AMFeJb°TiP8HRM }       On  the  Detection  of  Ammonia.  67 
ON  THE  DETECTION  OF  AMMONIA.* 
By  G.  C.  Wittstein. 
Scientific  journals  have  recently  often  mentioned  Nessler's  reagent 
for  ammonia,  which  consists  in  a  strongly  alkaline  solution  of  mercu- 
ric iodide  in  aqueous  iodide  of  potassium.  It  is  asserted  to  surpass 
in  delicacy  all  other  tests  for  this  volatile  alkali;  my  experience,  how- 
ever, does  not  coincide  with  these  statements. 
The  well-known  behavior  of  a  solution  of  corrosive  sublimate  to 
ammonia  was  first  recommended  by  Einbrodtf  for  the  qualitative  de- 
tection of  ammonia,  who  likewise  determined  the  necessary  cautels. 
It  had  been  supposed,  up  to  that  time,  that  the  white  precipitate, 
which  is  thus  formed,  was  soluble  in  600  parts  of  water,  and  not  ex- 
pecting a  sufficient  delicacy  of  the  new  test,  I  made  some  experi- 
ments,! which,  however,  proved  that  one-half  of  a  millionth  of  free 
ammonia  could  thereby  be  detected,  and  that,  consequently,  white 
precipitate  is  totally  insoluble  in  water.  I  then  concluded  my  report 
with  the  following  remarks :  "  There  may  be  cases  in  which  this 
delicate  reagent  for  free  ammonia  will  be  unsuccessful.  If,  for  in- 
stance, the  liquid  contains  an  iodide,  the  solution  of  corrosive  subli- 
mate will  produce  a  yellowish  precipitate,  which  disappears  on  agita- 
tion, without  producing  the  reaction  of  ammonia,  even  though  this 
compound  may  be  perceptible  to  the  smell.  Continuing  the  addition 
of  the  corrosive  sublimate,  a  permanent  red  precipitate  of  iodide  of 
mercury  occurs  ;  the  chloride  of  mercury,  therefore,  is  decomposed 
by  the  iodide  before  it  is  acted  upon  by  the  ammonia,  and  the  latter 
may,  for  this  reason,  be  still  recognized  by  its  odor  or  by  a  glass  rod 
moistened  with  acetic  acid." 
This  observation  may,  I  believe,  now  be  more  precisely  explained 
as  follows  :  the  double  iodide  of  potassium  and  mercury  which  is  at 
first  formed,  is  subsequently  decomposed  by  the  free  ammonia,  and 
iodide  of  mercury  is  separated  as  a  red  or  yellowish  red  precipitate, 
from  a  concentrated  solution,  or  in  a  very  finely  divided  condition,  so 
that  only  a  more  or  less  intense  coloration  takes  place. 
From  the  above  observation  and  its  explanation,  it  seems  evident 
that  Nessler's  is  by  far  less  delicate  than  Einbrodt's  test,  and  this 
*  Translated  from  Archiv  d.  Pharm.,  1873,  Nov.,  397—399. 
f  Journal  f.  praktische  Chemie,  1852,  lvii,  180. 
%  Wittstein's  Yierteljahresschrift,  1853,  ii,  111. 
