ANoVJeTb'erhi™'}  Properties  and  Uses  of  Formaldehyde.  621 
one-half  parts  of  the  40  per  cent,  solution  may  replace  each  part 
of  formaldehyde. 
The  practice  of  employing  formaldehyde  as  a  preservative  for 
milk  has  of  late  become  very  general  amongst  dairymen,  with  the 
result  that  much  attention  has  been  directed  to  methods  for  its 
detection.  As  will  be  seen  from  the  above  table,  the  proportion 
necessary  is  so  small  that  any  test  to  be  of  service  must  be  excep- 
tionally delicate,  and  although  the  qualitative  reactions  in  use  do 
not  generally  fail  in  this  respect,  most  of  them  possess  the  disad- 
vantage of  being  common  to  aldehydes  generally.  The  more  im- 
portant are  the  following  :  Ammonio-nitrate  of  silver,  Schiff's 
reagent,  diphenylamine,  Hehner's  casein-sulphuric  test,  the  phenol- 
sulphuric  test,  the  chromate  test,  the  micro  test  with  ammonia,  and 
Nessler's  reagent. 
QUALITATIVE  TESTS  FOR  FORMALDEHYDE. 
Ammonio-nitrate  of  Silver. — This  reaction  is  moderately- deli- 
cate, but  is  given  by  other  aldehydes.  It  will  not  detect  the  faint 
traces  which  are  readily  recognizable  by  Hehner's  casein-sulphuric 
acid  and  the  phenol-sulphuric  test.  The  distillate  from  many 
organic  liquids  gives  a  slight  browning  with  this  reagent  which 
detracts  from  its  usefulness. 
Schiff's  Reagent  (rosaniline,  sodium  sulphite  and  hydrochloric 
acid)  is  colorless,  but  with  aldehydes  gives  a  reddish  violet  color. 
Richmond  and  Boseley  point  out  that  the  solution  should  always 
contain  excess  of  acid,  as  alkalis  turn  it  pink,  and  free  acid  does 
not  interfere  with  the  violet  tint  due  to  aldehyde.  According  to 
Hehner  the  best  way  of  operating  is  as  follows:  Distil  25  c.c. 
from  the  liquid  under  examination,  place  in  a  stoppered  cylinder, 
and  next  morning  add  a  few  drops  of  sulphurous  acid.  The  rose 
color  due  to  oxidation  disappears,  whilst  the  violet  tint  due  to  alde- 
hyde remains.  This  test  is  only  useful  for  confirmation,  as  it  reacts 
with  other  aldehydes. 
The  Diphenylamine  Test  (Richmond  and  Boseley). — Distil  the 
liquid,  such  as  milk,  etc.,  into  an  aqueous  solution  of  diphenylamine, 
in  the  preparation  of  which  just  sufficient  sulphuric  acid  has  been 
used  to  effect  solution.  If  formaldehyde  be  present,  a  white,  floccu- 
lent  precipitate  is  deposited,  which  often  has  a  green  tinge.  This 
test  is  inferior  to  Hehner's  and  Schiff's,  and  is  not  very  delicate. 
