ELECTROPHYSIOLOGY 341 



ions, such as H+, are soon in advance of the others results in an 

 unequal distribution of ions and therefore of electric charges, 

 which produces a potential gradient at or near the diffusion 

 front where the faster ions are in the lead. Except for the rapid 

 H+ ion and the moderately rapid OH" ion, all potentials due to 

 differences in velocity of ions are not great and will, in any case, 

 be quickly obliterated through equaUzation in concentration 

 when the slower ions catch up; furthermore, one ionic species 

 cannot separate far from the other without setting up a very 

 great potential difference. The electric force thus produced 

 tends to slow up the faster ions and hasten somewhat the slower 

 ones. However, if the fluid boundary is maintained by a con- 

 stant flow of the two solutions against one other, a measurable 

 potential is established. 



Liquid Junction Potentials. — A liquid junction potential is set 

 up at the interface between two immiscible liquids. Such an 

 interface maintains itself permanently. The potential results 

 not because of different rates of diffusion of ions across the 

 boundary, as in the case of diffusion potentials, but because of 

 different degrees of solubility of ions in the two phases. For 

 example, water and amyl alcohol are immiscible, and H+ and Cl~ 

 ions are not equally soluble in them. If they are shaken with 

 hydrochloric acid, the two hquids (water and amyl alcohol) 

 separate and form a sharp boundary where a potential is estab- 

 lished owing to the unequal solubility of the H+ and CI" ions. 



Oxidation-reduction Potentials. — Priestley, in 1774, dis- 

 covered that a particular constituent of air, which he called 

 "dephlogisten," is necessary to life; Lavoisier called this sub- 

 stance oxygen. Since then, the importance of oxygen in physi- 

 ological reactions has become increasingly evident. The prime 

 use to which oxygen is put by organisms is the oxidation of foods, 

 among which the sugars have been most studied, though fat and 

 protein oxidation is quite as important. For sugar, the equation is 



CeHiaOe + 6O2 = 6CO2 + 6H2O 

 When sugar is formed by plants through photosynthesis, oxygen 

 is liberated: 



6CO2 + 6H2O - C6H12O6 + 6O2 



Because this latter process involves the release of oxygen, it is 

 known as reduction, which is defined as the reverse of oxidation. 



