^'°ApriU9n*'"'  }  Quantitative  Methods.  189 
diluted  with  10  c.c.  of  water  and  let  stand  with  occasional  stirring 
for  about  two  minutes.  Acidify  with  acetic  acid ;  add  25  c.c.  of 
N/io  iodine  and  after  all  the  precipitated  mercury  has  combined 
with  the  iodine,  titrate  the  excess  with  N/io  sodium  thiosulphate 
solution. 
Mercurous  salts  must  be  converted  into  mercuric  before  precipi- 
tation. 
In  the  case  of  mercuric  cyanide,  sulphuric  acid  should  be  used 
instead  of  acetic  in  order  to  decompose  any  cyanogen  iodide  which 
may  have  formed. 
Method  No.  5. 
Suggested  by  Mr.  F.  O.  Taylor. 
(See  C.  J.  Pretzfeld — J.  A.  C.  S.,  1903,  page  198.) 
Estimation  of  Mercury  as  Arsenate. 
The  mercury  must  be  present  as  a  mercuric  salt,  and  preferably 
as  a  nitrate,  as  mercuric  arsenate  is  not  precipitated  from  the 
chloride  solution.  A  small  amount  of  free  nitric  acid  does  not 
interfere  with  the  accuracy  of  the  results.  To  a  cold  solution 
containing  about  0.25  gramme  Hg  in  100  c.c,  add  20  c.c.  of 
saturated  solution  sodium  arsenate.  The  heavy  yellowish-white 
precipitate  of  mercuric  arsenate  immediately  forms  and  settles 
rapidly,  but  for  greater  accuracy  the  author  of  the  method  recom- 
mends that  the  solution  stand  for  several  hours ;  then  filter  through  a 
Gooch  filter,  wash  thoroughly  with  cold  water  and  dry  at  100°. 
Method  No.  6. 
Suggested  by  Mr.  F.  O.  Taylor. 
(See  same  paper  as  above.) 
Estimation  of  Mercury  as  Chloride. 
To  a  solution  of  tlie  mercuric  salt,  preferably  in  the  form  of 
a  nitrate  containing  about  0.25  to  0.4  gramme  Hg,  add  a  slight 
excess  of  a  mixture  containing  one  drop  of  hypophosphorus  acid 
to  each  c.c.  of  H„0.,  and  then  immediately  an  excess  of  solution 
sodium  chloride.  Let  stand  for  one  hour,  filter  off  the  precipitated 
HgCl,  wash  thoroughly,  dry  at  100°  and  weigh. 
