622  Properties  and  Uses  of  Formaldehyde.  {^ovembe^S" 
Hehner's  Casein- sulphuric  Test. — Add  one  drop  of  milk  to  the 
sample  and  pour  the  mixture  carefully  on  the  surface  of  strong  sul- 
phuric acid  contained  in  a  test  tube.  A  blue  ring  appears  at  the 
zone  of  contact  when  a  trace  of  formaldehyde  is  present,  but  the 
reaction  is  not  obtained  with  ordinary  aldehyde,  nor  does  much 
formaldehyde  give  it.  Care  being  taken  that  the  solution  is  very 
dilute,  this  appears  to  be  a  thoroughly  distinctive  test ;  it  is  very 
sensitive,  and  will  detect  I  part  of  formaldehyde  in  200,000.  The 
production  of  the  blue  color  probably  depends  on  the  presence  of 
casein  in  the  milk.  In  testing  milk  for  formaldehyde,  distillation 
may  be  avoided  by  diluting  the  milk  with  an  equal  volume  of  water 
and  carefully  adding  sulphuric  acid  of  90  to  94  per  cent,  strength. 
In  the  absence  of  formaldehyde  a  faint  greenish  tinge  appears  at 
the  junction  of  the  two  liquids,  but  if  present  a  violet  ring  is 
developed. 
The  Phenol  Sulphuric  Test  is  described  by  O.  Hehner1  as  being 
equally  as  sensitive  and  characteristic  as  the  foregoing,  with  the 
further  recommendation  of  answering  with  all  proportions  of  formal- 
dehyde. To  the  distillate  from  the  sample  of  milk,  or  other  liquid 
under  examination,  add  one  drop  of  a  dilute  aqueous  solution  of 
phenol,  and  pour  the  mixture  on  the  surface  of  strong  sulphuric  acid 
contained  in  a  test  tube.  A  bright  crimson  color  appears  at  the  line 
of  contact.  This  detects  I  in  200,000,  and  if  more  than  1  in 
100,000  be  present,  there  is  seen  above  the  red  ring  a  white,  milky 
zone,  and,  in  stronger  solution,  a  white  or  pinkish  precipitate.  It  is 
important  that  only  a  trace  of  phenol  be  used  (I  find  y2  grain  to  the 
ounce  a  suitable  strength  for  the  solution)  and  that  the  test  be 
carried  out  exactly  as  described.  A  dilute  solution  of  phenol,  with- 
out formaldehyde,  poured  on  strong  sulphuric  acid,  gives  a  colored 
zone,  crimson  above  and  green  below,  therefore  excess  must  be 
avoided.  The  colored  ring  due  to  formaldehyde  is  crimson  below, 
merging  into  yellow  above.  Acetaldehyde  gives  an  orange  precipi- 
tate, not  a  crimson  color.  Other  benzene  hydroxy-derivatives  may 
be  used  in  place  of  phenol,  as  salicylic  acid,  resorcinol  and  pyro- 
gallol ;  they  all  yield  the  red  color. 
The  Chromate  Test  (C.  D.  Mitchell). —  1  c.c.  nitric  acid  with  1  c.c. 
of  a  solution  of  bichromate  of  potash  forms  a  reagent  which,  added 
1  Analyst,  April,  1896. 
