Am.  Jour,  Pharm. 
Oct.,  1921. 
Methyl-Orange 
677 
methyl-orange.  Two  years  ago  Jf.  A.  Af  Hallstrom  upon  my  sug- 
gestion applied  the  same  addition  for  the  same  purpose.  As  the 
intended  detailed  publication  of  this  artifice  was  postponed  until  the 
completion  of  some  additional  experiments,  a  few  statements  may 
be  communicated  at  this  time. 
"The  addition  of  indigo  to  yellow  colored  solutions  undoubtedly 
increases  the  sharpness  of  the  end-reaction.  But  we  must  not  for- 
get that  the  quantity  of  indigo-carmine  to  be  added  must  be  pro- 
portional to  the  yellow  coloration.  The  more  intense  the  yellow 
color  the  sooner  will  appear  the  change  of  color  from  violet  to 
green  and  so  much  later  the  change  from  green  to  violet.  A  def- 
inite formula  for  the  mixture  suitable  for  all  degrees  of  yellow 
colorations  is  not  possible.  In  case  of  approximately  uniform  yellow 
color  solutions  and,  especially,  in  cases  of  somewhat  concentrated 
solutions  for  titrations  this  condition  need  hardly  be  considered, 
as  the  addition  of  indigo-carmine  allows  titration  which  would  other- 
wise be  impossible. 
"It  appears  to  me  that  the  addition  of  indigo-carmine  in  the  ac- 
curate titration  of  colorless  solutions  with  methyl-  or  ethyl-orange 
is  of  even  greater  value  than  in  the  titration  of  yellow  colored 
solutions. 
''To  obtain  distinct  end-reactions  in  titrations  with  alkaline 
hydroxides  containing  carbonate  the  suggestion  of  Kiister  is  followed 
by  preparing  a  'normal  color'  by  saturating  the  methyl-orange  solu- 
tion with  carbon  dioxide  and  titrating  to  this  color.  As  the  changes 
in  color,  red,  orange,  yellow,  especially  in  dilute  solutions,  are  gradual 
the  end-reaction  may  be  doubtful.  In  these  cases  indigo-carmine  is 
of  advantage  as  its  color  is  practically  complementary  to  the 
'normal  color/ 
"It  is  possible  by  a  few  trials  to  obtain  the  proper  mixture  of 
methyl-orange  and  indigo-carmine  which  will  result  in  giving  an  al- 
most neutral  gray  with  carbon  dioxide;  sufficiently  dilute  solutions 
of  such  a  mixture  will  then  appear  almost  colorless. 
"The  change  in  color  from  violet,  trfpugh  colorless,  to  green  is 
very  pronounced  and  titrations  in  dilute  solutions,  especially,  are 
more  easily  made;  the  end-reaction  is  colorless  (gray.) 
"As  indigo-carmine  with  excess  of  alkali  changes  to  yellow,  the 
complete  color  changes  obtainable  in  titrating  an  alkali  with  acid  is 
as  follows:  yellow,  green,  colorless  (gray),  violet.  The  multiplicity 
of  color  changes  has  the  advantage  of  preparing  one  for  the  ap- 
proach of  the  end-point ;  overstepping  the  end  is  therefore,  even  in 
rapid  work,  easily  avoided. 
"Very  few  references  in  literature  were  found  mentioning  the 
yellow  color  produced  by  dilute  alkalies  on  indigo  derivatives." 
