460  Note  on  Assay  of  Formaldehyde.       [  ^'^)"cirer  mY 
formaldehyde  is  not  complete,  we  must  assume  that  the  value  of 
^\Px^i      tl^^  general  equation 
Y  —  V  —  —  cpq  —  c.p^q^, 
which  is  the  fundamental  equation  underlying  all  chemical  dynamics/^ 
is  not  negligible  but  has  a  value  which  would  account  for  the  lower 
results  obtained  by  the  KCN  method.  That  is,  in  the  reaction  be- 
tween the  formaldehyde  and  the  KCN,  which  may  be  represented  by 
the  equation 
H 
CH2O  +  KCN  ^  N  -  C-C-O-K 
H 
we  must  assume  that  the  velocity  of  the  reaction  which  tends  to 
decompose  the  condensation  product  back  into  formaldehyde  and 
KCN  has  an  appreciable  value.  From  the  law  of  mass  action  it 
would  follow,  therefore,  that  if  we  were  to  vary  the  mass  of  one 
of  the  reacting  substances  the  point  at  which  equilibrium  is  estab- 
lished should  also  vary;  just  as  in  the  classical  instance  of  an 
incomplete  reaction,  namely,  the  reaction  between  acetic  acid  and 
ethyl  alcohol,  in  which  case  Berthelot  and  Pean  de  Saint  Gilles  ^* 
found  that  by  varying  the  amount  of  alcohol  with  respect  to  that  of 
the  acid  the  amount  of  ester  produced  also  varied,  being  only  66.5 
per  cent,  of  the  theory  when  one  equivalent  of  alcohol  was  used  but 
increasing  to  82.8  per  cent,  when  tw^o  equivalents  of  the  alcohol  were 
added.  In  order,  therefore,  to  determine  whether  any  such  varia- 
tion occurs  in  the  case  of  the  reaction  between  formaldehyde  and 
KCN,  the  following  experiments  were  carried  out  on  mixtures  of 
formaldehyde  and  KCN,  in  which  the  amount  of  KCN  added  varied 
from  approximately  one  to  three  equivalents. 
Mode  of  Procedure. 
A  formaldehyde  solution  was  prepared  by  diluting  21.1879  Gms. 
of  a  sample  of  U.S. P.  solution  of  formaldehyde  to  2000  c.c.  with 
distilled  water.  Portions  of  15  c.c.  of  this  solution  were  mixed 
at  room-temperature  (22-25°  C.)  with  varying  amounts  of  approxi- 
mately n/io  KCN,  the  minimum  amount  of  which  was  just  a  little 
in  excess  of  that  theoretically  required  to  combine  with  all  of  the 
formaldehyde  taken,  while  the  maximum  amount  of  KCN  used  was 
about  two  equivalents  more  than  necessary  for  a  complete  equi- 
"Jones:  Elements  of  Physical  Chemistry,  3rd  ed.,  p.  529  (iW)- 
Ibid.  p.  522. 
