The  reactions  most  suitable  for  characterizing  small  quan-
tities of  formaldehyde  are  as  follows  : —

Rimini's  test  consists  in  adding  2.  drops  of  phenylhydra-
zine  hydrochloride,  2  drops  of  sodium  nitroprusside  solution,
and  I  c.c.  of  sodium  hydroxide  to  I  c.c.  of  the  liquid  to  be
tested.  A  blue  colour  is  formed,  which  changes  rapidly
through  green  and  brown  to  red.  Schryver*  has  modified
this  test  and  made  it  much  more  sensitive ;  he  recommends
the  following  method:  to  10  c.c.  of  the  liquid  to  be  tested
add  2  c.c.  of  a  i  per  cent  solution  of  phenylhydrazine  hydro-
chloride  freshly  made  up  and  filtered  ;  then  add  I  c.c.  of  a  5
per  cent  solution  of  sodium  ferricyanide,  also  freshly  made  up,
and  5  c.c.  of  hydrochloric  acid  ;  a  brilliant  magenta  colour  is

*  Schryver:  "  Proc.  Roy.  Soc.,  Lond.,"  B.,  1910,  82,  226.
DETECTION  OF  FORMALDEHYDE  153

produced.  The  test  is  a  very  delicate  one  and  will  detect
quantities  of  formaldehyde  varying  from  I  part  in  1,000,000
to  i  part  in  100,000.  Acetic  aldehyde  gives  no  colour  with
this  reagent.
The  following  test,  due  to  Deniges,*  is  sensitive  for  formal-
dehyde, even  in  presence  of  acetic  aldehyde  up  to  2  per  cent ;
5  c.c.  of  an  aqueous  solution  of  formaldehyde  are  mixed  with
I  '2  c.c.  of  pure  sulphuric  acid  (sp.  gr.  1 166)  and  5  c.c.  of  Schiff's
reagent.  An  intense  violet  colour  having  an  absorption  band
in  the  orange  is  produced.  Schiffs  reagent  may  be  prepared
by  adding  a  litre  of  O-QI  per  cent  of  solution  of  magenta  to
20  c.c.  of  sodium  hydrogen  sulphite  solution  (sp.  gr.  I  -3),  and
after  five  minutes  adding  20  c.c.  of  hydrochloric  acid  (sp.  gr.
I -1 8).
