VIII. BIOELECTRIC M E A S U K E M E N T S 239 



such a way that the two sohitions come in contact with each other, a 

 potential will be found to exist between the two solutions as long as 

 the difference in concentration persists. These potentials are known 

 as liquid junction potentials. In 1889 Nernst derived an equation 

 expressing the voltage, E, obtained from a liquid junction: 



E = -— In -- (1) 



n u -\- V C 2 



where Ci and C^ are the concentrations in the two solutions, u and v 

 the mobilities of cation and anion, respectively, R the gas constant, 

 T the absolute temperature, and n the quantity of electricity carried 

 by one gram equivalent. It will be noted that the voltage depends 

 on the difference in mobility of the two ions. For example, in the 

 case of hydrochloric acid, the hydrogen ion can diffuse about five 

 times as fast as the chloride ion. At a liquid junction both ions 

 would tend to diffuse from the concentrated solution to the dilute, 

 but the hydrogen going faster would make the dilute solution positive 

 with respect to the concentrated solution by an amount given by 

 equation (1). If the mobilities of the two ions are practically the 

 same, as is the case with potassium chloride, the liquid junction po- 

 tential practically vanishes. 



Equation (1) has been found to be very nearly correct for the poten- 

 tial between two solutions having practically the same activity coef- 

 ficient. If this condition is not fulfilled, equation (1) must be some- 

 what modified. 



The question arises as to what part, if any, these liquid junction 

 potentials play in the potentials commonly measured in biological 

 systems. As we proceed with a discussion of methods of measuring 

 these potentials it will become evident that, in any actual measuring 

 set-up, liquid junctions play quite a prominent part in determining 

 the measured potential. However, whether liquid junction poten- 

 tials normally exist in and around normal single cells is quite 

 doubtful. The distances involved are usually small and diffusion 

 equilibrium could take place quite rapidly. Thus it is usually felt 

 that except for special cases liquid junction potentials as they nor- 

 mally appear represent measurement artifacts. 



If, however, one ion of an electrolyte is constrained so that it can- 

 not move, while the other is free to migrate, it is clear that an appre- 

 ciable potential could develop. Such a situation exists when two 

 solutions are separated by a membrane that will allow only one of 



