4^2  Note  on  Assay  of  Formaldehyde.  -j 
Am.  Jour.  Phann. 
October.  1911. 
The  results  given  in  Table  I  show  that  the  maximum  result  was 
reached  when  the  excess  of  KCN  was  about  one-half  of  an  equiva- 
lent (exp.  8),  while  further  increasing  the  amount  of  KCN  up  to 
approximately  three  equivalents  did  not  appreciably  alter  the  result. 
The  reaction  may  therefore  be  regarded  as  complete,  at  least  for  all 
practical  purposes,  when  the  excess  of  KCN  is  as  much  as  one-half 
of  an  equivalent.  On  the  other  hand,  when  the  excess  of  KCN  is 
less  than  one-fourth  of  an  equivalent  (exps.  i-6),  the  reaction  is 
incomplete ;  and  when  the  amount  of  KCN  added  is  only  slightly  in 
excess  of  that  recjuired  to  combine  wnth  all  of  the  formaldehyde 
present,  the  result  is  about  3  or  4  per  cent,  too  low  (exps.  1-2). 
That  the  excess  of  KCN  must  be  above  a  certain  minimum  in  order 
to  insure  a  complete  reaction  is  not  brought  out  either  in  the  original 
paper  of  Romijn  or  those  of  Smith  or  Williams.  In  fact,  none  of  the 
literature  examined  has  any  reference  to  this  point ;  while  from  the 
statement  of  Romijn  that  the  method  is  based  on  the  property  of 
formaldehyde  to  iinmediately  add  itself  to  potassium  cyanide  and  his 
unmodified  general  statement  that  when  the  KCN  is  in  excess 
so  much  of  the  KCN  becomes  combined  as  to  correspond  to  one 
molecule  of  KCN  for  every  molecule  of  formaldehyde,  one  might 
suppose  that  the  relative  amount  of  the  excess  of  KCN  to  be  added 
does  not  matter.  This  conclusion  would  gain  further  support  from 
the  statement  of  Smith  that  the  determination  is  carried  out  by 
mixing  the  formaldehyde  with  a  known  quantity  of  potassium  cya- 
nide, the  latter  being  in.  excess." 
In  order  to  determine  the  effect  of  varying  the  time  or  the' 
temperature,  two  series  of  experiments  were  carried  out.  In  one 
of  these,  the  mixtures  of  formaldehyde  and  KCN  were  allowed  to 
stand  for  different  intervals  of  time  at  the  constant  temperature  of 
15°  C.  and  the  residual  cyanide  then  determined  in  the  usual  way. 
In  the  other,  the  time  was  constant  but  the  temperature  was  made 
to  vary  between  5°  C.  and  40^  C.  In  those  cases  where  the  mixt- 
ures had  to  stand  for  a  considerable  length  of  time,  the  flasks  con- 
taining same  were  tightly  stoppered  by  means  of  glass  or  rubber 
'"Zr?7.  anal.  Chem.,  36,  18  (1897)  :  "die  Eigenschaft  des  Formaldehyds, 
das  Cyankalium  sofort  zu  addiren." 
Ibid. :  '*  wenn  ein  Ueberschuss  von  Cyankalium  vorhanden  war,  wird 
von  dem  Cyankalium  so  viel  gebunden,  dass  auf  ein  Moleciil  Formaldehyd 
ein  Moleciil  Cyankalium  kommt."  > 
"Jour.  Amer.  Chem.  Soc./ 25,  1032  (1903)-  •  " "  ' 
