372 Mr. A. Frumkin on the 



there are ions of two kinds : Zn" and S0 4 '', so that we can 



put e=(-r Zll .. + r S o 4 ,)F; 



further -n __p , r 



l Zn — X Zn" ~1~ •*- ZnS0 4 ' 



and if we neglect the influence of undissociated molecules 

 whence 



r — r 



X Zn - "-*" Zn"? 



=r S0 „F* (5) 



With the help of this equation we can calculate the 

 maximum possible value which .- — \- can have when zinc 



amalgam is immersed in w/1000 ZnS0 4 . The amalgam is 

 negatively charged, in the surface-layer there is therefore a 

 deficiency of S0 4 " ions. We may assume the thickness of 

 the double layer to be approximately equal to 10~ 7 -cm. 

 The absolute A^alue of T so „ is at any rate less than 10~ 7 cm. 

 X 10" 6 gr. eq./c.cm. = 10~ 13 gr. eq./cm. 2 ; this corresponds to a 



value olVj equal to 10" 8 coulomb/cm. 2 = 10 _1 dyne/volt cm. j 



which can be neglected. The surface-tension of a zinc 

 amalgam immersed in a dilute solution of a zinc salt does not 

 depend therefore on its potential, nor, in consequence, on 

 the concentration of the solution. In this particular case 



^-. is zero because the quantities e and T-^gF have equal 



absolute values and opposite signs. 



Let us suppose now that in the solution there is a great 

 excess of a salt with another cation, for instance, Na 2 S0 4 , then 



e=-(r Na .-r Zll ..+r S0 ,,)F, 



or, if the concentration of ZnS0 4 is low as compared with 

 that o£ Na 2 S0 4 , 



*=(-iV+r S0 ,,)F. 



As there is now an excess of Na* in the surface-layer, the 

 value of € is no longer limited and the surface-tension will 

 vary with tJt and in consequence with the concentration of 

 ZnS0 4 . To verify this inference I measured with the 



* The reasoning above is not quite correct, the concentration of Zn" 

 Deing, contrary to the assumption we have made previously, of the same 

 order of magnitude as the concentration of 80 4 ". Nevertheless, as it is 

 easy to show, equation (5) holds if we denote by yjr the potential 

 difference between an imaginary SO/' electrode and the zinc electrode. 

 Besides that does not affect the following considerations. 



