CHAPTER II 



HYDROGEN ION CONCENTRATION AND pR ; 

 OXIDATION-REDUCTION POTENTIALS 



Hydrogen Ion Concentration. — The course of all biological 

 processes is profoundly influenced by the degree of acidity 

 or alkalinity of the fluid in which they take place — whether 

 the fluid ])e tlie cell contents, or a circumambient fluid 

 like blood, or a cidture medium in which micro-organisms 

 are growing, or a solution in wliicli enzymes are acting. 

 As a notable example one may consider the blood. Should 

 it become very slightly acid death in coma will result, with 

 the heart muscle relaxed ; on the other hand, if it should 

 become but slightly alkaline tetany results and the heart 

 will cease to function with the muscle contracted. The 

 heart will only function properly when the blood is within 

 a very narrow range between acidity and alkalinity. 

 Similar, though as a rule not such dramatic, changes 

 follow alteration of the normal state of other biologically 

 concerned fluids. Some bacteria, for instance, thrive in 

 quite strongly acid solutions but raj)idly die out in 

 alkaline conditions ; others, like the cholera vibrio, 

 develop in alkaline but not in acid media. The same 

 applies to enzymes ; the gastric enzyme, pepsin, is only 

 active in breaking down j)roteins when in acid solution, 

 whilst trypsin, in the pancreatic juice, requires an alkaline 

 medium for its activity. 



According to modern views an acid is defined as a 

 substance which tends to lose a proton (hydrogen nucleus 



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