Microbes Must Eat 65 



PREREQUISITE III: THE CORRECT ACID-BASE BALANCE- 



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If all of the other requirements for a good microbial medium 

 are supphed, but the acidity or the alkahnity is not properly con- 

 trolled, poor growth of organisms, or no growth at all, may result 

 The usual ingredients incorporated into common media are acid in 

 character, and we must neutralize at least part of this acidity by 

 adding alkaline substances called bases. Most organisms prefer a 

 medium that is close to neutrality, but as might be expected, some 

 species require marked deviations from neutrality before thev find 

 optimum growth conditions. All organisms have a maximum, 

 an optimum, and a minimum chemical reaction with respect to 

 growth of that species. 



We learn in chemistry that it is the concentration of dissociated 

 or ionized hydrogen (H) or hydroxyl (OH) that determines the 

 effective acidity or alkalinity of a solution. The theory of dis- 

 sociation of electrolytes was formulated in 1887 bv the Swedish 

 chemist, Svante Arrhenius (1859-1927), and measurement of the 

 acid-base reaction of a medium in microbiology is determined by 

 the HYDROGEN ION CONCENTRATION, abbreviated pH. Equal con- 

 centrations of so-called weak acids and strong acids show marked 



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diflferences in true acidity, or sourness. This difference depends 

 upon the ability of the acid to ionize, or dissociate. Strong acids 

 dissociate into relatively large numbers of hydrogen ions, while 

 weak acids do not ionize to such a degree. 



Pure water dissociates very slightly into equal numbers of 

 hydrogen ( H ) ions and hydroxyl ( OH ) ions, and hence such water 

 is neutral in reaction. At 22°C. the hydrogen ion concentration 

 of pure water is 1 X 10"^ gram ions per liter. Hydrogen ion 

 potential is expressed as the logarithm of the reciprocal of the 

 hydrogen ion concentration, and this expression was given the 

 symbol pH by S. P. L. Sorensen in 1909. In other words, pH can 



be determined bv the formula pH = Loe — - 



