390 
Ortho-Tolidine  Test  for  Gold.       {Am.  Jour.  Pharm. 
*  1      June,  1919. 
THE  USE  OF  ORTHO-TOLIDINE  AS  A  COLORIMETRIC 
TEST  FOR  GOLD.1 
By  W.  B.  Pollard. 
Ortho-tolidine  dissolved  in  dilute  acetic  acid  was  suggested  by 
E.  B.  Phelps2  as  a  delicate  color  test  for  free  chlorine  in  water.  This 
test  was  modified  by  J.  W.  Ellms  and  S.  J.  Hauser,3  who  showed 
that  a  hydrochloric  acid  solution  of  ortho-tolidine  was  better  adapted 
for  the  purpose.  Their  reagent  was  prepared  by  dissolving  one 
Gm.  of  ortho-tolidine  in  a  liter  of  10  per  cent,  hydrochloric  acid. 
The  reagent  in  its  modified  form  has  now  been  found  to  be  a 
delicate  test  for  aurichloric  acid.  A  solution  of  1  part  of  gold  in  a 
million  parts  of  water  gave  a  bright  yellow  color  on  addition  to  1 
Cc.  of  the  reagent.  With  a  solution  containing  1  part  of  gold  and 
20  million  parts  of  water  the  yellow  color  can  just  be  detected  in  a 
depth  of  10  Cm.  of  liquid. 
Ellms  and  Hauser  found  that  in  the  case  of  dilute  solutions  con- 
taining free  chlorine  the  color  took  about  three  minutes  to  fully 
develop,  and  was  then  permanent  for  about  half  an  hour,  after 
which  it  slowly  faded.  This  was  also  found  to  be  the  case  with 
aurichloric  acid. 
In  making  the  test  large  amounts  of  strong  mineral  acid  should 
not  be  present  as  the  reaction  becomes  less  delicate. 
The  following  metals  when  present  as  chlorides  in  a  dilute  hydro- 
chloric acid  solution,  were  found  to  give  no  reaction  with  ortho- 
tolidine:  Al,  Sb  (ic),  Ba,  Bi,  Cd,  Ca,  Cr,  Co,  Cu,  If,  Pb,  Mg,  Hg, 
Mn  (ous),  Ni,  Pt,  K,  Rh,  Na,  Sr,  Sn  (ic),  U  and  Zn. 
In  a  second  paper  Ellms  and  Hauser  point  out  that  iron  in  the 
ferric  condition  also  reacts  with  ortho-tolidine.  The  author  found 
that  in  the  case  of  ruthenium  a  yellow  color  was  also  formed. 
Osmic  acid  gives  a  yellow  color,  but  this  changes  to  a  green  on  stand- 
ing. Vanadates  acidified  with  dilute  hydrochloric  acid  give  a  re- 
action.   Molybdates  acidified  with  hydrochloric  acid  do  not  react. 
Sodium  tungstate  acidified  with  dilute  hydrochloric  acid  gives  a 
precipitate  on  addition  of  ortho-tolidine,  but  no  yellow  color 
develops. 
1  Reprinted  from  The  Analyst,  March,  1919. 
2  Bulletin  No.  1,  Ohio  State  Board  of  Health,  January,  1913. 
3  Analyst,  191 4,  39,  454. 
