﻿458 
  R. 
  G. 
  Van 
  Name 
  — 
  Temperature 
  Coefficient 
  

  

  urements 
  have 
  been 
  made 
  at 
  temperatures 
  above 
  25°. 
  Nernst* 
  

   gives 
  the 
  equation 
  D 
  t 
  = 
  D 
  (1 
  + 
  0*026 
  (t 
  — 
  18°) 
  ) 
  as 
  approxi- 
  

   mately 
  expressing 
  the 
  experimental 
  results 
  in 
  the 
  case 
  of 
  salts. 
  

   For 
  acids 
  and 
  bases 
  the 
  numerical 
  coefficient 
  is 
  0*024 
  instead 
  of 
  

  

  0*026. 
  Calculating 
  from 
  this 
  equation 
  the 
  value 
  of 
  — 
  * 
  + 
  10 
  ° 
  for 
  

  

  the 
  different 
  intervals 
  in 
  the 
  range 
  covered 
  by 
  the 
  reaction 
  

   velocity 
  measurements 
  we 
  obtain 
  the 
  following 
  results: 
  

  

  2-5°-12-5° 
  15°-25° 
  25°-35° 
  35°-45° 
  45°-55° 
  55°-65° 
  

  

  D 
  t 
  

  

  1*436 
  1*282 
  1*220 
  1*180 
  1*153 
  1*132 
  

  

  These 
  figures 
  can 
  only 
  be 
  regarded 
  as 
  rough 
  approximations, 
  

   since 
  the 
  change 
  in 
  D 
  with 
  the 
  temperature 
  is 
  not, 
  in 
  reality, 
  

   strictly 
  linear 
  as 
  the 
  equation 
  assumes. 
  Moreover 
  Oholmf 
  has 
  

   shown 
  that 
  the 
  numerical 
  coefficient 
  is 
  not 
  the 
  same 
  for 
  all 
  

   salts, 
  but 
  varies 
  between 
  the 
  limits 
  0*023 
  and 
  0*027. 
  

  

  On 
  comparing 
  these 
  results 
  with 
  corresponding 
  ratios 
  for 
  the 
  

   reaction 
  velocity, 
  as 
  given 
  in 
  Table 
  IV, 
  we 
  see 
  that 
  with 
  the 
  

   exception 
  of 
  the 
  lowest 
  value 
  the 
  increase 
  in 
  the 
  reaction 
  veloc- 
  

   ity 
  with 
  the 
  temperature 
  is 
  throughout 
  slightly 
  more 
  rapid 
  

   than 
  that 
  of 
  the 
  rate 
  of 
  diffusion. 
  This 
  appears 
  to 
  be 
  at 
  least 
  

   a 
  partial 
  confirmation 
  of 
  the 
  predictions 
  of 
  the 
  diffusion 
  theory, 
  

   since 
  the 
  increase 
  in 
  the 
  fluidity 
  with 
  the 
  temperature 
  probably 
  

   causes 
  some 
  decrease 
  in 
  the 
  thickness 
  of 
  the 
  diffusion 
  layer.*]: 
  

   No 
  great 
  significance 
  can 
  be 
  ascribed 
  to 
  this 
  result, 
  however, 
  

   because 
  of 
  the 
  above 
  mentioned 
  uncertainty 
  in 
  the 
  calculated 
  

   values 
  of 
  the 
  diffusion 
  coefficients, 
  and 
  because 
  the 
  case 
  of 
  

   iodine 
  diffusing 
  in 
  an 
  iodide 
  solution 
  is 
  not 
  wholly 
  analogous 
  

   to 
  that 
  of 
  the 
  diffusion 
  of 
  a 
  simple 
  electrolyte. 
  Although 
  the 
  

   iodine 
  is 
  largely 
  in 
  the 
  form 
  of 
  tri-iodide 
  ion 
  a 
  probability 
  of 
  

   complications 
  is 
  introduced 
  by 
  the 
  fact 
  that 
  this 
  ion 
  is 
  itself 
  in 
  

   dissociation 
  equilibrium 
  with 
  free 
  iodine. 
  It 
  has 
  been 
  proved, 
  

   for 
  example, 
  that 
  the 
  rate 
  of 
  diffusion 
  of 
  iodine 
  in 
  potassium 
  

   iodide 
  solutions 
  increases 
  with 
  the 
  concentration 
  of 
  the 
  salt,§ 
  

   while 
  the 
  reverse 
  would 
  be 
  expected 
  if 
  the 
  case 
  were 
  normal. 
  | 
  

  

  * 
  Zeitschr. 
  phys. 
  Chem. 
  ii, 
  625, 
  1888. 
  

  

  f 
  Zeitschr. 
  phys. 
  Chem. 
  1, 
  309, 
  1905. 
  

  

  \ 
  That 
  the 
  effect 
  would 
  be 
  in 
  this 
  direction 
  is 
  practically 
  certain, 
  but 
  noth- 
  

   ing 
  definite 
  is 
  known 
  about 
  its 
  magnitude. 
  The 
  results 
  of 
  Van 
  Name 
  and 
  

   Hill 
  (this 
  Journal 
  (4) 
  xxxvi, 
  543, 
  1913) 
  would 
  seem 
  to 
  show, 
  however, 
  that 
  the 
  

   effect 
  of 
  viscosity 
  changes 
  upon 
  the 
  thickness 
  of 
  the 
  diffusion 
  layer 
  is 
  rela 
  

   tively 
  small. 
  

  

  § 
  Edgar 
  and 
  Diggs, 
  Jour. 
  Amer. 
  Chem. 
  Soc, 
  xxxviii, 
  253, 
  1916. 
  

  

  || 
  This 
  follows 
  from 
  the 
  fact 
  that 
  the 
  velocity 
  of 
  the 
  tri-iodide 
  ion 
  is 
  con- 
  

   siderably 
  lower 
  than 
  that 
  of 
  the 
  potassium 
  ion, 
  together 
  with 
  the 
  rule 
  (Abegg 
  

   and 
  Bose, 
  Zeitschr. 
  phys. 
  Chem., 
  xxx, 
  551, 
  1899), 
  that 
  the 
  presence 
  of 
  an 
  

   excess 
  of 
  a 
  common 
  ion 
  tends 
  to 
  impart 
  to 
  a 
  diffusing 
  salt 
  the 
  velocity 
  char- 
  

   acteristic 
  of 
  its 
  other 
  ion. 
  

  

  