218     Preparation  of  Pare  Protiodide  of  Mercury.  { A\J™1  gSf^ 
ON  THE  PREPARATION  OF  PURE  PROTIODIDE  OF  MERCURY  * 
By  Jules  Lefort. 
Protiodide  of  mercury  as  at  present  prepared,  is  often  a  mixture  of 
protiodide  and  metallic  mercury  with  more  or  less  of  biniodide,  and 
though  it  is  not  difficult  to  remove  the  last  named  compound,  the  con- 
taminating metal  cannot  be  separated.  To  prepare  the  protiodide  by 
double  decomposition,  it  was  necessary  to  find  a  mercurous  salt,  neu- 
tral in  its  reaction  and  readily  soluble  in  water.  The  author  has 
found  these  conditions  in  the  hitherto  unknown  double  salt  of  pyro- 
phosphate of  sodium  and  mercurous  acetate,  which  must  be  placed  in 
the  same  category  with  the  double  pyrophosphates  investigated  by 
Persozf  and  PahlJ. 
The  new  double  salt  crystallizes  in  handsome  needles,  which  alter 
on  prolonged  contact  with  the  air,  but  dissolve  readily  in  water  without" 
decomposition.  The  solution  yields  with  potassium  iodide  a  greenish 
yellow  precipitate  of  mercurous  iodide,  having  exactly  the  composition 
Hgl ;  in  the  reaction,  the  pyrophosphate  of  sodium  plays  no  other 
part  except  that  of  a  solvent  for  the  mercurous  acetate. 
To  prepare  the  double  salt  60  grams  of  pure  crystallized  pyrophos- 
phate of  sodium  are  dissolved,  with  the  aid  of  heat,  in  300  grams  of 
distilled  water ;  when  cool,  30  grams  of  mercurous  acetate  are  added 
to  the  solution,  and  the  mixture  stirred  from  time  to  time.  If  the 
pyrophosphate  is  chemically  pure,  the  mercurous  salt  will  completely 
dissolve  without  the  least  decomposition.  Usually,  however,  a  partial 
decomposition  has  occurred  during  the  conversion  of  the  ordinary 
phosphate  into  the  pyrophosphate  of  sodium  by  the  prolonged  heat, 
and  a  small  quantity  of  the  mercurous  acetate  is  then  decomposed 
into  mercuric  salt  and  metallic  mercury,  which,  however,  has  no  other 
effect  upon  the  mercurous  iodide  except  to  somewhat  lessen  the  yield. 
To  the  filtered  solution  an  equal  volume  of  distilled  water  is  added, 
and  afterwards  in  small  quantities  with  continued  agitation,  a  solution 
of  30  grams  iodide  of  potassium  in  one  litre  of  distilled  water.  The 
precipitate  is  at  first  brownish  green,  afterwards  green,  resembling 
the  green  oxide  of  chromium  ;  but  after  settling,  it  has  a  greenish 
yellow  color,  so  that  the  salt  is  probably  polychromatic. 
*  Abstract  of  a  paper  read  before  the  Pharmaceutical  Society  of  Paris,  and 
published  in  the  Journal  de  Pharmacie  et  de  Chimie,  1873,  April,  p.  267 — 270. 
t  Journal  de  Pharmacie  et  de  Chimie,  3  ser.  xii,  p.  218. 
X  Bulletin  de  la  Societe  Chimique,  xix,  1873,  p.  115. 
