In  many  cases  the  test  may  be  carried  out  by  simply  dissolving
a  small  quantity  of  the  dyestuff  in  water,  adding  a  few  drops
of  nitric  acid  (to  prevent  the  precipitation  of  any  other  salt  of
silver  besides  the  chloride),  and  then  a  few  drops  of  a  solution
of  silver  nitrate,  when  the  production  of  a  white  precipitate  will
indicate  the  presence  of  a  chloride.  This  method,  however,
is  not  always  reliable,  as  many  dyestuffs  are  hydrochlorides
of  color-bases  (basic  dyes),  or  give  insoluble  salts  of  silver,  in
which  cases  the  formation  of  a  white  precipitate  would  not
necessarily  indicate  the  presence  of  common-salt.  It  is  best  to
ignite  a  portion  of  the  dyestuff  in  a  porcelain  crucible,  so  as  to
burn  off  all  volatile  and  organic  matter,  leaving  only  mineral
matter  in  the  ash.  Dissolve  the  ash  in  water,  add  a  few  drops
of  nitric  acid  and  then  the  silver  nitrate;  a  white  precipitate
of  silver  chloride  will  be  formed  if  common-salt  is  present.  A
few  dyes,  such  as  the  eosins,  leave  chlorides  (or  bromides  or
iodides)  in  the  ash  after  ignition;  hence  this  test  would  not  be
satisfactory.  For  such  dyes,  the  aqueous  solution  of  the  coloring-
matter  should  be  acidulated  with  dilute  sulphuric  acid,  then
shaken  up  with  ether.  The  dyestuff  will  be  dissolved  out  by  the


THE   TESTING  OF  DYESTUFFS.  2/5

ether,  leaving  any  common-salt  which  may  be  present  in  the
aqueous  layer.  The  latter  may  be  removed  and  tested  with
silver  nitrate  as  described  above.  If  the  dyestuff  is  soluble  in
alcohol,  the  coloring-matter  may  first  be  extracted  by  warming
with  this  solvent,  and  the  test  for  common-salt  may  then  be
made  with  the  residue.
