4 
Mercuric  Chlorid  Tablets.  \ A™-  Jour-  ftj*- 
l    January,  1914. 
power  in  slight  but  distinct  degree.  Hence  a  standard  thiosulphate 
solution  should  be  made  the  basal  standard  against  which  the  iodin 
solution  is  standardized  by  running  a  blank  assay  with  the  other 
solutions  and  reagents  intended  for  actual  use. 
The  method  finally  chosen  for  tablets  after  Wilson's  formula  is 
as  follows :  Weigh  5  tablets,  dissolve  in  water,  dilute  to  100  c.c,  and 
pass  through  a  dry  filter,  discarding  the  first  20  c.c.  of  filtrate.  From 
the  remainder  pipette  10  c.c.  (equivalent  to  one-half  tablet)  into  a 
glass-stoppered  250  c.c.  Erlenmeyer  flask,  add  2l/2  grams  pure  pow- 
dered potassium  iodid,  mix  to  entirely  dissolve,  and  then  wash 
down  the  sides  of  the  flask  with  20  c.c.  of  normal  caustic  alkali. 
Add  exactly  3  c.c.  of  37  per  cent,  formaldehyde  solution,  mix 
thoroughly,  and  let  stand  for  at  least  ten  minutes,  swirling  the 
flask  occasionally.  Then  wash  down  the  sides  of  the  flask  with 
a  mixture  of  5  c.c.  of  36  per  cent,  acetic  acid  with  25  c.c.  water;  mix, 
and  without  delay  run  in  from  a  burette  25  c.c.  of  tenth-normal 
iodin  while  constantly  swirling  the  flask.  Stopper  the  flask  tightly, 
shake  vigorously  for  three  minutes,  then  after  giving  the  contents  a 
final  swirling  motion  leave  at  rest  for  two  minutes.  If  then  no  undis- 
solved mercury  can  be  detected  at  the  bottom  the  stopper  is  removed, 
rinsed,  together  with  the  neck  of  the  flask,  with  a  stream  from  a 
wash-bottle,  and  the  excess  iodin  titrated  with  tenth-normal  sodium 
thiosulphate,  adding  starch  solution  only  when  the  iodin  is  nearly 
consumed. 
Standardize  the  iodin  solution  by  running  a  blank  assay  on  10  c.c. 
distilled  water. 
Subtract  the  volume  of  thiosulphate  solution  used  in  the  assay 
from  that  used  in  the  blank.  The  difference  multiplied  by  the  factor 
0.0271  for  strictly  tenth-normal  sodium  thiosulphate  will  give  the 
average  weight  of  mercuric  chlorid  per  tablet.  For  a  direct  check 
upon  the  value  of  the  sodium  thiosulphate  solution  run  an  assay  on 
10  c.c.  of  a  2.y2  per  cent,  solution  of  mercuric  chlorid  of  known 
purity. 
While  mercuric  chlorid  is  the  important  active  ingredient  of  tab- 
lets made  according  to  Wilson's  formula,  nevertheless  ammonium 
chlorid  is  an  essential  part  of  the  formula,  added  in  order  to  render 
the  tablets  easily  soluble  and  to  inhibit  the  formation  of  insoluble, 
and  hence  inactive,  compounds  of  mercury.   An  assay  of  such  tablets 
