if several dye ranges overlap on the pH scale. It depends upon 

 the particular use to which the dye is to be put, but when at- 

 tempting to adjust the pH of a medium, choose an indicator whose 

 mid-point is close to the final desired pH of your medium. The 

 degree of color change at the mid-point with each minor alteration 

 in pH is more pronounced than are the color changes at the low 

 end or at the high end of the indicator's effective range. 



One of the characteristics of many microorganisms is their 

 ability to attack sugars or proteins present in a medium, and the 

 resulting concentrations of acids or alkalies may be sufficient to kill 

 the very cells that produced them. When attempting to harvest 



