AmAp0df;Srm'}   Determination  of  Pyrophosphoric  Acid.  2 1  3 
because  very  frequently  we  have  to  do  with  a  mixture  of  ortho-  and 
pyrophosphates. 
Na2  HPO4  -f-  NH4  OH  -|-  MgSO4  = 
Mg  (NH4)  PO4-  Na2  SO4  +  H20. 
(11)  Neither  Fresenius,  Fliickiger  nor  E.  Schmidt  men- 
tions in  their  well-known  respective  text-books  how  sensitive  the 
silver  nitrate  test  is  for  orthophosphates.  I  do  not  know  if  this  has 
been  given  elsewhere.  Since  it  is  valuable  to  know,  from  a  pharma- 
ceutical and  chemical  standpoint,  "  about "  how  much  orthophos- 
phates a  silver  solution  detects,  experiments  were  made — o-ooi  g. 
could  be  found.  The  reaction  with  molybdate  solution  is  sensitive 
on  orthophosphates  in  the  fifth  decimal  place.  Quantitative  estima- 
tions of  phosphoric  acid  are  uncalled  for. 
"  Wer  dies  aber  wissen  muss, 
Der  kauft  sich  ein  Fresenius." 
(12)  Strongly  acidulating  with  hydrochloric  acid  and  then  adding 
magnesia  mixture  in  unlimited  quantity  [see  (7)  ]  is  another  source  of 
error.  Fresenius  teaches  that  magnesia  mixture  should  not  be 
used  in  too  excessive  quantities. 
In  one  sample  (No.  6),  which  was  nearly  pure  pyrophosphate,  five 
(5)  c.c.  magnesia  mixture  and  one  (1)  c.c.  ammonia  water,  gave  a 
precipitate  which  did  not  visibly  dissolve.  On  the  authority  of  the 
U.  S.  P.,  this  sample  would  have,  tlieref ore,  been  condemned.  This 
precipitate  gave  a  white  color  reaction  with  silver  nitrate  solution, 
after  it  was  collected,  washed,  dissolved  in  dilute  nitric  acid,  repre- 
cipitated  by  ammonia  and  redissolved  in  dilute  acetic  acid. 
It  is  repeated  here  that  the  pyrophosphate  was  "  nearly  "  pure  ; 
that  the  precipitate  did  not  visibly  dissolve,  because  : 
(1)  The  colorless  filtrate  of  the  mixture  of  the  salt  with  potas- 
sium hydrate  solution  remained  clear  with  two  (2)  c.c.  of  magnesia 
mixture  and  five  (5)  drops  ammonia  water,  for  a  day.  With  o  5  c  c. 
ammonia  water,  the  same  results. 
(2)  The  molybdate  test  showed  only  traces  of  orthophosphoric 
acid. 
(3)  The  experienced  eye  detected  a  yellowish  tinge  in  the  white 
silver  precipitate,  when  a  larger  quantity  of  the  silver  solution  was 
added  after  the  first  portion,  whereby  the  white  precipitate  was 
thrown  down. 
