^ovemberS'}  Properties  and  Uses  of  Formaldehyde.  623 
to  the  distillate  from  a  liquid  containing  formaldehyde,  produces  an 
intense  violet  coloration.  This  does  not  appear  to  be  a  very  deli- 
cate test,  as  I  have  not  been  able  to  obtain  the  reaction  with  traces 
of  formaldehyde. 
Nessler's  Reagent  gives  a  distinct  yellow  precipitate  with  a  solu- 
tion containing  1  part  of  formaldehyde  in  200,000.  It,  therefore, 
lacks  nothing  in  delicacy,  and  as  the  reagent  is  always  to  be  found 
on  the  shelves  of  the  pharmacy,  it  may  be  recommended  as  the  most 
convenient  test.  With  solutions  of  1  part  of  formaldehyde  in 
^0,000  to  100,000  a  yellowish  opalescence  first  appears,  quickly 
changing  into  a  copious  orange  precipitate,  which,  on  standing  or 
gently  heating,  becomes  darkened  in  color  and  reduced.  From  I 
in  100,000  to  1  in  200,000  the  opalescence  develops  into  a  lemon- 
yellow  precipitate  on  standing,  and  little  or  no  reduction  takes  place. 
The  effect  produced  by  formaldehyde  is  quite  distinct  from  that  of 
a  weak  solution  of  ammonia  giving  about  the  same  depth  of  color. 
The  latter  remains  quite  clear  and  transparent  even  on  boiling. 
With  strong  solutions  of  acetaldehyde  an  orange  precipitate  and 
immediate  reduction  occurs,  the  same  as  with  formaldehyde,  but 
traces  hardly  affect  Nessler's  reagent,  and  when  acetaldehyde  is 
present  in  sufficient  quantity  to  even  communicate  its  odor  to  the 
liquid  a  yellow  precipitate  is  the  only  result.  The  reaction  of  ace- 
taldehyde with  Nessler's  reagent  is  very  much  less  energetic  than  that 
of  formaldehyde,  and  there  is  little  risk  of  confounding  the  two  alde- 
hydes, when  traces  only  are  present,  such  as  would  be  met  with  in 
the  ordinary  course  of  analysis. 
The  Ammonia  Micro  Test  (Remijn). — The  substance  or  liquid 
suspected  to  contain  formaldehyde  is  distilled  with  water,  a  drop  of 
the  distillate  evaporated  on  a  slide  with  a  drop  of  ammonia,  and 
the  crystalline  residue  tested  with  mercuric  chloride  in  excess. 
Crystals  are  immediately  formed,  hexahedral  at  first,  but  becoming 
octahedral  after  a  time.  The  crystals  are  easily  obtained  from  a  I 
in  10,000  solution,  and  are  visible  in  a  1  in  100,000  solution.  The 
evaporated  residue  also  gives  precipitates  with  platinic  chloride, 
phosphomolybdic  acid  and  other  alkaloidal  reagents. 
THE  QUANTITATIVE  DETERMINATION  OF  FORMALDEHYDE. 
There  are  several  methods  by  which  formaldehyde  may  be  quan- 
titatively determined  : 
