676 
Methyl-Orange 
combination  was  claimed  quoting  an  article  by  J.  A.  Af  Hallstrom 
in  Berichte,  1905,  page  228S. 
The  several  original  articles  in  the  order  of  their  publication 
contain  the  following  information: 
(1)  J.  A.  Af  Hallstrom  in  an  investigation  found  it  necessary 
to  determine  alkaline  carbonates  and  bicarbonates  in  presence  of 
phloroglucin ;  as  the  solutions  were  always  colored,  the  titration  with 
HCI  V.  S.  (ethyl-orange  indicator)  was  unsatisfactory.  Prof.  R. 
Luther,  under  whose  direction  the  investigation  was  made,  suggested 
the  addition  of  indigo  and  this  then  .enabled  more  satisfactory  titra- 
tions. 
(2)  C.  Kirschnick  in  Chem.  Ztg.,  1907,  page  960,  published  an 
article  under  the  title  of  "Indicator  for  the  determination  of  free 
acid  in  zinc  chloride  solutions,  etc."  The  demand  for  an  indicator 
which  combined  the  sensitiveness  of  methyl-orange  with  a  better 
end-reaction  in  turbid  or  light-colored  solutions  induced  the  author 
to  bring  about  a  change  in  color  of  the  methyl-orange  solution.  It 
was  found  that  an  addition  of  indigo-carmine  under  certain  condi- 
tions of  dilution  produced  a  pure  green  color  which  by  the  minutest 
addition  of  normal  acid  changed  to  a  violet-red.  The  following 
dilution  proved  practical :  One  gram  of  methyl-orange  and  one  gram 
of  indigo-carmine  were  each  dissolved  in  one  liter  of  water ;  20  cc.  of 
the  methyl-orange  and  60  cc.  of  the  indigo-carmine  solutions  were 
mixed  and  diluted  to  one  liter. 
This  mixed  indicator  was  used  in  testing  zinc  chloride  in  the 
following  manner:  10  cc.  of  the  indicator  was  diluted  with  100  cc. 
of  water  and  10  cc.  of  the  zinc  chloride  solution  (25%  Zh)  added; 
an  unchanged  green  color  shows  basicity,  a  resulting  red  color  shows 
acidity  of  the  zinc  chloride;  by  titration,  the  bacisity  or  acidity  of 
the  salt  can  be  determined.  This  indicator  can  also  be  used  to  de- 
termine the  free  acid  in  superphosphate.  Pure  indigotin  dissolved 
in  sulphuric  acid  and  carefully  neutralized  can  replace  the  indigo- 
carmine. 
(3)  B.  Luther  in  Chem.  Ztg.,  1907,  page  1172,  furnishes  more 
information  under  the  title  of  "On  the  addition  of  Indigo  in  titrating 
with  methyl-  or  ethyl-orange." 
"In  the  Chem.  Ztg.  1907,  page  960,  C.  Kirschnick  recommends 
an  addition  of  indigo  in  titrating  yellow  colored  solutions  with 
