This  substance  is  so  called  from  the  fact  that  it  is  obtained
by  the  destructive  distillation  of  catechu,  the  sap  of  Mimosa
Catechu  ;  it  is  also  obtained  by  the  fusion  with  potash  of  other
tanno-resins  such  as  kino,  the  sap  of  various  species  of  Ptero-
carpus,  Butea  or  Eucalyptus  ;  also  it  occurs  in  small  quantities
combined  with  sulphuric  acid  in  the  urine  of  horses  and  of
human  beings.  It  crystallizes  from  benzene  in  colourless
glistening  plates  and  melts  at  140°.
*  Although  behaving  normally  as  a  trihydric  phenol,  phloroglucinol  reacts
with  certain  ketonic  reagents  as  though  it  had  the  following  constitution : —

CO
/\


CH2    CH

CO       CO

\/

CH2


202  TANNINS

Reactions.
1.  Pyrocatechol  is  precipitated  from  aqueous  solution  by
lead  acetate.     (Distinction  from  resorcin  and  hydroquinone.)

2.  With  ferric  chloride  it  gives  a  green  colour  which  is
changed  to  violet  on  the  addition  of  sodium  acetate.
3.  Like  pyrogallol  it  reduces  silver  nitrate  in  the  cold  and
has  therefore  been  used  as  a  photographic  developer.
4.  It  reduces  Fehling's  solution  on  warming.
RESORCINOL.     C6H4(OH)2.
This  is  isomeric  with  pyrocatechol  (for  constitutional  formula
see  page  200) ;  it  does  not  generally  occur  in  tannins  *  but  in
certain  resins,  notably  galbanum  resin  and  asafoetida.
It  is  used  commercially  in  the  manufacture  of  dye-stuffs,
and  when  heated  with  sodium  nitrite  gives  the  indicator
known  as  Lacmoid.
Resorcinol  crystallizes  from  benzene  in  colourless  needles
and  melts  at  1 19° ;  it  is  somewhat  soluble  in  water,  the  solution
having  a  sweetish  taste.
Reactions.
1.  It  is  not  precipitated  from  solution  by  lead  acetate.
2.  With  ferric  chloride  it  gives  a  dark  violet  colour  which
is  destroyed  by  the  addition  of  sodium  acetate.
3.  It  reduces  ammoniacal  silver  nitrate  or  Fehling's  solution
on  warming.
HYDROQUINONE.
