Alternating  Current  on  a  Polarizable  Electrode.       337 
Special  treatment  of  the  case  of  the 
Solution  of  a  Complex  Salt. 
It  is  necessary  to  treat  o£  solutions  of  complex  salts 
separately.  The  polarization  capacity  of  a  mercury  electrode 
lias  been  worked  out  in  this  case  theoretically  and  experi- 
mentally by  Kriiger  *. 
In  the  case  of  complex  salts  there  is  a  third  element  which 
comes  into  play  in  addition  to  the  double-layer  of  Helmholtz 
and  the  effect  connected  with  diffusion,  considered  by 
Warburg.  This  is  due  to  the  fact  that  the  mercury  ions  are 
formed  from  complex  ions  by  dissociation  when  the  current 
brings  ions  out  of  solution,  and  the  mercury  ions,  when 
brought  into  solution,  disappear  to  form  complex  ions,  and 
these  ionic  reactions  do  not  take  place  with  infinite  velocity. 
Kriiger  considers  three  limiting  cases: — 
(1)  The  concentration  of  the  mercury  salt  is  very  small. 
Tn  this  case  the  double-layer  plays  the  most  important  part, 
as  the  ion  quantity  supplied  by  dissociation  is  small  compared 
with  that  required  to  form  the  double-layer. 
(2)  The  solution  is  of  medium  strength.  With  increase  of 
concentration,  the  diffusion  and  dissociation  come  into  im- 
portance. The  influence  of  diffusion,  especially  that  of  the 
undissociated  complex,  is  the  important  factor. 
(3)  The  limiting  case,  where  the  concentration  of  the  salt 
is  very  great.  If  the  velocity  of  reaction  were  infinite,  the 
electrode  would  be  unpolarizable.  This  velocity  is,  however, 
even  if  very  great,  by  no  means  infinitely  great,  and  for 
frequencies  so  high  as  1000^  to  5000^,  probably  the  velocity 
of  reaction  is  an  important  factor. 
The  three  cases  are  characterized  by  the  theoretical  result 
that  the  polarization  capacity  in  the  first  is  independent  of 
the  frequency,  in  the  second  is  proportional  to  — -p=,  and  in 
1  ...     v  ^ 
the  third  to  ^-.      Kriiger  gives   cases  in  which  these  three 
limiting  cases  are  realized  or  at  any  rate  nearly  approached. 
In  considering  the  asymmetrical  action  in  the  case  of 
complex  salts,  we  ma}^  put  aside  the  first  limiting  case  as 
presenting  nothing  new,  being  the  same  as  with  typical  salts 
of  small  concentration. 
Also  in  the  second  case,  it  is  clear  that  the  corresponding 
rase  with  the  solution  of  a  typical  salt  is  only  so  far  altered, 
that  the  diffusion  is  that  of  the  complex,  and  not  that  of  the 
mercury  ion.     The  finiteness  of  the  velocity  of  reaction  has 
*  Kriig'er,  toe.  cit. 
