^"■j^uy^m*'^"'' }  Volumetric  Determination  of  Mercury.  313 
late  with  about  5  c.c.  of  36  per  cent,  acetic  acid.  Add  50  c.c.  of 
n/io  iodine  solution  and  shake  vigorously  in  the  stoppered  flask 
until  the  mercury  is  dissolved.  Titrate  the  excess  of  iodine  with 
n/io  sodium  thiosulphate  solution,  adding  starch  solution  when  the 
liquid  is  nearly  decolorized.  Each  c.c.  of  n/io  iodine  solution  cor- 
responds to  0.01355  Gm.  of  mercuric  chloride. 
Chemically  pure  mercuric  chloride  gave  by  this  method  99.8  to 
100.3  P^r  cent.,  with  an  average  of  100.  i  per  cent,  in  5  determina- 
tions. Similar  results  were  obtained  with  mercuric  iodide,  oxide, 
ammoniated  mercury,  and  mixtures  of  mercuric  chloride  and  am- 
monium chloride  colored  with  aniline  dyes.  It  is  the  official  method 
of  the  German  Pharmacopoeia,  5th  revision,  1910,  for  the  assay  of 
mercuric  chloride  tablets  and  ointment  of  ammoniated  mercury. 
The  third  method  alluded  to  above  consists  in  the  titration  of 
mercuric  compounds,  in  nitric  acid  solution,  with  sulphocyanate  in 
exactly  the  same  way  as  the  titration  of  silver.  It  is  not  applicable 
in  presence  of  chlorides  and  probably  not  in  presence  of  other 
halogens.  It  was  first  made  serviceable  for  accurate  work  by 
R.  Cohn  {Ber.  d.  d.  chem.  Ges.,  1901,  v.  34,  p.  3502)  and  simplified 
by  Rupp  and  Kraus  (Ibid.,  1902,  v.  35,  p.  2015).  For  illustrations 
of  its  application  see  below.  The  writer  has  no  personal  experience 
with  this  method,  but  his  associates  have  found  it  accurate  and  use- 
ful for  the  assay  of  technical  mercuric  oxide  containing  iron.  The 
German  Pharmacopoeia  uses  it  for  the  assay  of  several  galenical 
preparations.  For  the  convenience  of  any  readers  of  this  Journal 
interested  in  this  subject,  who  may  not  have  access  to  this  book, 
the  assay  methods  for  mercury  preparations  prescribed  therein  are 
given  here : 
Mercuric  Chloride  Tablets. — Composed  of  equal  parts  of  mer- 
curic chloride  and  sodium  chloride,  colored  with  aniline  dye.  Dis- 
solve 2  tablets  of  about  i  Gm.  each,  accurately  weighed,  in  water 
and  dilute  to  100  c.c.  In  20  c.c.  of  the  solution  dissolve  i  Gm.  of 
potassium  iodide,  add  10  c.c.  of  a  15  per  cent,  solution  of  potassium 
hydrate  and  3  c.c.  of  a  40  per  cent,  solution  of  formaldehyde  with 
10  c.c.  of  water.  After  one  minute  add  25  c.c.  of  30  per  cent,  acetic 
acid,  25  c.c.  of  n/io  iodine  solution,  and  shake  until  the  mercury  is 
dissolved.  Titrate  the  excess  of  iodine  with  n/io  sodium  thio- 
sulphate solution,  using  starch  solution  as  indicator.  Each  c.c.  of 
n/io  iodine  solution  consumed  corresponds  to  0.01355  Gm.  of 
mercuric  chloride. 
