340 



PROTOPLASM 



one direction and carry with them a definite quantity of negative 

 electricity; and cations migrate in the other direction, carrying 

 positive electricity. 



Electrode potentials as such do not enter into living systems 

 normally, but, owing to polarization, they are often bothersome 

 when measurements of other potentials are being made with the 

 aid of metal electrodes. For this reason, metal electrodes are not 

 satisfactory for measuring potentials in living tissues. Special 

 contacts or junctions {e.g., of agar saturated with salt) have to 

 be resorted to (Fig. 42). 



Contact and polarization potentials are potentials not unlike 

 electrode potentials but with other names. When a current is 

 passed through a circuit, the source of electromotive force then 

 removed, and the circuit closed again, a current may begin to 

 flow in the opposite direction, as in an accumulator. This back 

 current presupposes a polarization potential. 



Concentration Potentials. — The electrodes of a galvanic cell 

 are of two different metals in a common solution. If electrodes 

 of the same metal are put into separate solutions of the same salt 

 but of different concentrations, and the two electrodes joined 

 by a conductor, a flow of current will result due to a potential 

 difference between the electrodes. Such a potential is known 

 as a concentration potential, and the cell as a concentration cell. 



Diffusion Potentials. — A diffusion potential, sometimes referred 

 to as a liquid junction potential, is produced at the surface of 

 contact between two miscible solutions. It is due, as Nernst 

 first pointed out, to the unequal diffusion of ions across the 

 boundary from one to the other liquid due to the different 

 migration velocities of the ions. Some migration numbers are 

 given in the following table: 



The hydrogen ion moves at 0.00325 cm. a second, and the 

 nitrate ion at 0.00064 cm. a second. The fact that the faster 



