﻿20 
  

  

  J. 
  Stansfield 
  — 
  Retarded 
  Diffusion 
  and 
  

  

  potassium 
  chromate 
  it 
  is 
  convenient 
  to 
  speak 
  of 
  the 
  silver 
  ions 
  

   and 
  chromat-ions, 
  but 
  the 
  fact 
  should 
  not 
  be 
  lost 
  sight 
  of 
  that 
  

   these 
  ions 
  do 
  not 
  diffuse 
  as 
  isolated 
  entities, 
  but 
  each 
  charged 
  

   ion 
  must 
  be 
  associated 
  with 
  a 
  charge 
  of 
  equal 
  amount 
  and 
  

   opposite 
  sign 
  borne 
  by 
  some 
  other 
  ion 
  or 
  ions. 
  

  

  Suppose 
  that 
  the 
  concentrations 
  of 
  the 
  silver 
  and 
  chromate 
  

   ions 
  at 
  the 
  planes 
  from 
  which 
  they 
  begin 
  to 
  diffuse 
  are 
  a 
  and 
  c 
  

   respectively, 
  that 
  the 
  rates 
  of 
  diffusion 
  are 
  Y 
  a 
  and 
  Y 
  c 
  , 
  and 
  that 
  

  

  Fig. 
  10. 
  

  

  fCwi 
  

  

  > 
  -6 
  

  

  o 
  

  

  n 
  R,a 
  4 
  

  

  N 
  N 
  a 
  0H 
  

  

  T,ME 
  "" 
  2 
  ^ 
  

  

  Fig. 
  10. 
  Simple 
  diffusion. 
  

  

  a 
  precipitate 
  is 
  formed 
  at 
  some 
  plane 
  P 
  at 
  a 
  distance 
  d 
  from 
  the 
  

   original 
  silver 
  diffusing 
  front. 
  

  

  Then 
  the 
  concentrations 
  of 
  silver 
  and 
  chromate 
  ions 
  at 
  P 
  are 
  

  

  _j^i_ 
  and 
  c 
  ' 
  respectively. 
  But 
  the 
  product 
  of 
  these 
  two 
  is 
  

  

  V 
  Y 
  a 
  c 
  

   the 
  precipitation 
  value, 
  so 
  that 
  *' 
  c 
  ' 
  , 
  y 
  has 
  a 
  value 
  which 
  is 
  

  

  susceptible 
  of 
  measurement, 
  because 
  A 
  .C 
  = 
  k.X 
  . 
  (1) 
  

  

  Assuming 
  that 
  the 
  distance 
  D 
  remains 
  about 
  the 
  same 
  for 
  

   two 
  or 
  three 
  consecutive 
  bands, 
  the 
  condition 
  for 
  the 
  forma- 
  

   tion 
  of 
  a 
  precipitate 
  is 
  from 
  (1) 
  

  

  V^ax 
  = 
  , 
  x 
  (2) 
  

  

  or, 
  since 
  Y 
  a 
  and 
  Y 
  c 
  are 
  constants 
  for 
  given 
  concentrations 
  and 
  

   for 
  a 
  given 
  temperature, 
  the 
  relation 
  between 
  the 
  initial 
  con- 
  

  

  