OF VITAL PHENOMENA 107 



Calomel electrode | . 1 normal KC1| . 1 n HCl|n HCl[n KCl|cal. elec- 

 trode. 



The diffusion potential between the two acid solutions may be 

 calculated from Nernst's formula (in millivolts at 25 ) : 



u — v c 2 318—65 

 emf = 59 log = 59 log 10 = +39 mv on 



u+v c x 318+65 



the left. The two diffusion potentials between HC1 and KC1 may 



U l+ V 2 



be calculated by Planck's formula: emf = 59 log and 



u 2 +v t 



318+65 



for the first one is 59 log = 59 log 3.2 = +29 mv on 



65+65 



the left, and is neutralized by the second which is +29 mv on 

 the right. Hence the total emf is +39 mv on the left, but would 

 be less if the two KC1 solutions were of the same concentration. 

 In this concentration cell, the emf is due to the difference in 

 speeds of H* and CI'. If we increase this difference by inter- 

 posing between the two HC1 solutions a membrane permeable 

 to H" but impermeable to CI' the emf will be increased to 59 mv, 

 or if the membrane is permeable to CI' and impermeable to H* 

 the emf will be reversed in direction but still be 59 mv. In 

 studying the effect of a membrane on the relative speeds of the 

 two ions of an electrolyte, the use of KC1 is most convenient 

 wherever possible, since the two ions move at the same rate, and 

 danger of unbalanced diffusion potentials at the two ends of the 

 series is eliminated. 



The effect of porcelain membranes on the emf of concentra- 

 tion cells was studied by Bartell and Hocker (1916). Bartell 

 had observed that negative osmose only occurred if the speed 

 of the anion in free diffusion were greater than the cation, and 

 Hocker found that in negative osmose the speed of the anion 

 through the membrane is greater than the cation. Since the 

 dilute side receives the charge of the diffusing ions, the dilute 

 side is negative in negative osmose and positive in positive 

 osmosis. The porcelain itself is charged negatively except in 

 relatively more concentrated solutions of acids, or the salts of 

 certain polyvalent or heavy or rare metals, which neutralize or 

 reverse the charge and consequently alter the osmose. Appar- 



