i 
COMMERCIAL  VALUE  OF   INDIGO.  223 
ON  THE  DETERMINATION  OF  THE  COMMERCIAL  VALUE  OF 
INDIGO. 
Wittstein  advocates  the  use  of  the  deoxidation  test  proposed 
by  Berzelius  and  Pugh,  and  considers  that  the  discredit  into 
which  it  has  fallen  is  the  result  of  unfounded  prejudice.  He 
further  points  out  the  several  defects  of  the  other  modes  of  test- 
ing indigo.  The  use  of  sulphuric  acid  as  a  solvent  of  the  blue 
coloring  matter  is  objectionable ;  first,  because  it  takes  up  other 
constituents  of  indigo,  partly  organic,  and  worthless  as  dying 
materials,  partly  inorganic ;  secondly,  because  there  is  no  means 
of  separating  the  indigo  blue  from  the  other  dissolved  substances. 
He  considers  the  method  proposed  by  Reinsch,  of  determining 
the  commercial  value  of  indigo  by  observing  the  intensity  of 
color  of  the  sulphuric  acid  solution  at  a  certain  definite  point  of 
dilution  as  of  small  value,  inasmuch  as  it  gives  only  relative  re- 
sults, and  that  with  no  very  great  accuracy.  Of  the  actual  per 
centage  of  blue  coloring  matter  it  gives  no  indication. 
The  use  of  chlorine  he  considers  as  still  more  open  to  objec- 
tion ;  in  the  first  place,  because  it  furnishes  only  relative  results, 
and  in  order  to  obtain  absolute  results,  it  must  be  ascertained 
how  much  chlorine  a  certain  weight  of  pure  indigo  blue  requires 
for  its  decolorization ;  secondly,  commercial  indigo  contains 
various  other  organic  substances,  colored  and  colorless,  which 
likewise  take  up  chlorine,  consequently  the  percentage  of  indigo 
blue,  calculated  from  the  quantity  of  chlorine  consumed,  is 
always  too  high  and  requires  correction,  for  it  cannot  safely  be 
assumed  that  the  chlorine  acts  first  upon  the  indigo  blue. 
In  order  to  obtain  some  positive  data  connected  with  the  use 
of  the  chlorine  test,  Wittstein  made  some  experiments  to  deter- 
mine the  quantity  of  chlorine  requisite  for  the  decolorization  of 
a  certain  weight  of  pure  indigo  blue,  assuming  at  the  outset  that 
this  quantity  was  constant.  He  was,  however,  unable  to  arrive 
at  any  practically  available  results.  He  used  the  solution  of 
indigo  highly  diluted ;  the  operation  was  conducted  in  a  long- 
necked  flask,  in  order  to  avoid  any  interruption  of  the  action  by 
the  renewal  of  water  which  had  evaporated,  and  the  solution  of 
salt  was  added  at  regular  intervals ;  notwithstanding  all  these 
precautions  the  quantities  of  chlorate  of  potash  requisite  to  pro- 
duce decolorization  in  several  successive  experiments  were  very 
