6  Mercuric  Chlorid  Tablets.  {Aj^™£  PmT' 
For  a  direct  check  upon  the  value  of  the  tenth-normal  alkali  run 
an  assay  upon  5  c.c.  of  a  2]/2  per  cent,  solution  of  pure  ammonium 
chlorid. 
Solutions,  reagents,  and  water  used  should  be  free  from  carbon 
dioxid. 
Ordinarily  titration  B  is  very  small,  sometimes  zero,  but  usually 
calling  for  the  addition  of  a  few  drops  of  tenth-normal  acid. 
As  respects  the  end  points  with  the  indicators  it  is  only  possible 
to  state  that  up  to  the  present  time  no  blue  or  green  tablets  which 
have  been  received  by  the  writer  have  presented  the  slightest  difficulty. 
The  characteristic  colors  of  the  indicators  of  course  do  not  appear  in 
the  presence  of  other  coloring  matter,  but  the  change  of  tint,  if 
standards  of  comparison  are  used,  is  delicate  and  distinct.  No  red 
tablets  have  been  received  for  examination. 
The  reliability  of  the  method  may  be  shown  by  noting  a  few 
results. 
Experiment  2. — Four  tests  made  on  5  c.c.  of  a  solution  of  2  grams  each 
of  commercial  CP.  chlorids  of  mercury  and  ammonium  per  100  c.c.  gave  the 
following  figures  for  titration  A  (titration  B  —  O),  made  with  barium  hydrate 
solution  (tenth-normal  X  1.021)  which  had  been  standardized  against  a  labora- 
tory stock  solution  of  half-normal  hydrochloric  acid:  (1)  18.24  c.c;  (2)  18.26 
c.c;  (3)  18.29  cc-'>  (4)  18.26  c.c;  an  extreme  difference  of  0.05  c.c.  and  a 
recovery  of  99.6  to  99.9  per  cent,  of  the  ammonium  chlorid  employed. 
Experiment  3. — Three  titrations  made  on  5  c.c.  of  the  solution  of  commer- 
cial tablets  employed  in  Experiment  1  gave  titration  B  as  zero,  and  titration 
A  as  (1)  22.64  c.c;  (2)  22.58  c.c;  (3)  22.62  c.c;  an  extreme  difference  of 
0.06  c.c 
It  appears,  therefore,  that  assay  methods  are  now  available  for 
accurately  and  conveniently  estimating  mercuric  and  ammonium 
chlorid  in  commercial  tablets.  Knowing  the  weight  of  tablets  taken 
to  prepare  the  stock  solution  for  assay,  estimation  of  coloring  matter 
and  "  filler  "  is  merely  a  matter  of  subtraction.  Further  possible 
tests,  such  as  degree  of  solubility,  amount  of  insoluble  matter,  and 
uniformity  of  weight  of  individual  tablets,  are  a  matter  of  discretion 
with  the  analyst  and  need  no  discussion. 
