590  Pyrophosphoric  and  Phosphoric  Acid.  {AniDec"'Sarm' 
cipitate  could  be  obtained  of  pyrophosphate  of  magnesium.  The 
quantity  of  magnesium  ammonium  phosphate  precipitated,  and  the 
color  of  the  silver  phosphate  obtained  from  it,  confirmed  the  result 
of  the  pyrophosphate  test. 
I  would  therefore  suggest  that  either  the  method  of  separating 
the  acids  by  means  of  the  acetic  acid  reaction,  or  of  the  excess  of 
magnesium  sulphate  solution,  be  adopted  for  the  U.  S.  P.  The 
acetic  acid  test  appears  to  me  to  be  preferable  and  the  handier  to 
work. 
Detroit,  Anal.  Laboratory  of  Parke,  Davis  &  Co. 
NOTES  ON  PYROPHOSPHORIC  AND  PHOSPHORIC  ACID. 
By  Julius  Stieglitz,  A.M.,  Ph.D. 
The  general  text  books  of  pharmaceutical  laboratory  libraries 
give  very  little  information  concerning  methods  for  distinguishing 
pyrophosphoric  and  phosphoric  acid  in  presence  of  each  other, 
although  their  special  reactions  are  generally  briefly  noted.  Such 
short  notices  are  of  little  help  in  the  course  of  work,  for  special 
phenomena,  limits,  and  favorable  and  unfavorable  circumstances 
attending  reactions  are  often  as  important  as  reactions  themselves. 
In  seeking  a  practical  reaction  for  quickly  and  easily  separating  the 
two  acids  for  the  purposes  mentioned  in  the  previous  article,  I 
found  the  usual  reactions  in  very  different  degrees  serviceable. 
The  test  most  commonly  employed  is  the  silver  test.  Of  course, 
without  separating  the  acids,  a  small  percentage  of  pyrophosphate 
in  phosphate  does  not  perceptibly  change  the  color  of  the  silver 
precipitate,  and  can  therefore  not  be  detected  by  it.  ■  A  larger  per- 
centage (50  per  cent.)  very  distinctly  changes  the  usually  bright 
yellow  precipitate  of  silver  phosphate  to  a  very  pale  yellow  or  a 
yellowish  white.  On  the  other  hand  a  very  small  percentage  (1  to 
5  per  cent.)  of  phosphoric  in  pyrophosphoric  acid  can  be  detected 
by  precipitating  in  acetic  acid  solution  with  an  excess  of  silver 
nitrate,  filtering  from  the  white  precipitate  and  adding  to  the  filtrate 
a  drop  of  ammonia.  Bright  yellow  silver  phosphate  will  then 
appear,  undimned  by  the  pyrophosphate. 
The  test  with  molybdanic  acid  in  nitric  acid  solution  is  very  conve- 
nient for  finding  a  small  percentage  of  phosphate  in  pyrophosphates. 
The  result  of  the  test  must  be  taken  at  the  end  of  a  few  minutes,  as 
