88  Testing  of  Formaldehyde.  {A^Ir^mm- 
with  silver  nitrate.  It  involves  the  use  of  standard  solutions  of  sil- 
ver nitrate,  potassium  cyanide,  and  potassium  sulphocyanate.  This 
method  requires  much  more  care  and  attention  than  any  of  the 
others  tried,  and  a  few  trials  showed  conclusively  that  it  is  not  likely 
to  give  satisfactory  results  except  in  practiced  hands.  It  was,  there- 
fore, considered  useless  to  proceed  further  with  it. 
Fixed-Alkali  Method. — This  consists  in  heating  the  formaldehyde 
with  sodium  or  potassium  hydrate  solution  under  pressure,  in  a 
manner  similar  to  that  in  the  saponification  of  esters.  The  formal- 
dehyde is  converted  into  methyl  alcohol  and  formic  acid  as  follows: 
2  CH20  +  KOH  =  CH3OH  -f  KCH02. 
The  method  was  most  satisfactory  when  conducted  as  follows :  3 
grammes  of  the  sample  are  placed  into  a  strong  bottle  of  50  c.c. 
n 
capacity,  with  25  c.c.  of  soda  solution,  the  bottle  closed  with  a 
tight-fitting  rubber  stopper,  this  tied  down  with  a  cord,  and  the 
bottle,  after  wrapping  with  a  cloth,  immersed  in  boiling  water  for 
11 
one-half  hour.    After  cooling,  the  excess  of  soda  is  titrated  with  — 
sulphuric  acid  and  phenolphthalein,  each  c.c.  of  soda  solution  con- 
sumed indicating  0-06  grammes,  or,  if  3  grammes  be  taken,  0-5  per 
cent,  of  formaldehyde. 
The  time  required  to  complete  the  reaction  is  very  much  less 
than  is  stated  by  others  who  have  tried  the  method,  nor  were  some 
other  stated  disadvantages  noticed,  such  as  thickening  or  resinify- 
ing  of  the  solutions.  The  results  were  reasonably  accurate,  except 
in  the  case  of  one  commercial  sample.  The  following  figures  illus- 
trate the  duration  and  degree  of  heat  necessary.  They  were  all 
obtained  with  the  same  formaldehyde  solution  : 
Heated  in  boiling  water.         Per  cent.  Heated  over  boiling  water.      Per  cent. 
15  minutes  ...         ...  37*1  15  minutes  32*5 
30     "        ......  .37-3  30     <x        .  .  •  ^34-5 
1  hour   37 '6  1  hour   37-1 
2  hours   37  "2  2  hours     .  377 
3    "   37"3 
On  prolonged  heating  the  solution  frequently  darkens,  so  as  to 
make  dilution  necessary  before  titrating. 
The  influence  of  the  presence  of  acetone  and  methyl  alcohol  on 
the  accuracy  of  the  methods  was  determined. 
