440  Molybdic  Acid  as  a  Color  Reagent.  {Am-/^1^arm' 
approximating  on  tannin,  pyrogallol,  pyrogallo-carbonic  acid,  and 
gallic  acid.  As  all  four  substances  named  contain  oxy-groups  in 
an  ortho-position  to  each  other,  the  reagent  was  tried  also  for  other 
aromatic  compounds  in  which  the  same  case  occurs.  The  result 
was  that  ammonium  molybdate  is  a  specific  color  reagent  for  all 
aromatic  compounds  which  contain  two  or  more  oxy-groups  stand- 
ing in  the  ortho  position  to  each  other. 
Ammonium  molybdate  produces  intense  phenomena  of  colora- 
tion only  with  such  bodies,  which,  according  to  the  substance 
employed,  may  be  yellow,  reddish-brown,  or  blackish-brown.  On 
the  contrary,  in  all  organic  compounds  in  which  the  above-men- 
tioned condition  does  not  occur  no  coloration  is  recognized.  Thus, 
with  pyro-catechin  on  the  addition  of  the  molybdate  there  is  shown 
an  intense  reddish-brown  color,  which  does  not  appear  with  hydro- 
quinone  and  resorcine — a  behavior  which  shows  phloroglucine  in 
contrast  to  pyrogallol.  The  following  aromatic  compounds  display 
similar  colorations  :  Protocatechuic  acid,  caffeic  and  hydrocaffeic 
acid,  phenanthrenehydroquinone  and  retenehydroquinone ;  in 
alizarin,  purpurin,  anthragallol,  and  rufigallic  acid,  the  aqueous 
solutions  of  which  are  yellow  or  red,  the  tone  is  much  intensified 
by  an  addition  of  the  molybdate.  The  entrance  of  other  groups 
into  the  nucleus  does  not  generally  interfere ;  thus  the  reaction 
appears  in  the  bromo  and  nitro-  derivatives  of  pyrogallol  and  gallic 
acid,  but  not  in  the  phenol  ethers,  where  one  or  more  hydrogen 
atoms  of  the  oxy-group  are  replaced  by  alkyles,  as  guaiacol  and 
vanilline.  Very  slight  yellow  colorations  which  may  occur  with 
the  two  last-mentioned  bodies  are  probably  due  to  slight  impurities. 
Among  compounds  of  a  less  fully  known  constitution  the  reaction 
is  produced  by  quercitrin  and  its  scission  product  quercetin,  apo- 
morphin,  podophyllin,  koussein  and  aloin. 
The  aqueous  solution  of  free  molybdic  acid  behaves  exactly  like 
ammonium  molybdate.  The  latter,  in  presence  of  organic  sub- 
stances, passes  to  a  lower  stage  of  oxidation,  with  the  production 
of  a  blue,  yellow,  or  a  reddish-brown. 
The  origin  of  the  color  phenomena  in  question  depends  on  the 
oxidizing  action  of  molybdic  acid  and  its  ammonium  salt.  As 
those  aromatic  compounds  which  contain  oxy-groups  in  the  ortho- 
position  to  each  other  are  more  unstable  and  especially  more 
readily  oxidizable  than  their  isomers,  there  occurs  here  a  more 
