458  Estimation  of  Formaldehyde.  SiT0" 
are  added  2.5  c.c.  of  permanganate  solution  (2.0  grams  KMnC>4 
per  100  c.c.),  and  then  0.2  c.c.  of  strong  sulphuric  acid.  When 
the  reaction  has  proceeded  about  five  minutes,  0.5  c.c.  of  oxalic 
acid  solution  is  added  (0.6  grams  crystallized  acid  per  100  c.c). 
On  shaking  the  liquid  becomes  clear  and  nearly  colorless.  One 
c.c  of  strong  sulphuric  acid  is  now  run  in  and  well  mixed  with  the 
solution,  which  is  finally  treated  with  5  c.c.  of  SchifFs  reagent. 
A  violet  color  is  developed  in  the  course  of  a  few  minutes  unless 
mere  traces  of  methyl  alcohol  were  present,  when  twenty  or  thirty 
minutes  may  be  required. 
This  color  is  due,  of  course,  to  the  reaction  of  the  fuchsin 
solution  with  formaldehyde,  produced  by  the  oxidation  of  the 
methyl  alcohol.  A  sufficient  quantity  of  sulphuric  acid  is  present  to 
prevent  the  development  of  color  with  any  acetaldehyde  formed 
from  the  ethyl  alcohol  during  the  oxidation. 
A  preliminary  experiment  carried  out  as  described  serves  to 
detect  the  presence  of  methyl  alcohol,  if  it  is  not  already  known, 
and  to  give  some  idea  of  the  quantity.  According  to  the  indications 
thus  obtained,  another  part  of  the  prepared  liquid  is  further 
diluted,  if  necessary,  with  ethyl  alcohol  of  10  per  cent,  strength 
until  it  contains  from  0.001  to  0.004  grams  of  methyl  alcohol  in 
5  c.c. ;  the  experiment  is  repeated  side  by  side  with  two  or  more 
standards  for  comparison.  These  contain  0.001,  0.002,  0.003,  etc, 
gram  of  methyl  alcohol  in  5  c.c.  of  10  per  cent,  ethyl  alcohol.  The 
colors  produced  are  compared  in  small  Nessler  tubes  (25  c.c.)  or  in 
a  suitable  colorimeter. 
With  properly  sensitive  Schiff's  reagent,  0.0003  gram  methyl 
alcohol  in  the  5  c.c.  of  liquid  taken  is  readily  detected.  The  best 
depths  of  color  for  comparison,  however,  are  given  by  the  formal- 
dehyde produced  in  the  manner  described  from  quantities  of  0.001 
to  0.004  gram  of  methyl  alcohol. 
It  is  convenient  to  keep  a  standard  solution  (1  gram  per  liter) 
of  methyl  alcohol  in  10  per  cent,  ethyl  alcohol.  This  is  diluted  as 
required  with  10  per  cent,  alcohol  to  form  the  standards  for  com- 
parison. This  proportion  of  ethyl  alcohol  (10  per  cent.)  is  a  suit- 
able strength  for  general  work,  as  the  distillates  ordinarily  obtained 
are  stronger,  and  can  be  diluted  down  instead  of  having  to  be 
concentrated. 
The  process  has  the  advantage  of  (1)  being  rapidly  executed, 
(2)  requiring  only  a  small  quantity  of  material,  and  (3)  being 
