AFebruarVP?8a9r8m'}  Testing,  of  Formaldehyde,  87 
reaction.  This  large  excess  was  afterward  found  unnecessary,  50 
per  cent,  excess  being  quite  sufficient.)  After  standing  10  minutes 
the  liberated  acid  was  titrated,  requiring,  after  deducting  o-i  c.c. 
for  free  acid  in  the  hydroxylamine  solution,  10  7  c.c.  of  —  soda ; 
10 
1  c.c.  —  0-003  gramme  formaldehyde. 
107  X  0-003  X  100 
7   J    =  37-2  per  cent. 
0-0864  p/  * 
Several  trials  were  made  to  determine  the  time  limit  of  the 
reaction. 
Allowing  20  minutes  to  complete  the  reaction,  the  result  was  37-2 
per  cent. 
Allowing  45  minutes  to  complete  the  reaction,  the  result  was  37-3 
per  cent. 
Other  trials  showed  that  shaking  the  mixture  does  not  materially 
hasten  the  reaction,  and  that  it  is  practically  complete  in  7  to  8 
minutes  after  admixture. 
The  above  figures  agree  closely  with  those  obtained  on  the  same 
solution  by  other  methods. 
The  method  is  quick  and  accurate,  when  applied  to  pure  solutions, 
but  the  accuracy  is  interfered  with  by  the  presence  of  other  alde- 
hydes and  acetone,  as  stated  by  G.  Romijn  [Ztsch.  f.  Anal.  Chem.y 
vol.  36,  p.  18).  For  acetone  the  statement  was  verified  by  a  simple 
test-tube  experiment ;  aldehydes  were  not  tried. 
Iodine  Method. — Proposed  by  Romijn  [ibid.).  Nearly  identical 
with  Messenger's  process  for  estimating  acetone,  and  therefore  un- 
suitable for  the  assay  of  solutions  liable  to  contain  acetone.  It  was 
used  only  to  determine  the  strength  of  pure  solutions,  made  from 
para-formaldehyde,  and  for  this  purpose  was  found  accurate  and 
convenient.    A  very  dilute  solution  of  the  formaldehyde  is  mixed 
n  •  •  • 
with  an  excess  of          iodine  solution,  and  caustic  alkali  solution 
10 
added  until  the  iodine  is  decolorized.  After  standing  10  minutes, 
dilute  acid  is  added  to  liberate  the  excess  of  iodine,  and  this  esti- 
mated with  sodium  hyposulphite. 
Cyanide  Method. — Also  devised  by  Romijn  (ibid.).  It  is  based 
upon  the  formation  of  an  addition  product  of  formaldehyde  and 
potassium  cyanide,  from  which  the  cyanide  cannot  be  precipitated 
