2IO 
Determination  of  Pyrophosphoric  Acid. 
Am.  Jour.  Pharm. 
April,  189o. 
THE    DETERMINATION    OF    PYROPHOSPHORIC  ACID 
IN    THE    PRESENCE    OF  ORTHOPHOSPHORIC 
ACID  AS  APPLIED  TO  SOLUBLE  IRON  ^ 
(1)  Fresenius  teaches,  that  the  precipitate  obtained  upon  the 
addition  of  magnesium  sulphate  to  an  aqueous  solution  of  a  pyro- 
phosphate of  an  alkali,  Mg2P207,  is  soluble  in  an  excess  of  the 
pyrophosphate,  as  well  as  in  an  excess  of  the  magnesium  sulphate. 
(2)  Th  .  S  a  1  z  e  r  remarked,  in  the  Archiv  der  Pharmacie,  Band 
232,  Heft  5,  p.  371,  1894,  "Ueber  Natrium  pyrophosphate"  that  it 
is  "  doubtful  if  this  solubility  of  the  precipitate  is  a  characteristic 
property  of  pyrophosphates." 
He  published  the  remarkable  experience  that  he  found  an  ortho- 
phosphoric  acid,  which  showed  all  the  identity  reactions  of  a  pyro- 
phosphate, with  the  exception  of  the  one  with  potassium  perman- 
ganate. The  orthophosphoric  acid  that  led  to  this  discovery  was 
contaminated  with  one- fourth  of  an  acetyl  compound  of  one  of  the 
oxyacids  of  phosphorus.  A  sodium  salt  of  this  compound,  mixed 
with  5  parts  sodium  diorthophosphate,  gives  a  pure  white  precipi- 
tate with  silver  nitrate  solutions,  this  white  precipitate  being  the 
well-known  identity  reaction  for  pyrophosphoric  acid. 
Writing  phosphoric  acid,  according  to  Ge  rhardt's  type  theory, 
(3)  It  will  be  readily  conceded  that  S  a  1  z  e  r  '  s  experience  is  of 
importance  to  us,  and  that  a  wider  knowledge  of  it  may  end  the 
doubt  and  confusion  for  manufacturers  of  pyrophosphates,  and 
for  dispensers  of  it,  in  which,  unfortunately,  the  seventh  edition  of 
the  U.  S.  P.  leaves  its  readers  where  they  have  to  make  a  determi- 
nation of  pyrophosphates  in  the  presence  of  orthophosphates. 
PYROPHOSPHATE,   U.  S.  P. 
By  J.  B.  NageIvVOOrt. 
(Received  for  publication,  Jan.  16,  1895.) 
