Microbes and You 



Table 2 

 INDICATORS AND THEIR pR RANGES 



a maximum crop of organisms we must minimize pH changes in 

 our medium by incorporating substances called buffers. Buffers 

 may be defined as substances, which by their presence in solutions, 

 increase the amount of acid or alkali that must be added to cause 

 material change in pH of the solution. The word is derived from 

 the German word Puffer (plug or bung), and the most efficient 

 buffers are mixtures of weak acids or weak bases, in combination 

 with their salts and certain other amphoteric substances. Ampho- 

 teric substances are able to dissociate so that under one set of 

 conditions they yield hydrogen ions, and under another set of 

 conditions hydroxyl ions are liberated. Sometimes both types of 

 ions are released simultaneously. Alkaline dissociation pre- 

 dominates in an acid medium, and acidic dissociation of ampho- 

 teric substances can be expected when the medium is alkaline. 

 The hydrogen ion concentration at which this type of dissociation 

 is at a minimum is called the isoelectric point of the amphoteric 

 substances. Buffers do not stop pH changes, they merely retard 



