﻿Estimation 
  of 
  Boric 
  Acid. 
  129 
  

  

  certain 
  period 
  of 
  time 
  is 
  required 
  for 
  the 
  completion 
  of 
  the 
  

   reaction. 
  When 
  the 
  solution 
  is 
  of 
  small 
  volume 
  and 
  saturated 
  

   with 
  mannite, 
  the 
  reaction 
  ^oes 
  to 
  the 
  end 
  most 
  quickly 
  — 
  

   sometimes 
  almost 
  immediately 
  — 
  but 
  there 
  is 
  this 
  limitation 
  

   which 
  must 
  be 
  made 
  emphatic, 
  viz 
  : 
  that 
  if 
  the 
  solution 
  of 
  

   boric 
  acid 
  is 
  too 
  concentrated 
  — 
  near 
  saturation 
  — 
  the 
  boric 
  acid 
  

   alone, 
  when 
  the 
  iodate 
  and 
  iodide 
  are 
  added 
  to 
  destroy 
  any 
  

   other 
  free 
  acid 
  present, 
  throws 
  out 
  some 
  iodine 
  and 
  on 
  bleach- 
  

   ing 
  with 
  thiosulphate 
  a 
  starting 
  point 
  is 
  obtained 
  at 
  which 
  

   some 
  of 
  the 
  boric 
  acid 
  has 
  already 
  entered 
  into 
  combination. 
  

   The 
  amount 
  of 
  iodine 
  thus 
  liberated 
  by 
  the 
  boric 
  acid 
  is, 
  how- 
  

   ever, 
  not 
  large, 
  and 
  if 
  upon 
  the 
  addition 
  of 
  the 
  iodide 
  and 
  

   iodate, 
  the 
  iodine 
  thrown 
  out 
  by 
  the 
  free 
  hydrochloric 
  acid 
  

   present 
  is 
  immediately 
  bleached 
  by 
  thiosulphate 
  and 
  the 
  analy- 
  

   sis 
  proceeded 
  with 
  from 
  this 
  as 
  the 
  neutral 
  point, 
  even 
  in 
  con- 
  

   centrated 
  solutions 
  the 
  error 
  is 
  almost 
  inappreciable. 
  If, 
  how- 
  

   ever, 
  considerable 
  time 
  "intervenes 
  between 
  the 
  adding 
  of 
  the 
  

   iodide 
  and 
  iodate 
  and 
  the 
  determination 
  of 
  the 
  neutral 
  point 
  

   by 
  thiosulphate, 
  as 
  much 
  as 
  several 
  milligrams 
  of 
  boric 
  acid 
  

   may 
  have 
  liberated 
  its 
  amount 
  of 
  iodine 
  and 
  is, 
  therefore, 
  not 
  

   capable 
  of 
  being 
  registered 
  by 
  thiosulphate 
  after 
  the 
  addition 
  

   of 
  mannite. 
  This 
  difficulty 
  was 
  not 
  met 
  with 
  in 
  those 
  experi- 
  

   ments 
  in 
  which 
  the 
  iodide 
  and 
  iodate 
  were 
  added 
  at 
  a 
  dilution 
  

   little 
  greater 
  than 
  that 
  of 
  the 
  standard 
  solution 
  used 
  (7*738 
  

   grm. 
  per 
  liter), 
  but 
  in 
  an 
  attempt 
  to 
  estimate 
  the 
  boric 
  acid 
  in 
  

   colemanite, 
  where 
  the 
  solution 
  was 
  kept 
  as 
  concentrated 
  as 
  

   possible, 
  hoping 
  in 
  this 
  way 
  to 
  decrease 
  the 
  time 
  required 
  for 
  

   the 
  complete 
  liberation 
  of 
  iodine, 
  low 
  values 
  were 
  obtained 
  ; 
  

   that 
  is, 
  a 
  false 
  starting 
  point 
  was 
  used. 
  

  

  The 
  dilution 
  found 
  most 
  convenient 
  at 
  the 
  time 
  of 
  adding 
  

   the 
  iodide 
  and 
  iodate 
  is 
  not 
  less 
  than 
  25"^°'^ 
  for 
  each 
  decigram 
  

   of 
  boric 
  acid 
  (B^Og) 
  present 
  and 
  should 
  not 
  be 
  much 
  greater 
  

   than 
  two 
  or 
  three 
  times 
  that 
  amount. 
  This 
  limitation 
  as 
  

   regards 
  volume 
  is 
  equally 
  applicable, 
  whether 
  after 
  obtaining 
  

   the 
  neutral 
  point 
  and 
  treating 
  with 
  mannite, 
  the 
  boric 
  acid 
  is 
  

   to 
  be 
  measured 
  by 
  a 
  standard 
  solution 
  of 
  alkali 
  as 
  before 
  

   described 
  or 
  as 
  here 
  by 
  the 
  iodine 
  liberated. 
  As 
  has 
  been 
  sug- 
  

   gested, 
  a 
  large 
  volume, 
  even 
  though 
  saturated 
  with 
  mannite, 
  

   prolongs 
  the 
  time 
  of 
  standing 
  necessary 
  and 
  increases 
  the 
  effect 
  

   of 
  carbon 
  dioxide 
  upon 
  the 
  iodide 
  and 
  iodate 
  present, 
  for 
  car- 
  

   bon 
  dioxide, 
  whether 
  derived 
  from 
  the 
  atmosphere 
  or 
  existing 
  

   dissolved 
  in 
  the 
  solution, 
  upon 
  standing, 
  slowly 
  liberates 
  iodine. 
  

   The 
  amount, 
  however, 
  is 
  small 
  and, 
  in 
  the 
  time 
  required 
  for 
  

   the 
  completion 
  of 
  the 
  process, 
  has 
  never 
  been 
  found 
  equiva- 
  

   lent 
  to 
  more 
  than 
  a 
  single 
  drop 
  of 
  the 
  solution 
  of 
  thiosulphate 
  

   used. 
  Even 
  if 
  the 
  material 
  to 
  be 
  analyzed 
  contains 
  carbo- 
  

   nates, 
  after 
  acidifying 
  in 
  concentrated 
  solution 
  and 
  shaking 
  

  

  