592  Pyrophosphonc  and  Phosphoric  Acid.  { Am-^\^Tm' 
towards  acetic  acid.  The  pyrophosphates  of  the  alkaline  earths  are 
stated  to  be  difficultly  soluble  in  acetic  acid  :  magnesium  pyrophos- 
phate, the  salt  I  worked  with,  proved  comparatively  easily  soluble  in 
the  acid  when  freshly  precipitated  in  the  cold,  although  much  less 
easily  than  the  phosphate  salt ;  its  precipitation  is  prevented  with  still 
less  (about  half  the  quantity)  acetic  acid  if  the  latter  is  added  before 
the  addition  of  magnesium  sulphate.  But  I  found  that  on  boiling 
such  an  acetic  acid  solution  of  magnesium  pyrophosphate,  the  latter 
is  easily  precipitated,  unless  strong  acid  is  used  in  very  large  excess. 
Magnesium  phosphate  is  precipitated  by  boiling  from  solutions  con- 
taining only  the  slightest  excess  of  acetic  acid,  but  is  not  precipitated 
at  all  from  solutions  properly  acidified.  The  pyrophosphate  precipi- 
tated by  boiling  from  acetic  acid  solution  is  very  much  more  diffi- 
cultly soluble  in  this  acid  than  when  precipitated  by  any  means  in 
the  cold,  and  then  may  even  more  properly  be  called  "  difficultly 
soluble."  But  the  phosphate  obtained  by  boiling  in  the  presence 
of  very  little  acetic  acid  is  exceedingly  easily  soluble  in  the  latter, 
even  in  the  cold.  The  method  is  applied  by  adding  to  the  solution 
of  the  alkali  salts  an  excess  of  acetic  acid  (I  to  10  mg.  for  every  I 
mg.  P205  present)  and  boiling.  Phosphate  of  magnesium  will  remain 
dissolved  and  the  pyrophosphate  be  precipitated.  The  latter  is  washed 
with  hot  water  and  can  then  be  used  in  the  pure  state  for  any  fur- 
ther test  desired.  To  convert  it  into  the  silver  salt,  it  is,  on  account 
of  its  difficult  solubility  in  acetic  acid,  first  treated  with  nitric  acid 
and  ammonia  and  then  easily  dissolved  in  acetic  acid  before  the 
addition  of  silver  nitrate. 
In  a  solution  containing  one  part  of  P205  as  H4P207  to  20  parts  of 
P205  as  H3P04,  the  pyrophosphate  could  be  easily  detected  by  this 
method  without  special  precautions.  From  a  solution  containing 
only  one  part  (50  mg)  of  P205  as  H4P207  to  one  hundred  parts  of 
P205  as  H3PO4,  the  pyrophosphate  was  separated  pure  by  this  pro- 
cess after  making  two  fractional  precipitations  by  using  an  insufficient 
quantity  of  acetic  acid  for  keeping  all  the  phosphate  in  solution  on 
boiling  (for  the  first  precipitation  only  1  mg  C2H402  for  4  mg.  P205 
present  was  used).  A  sufficient  quantity  of  pyrophosphate  was 
obtained  for  further  tests.  This  compares  very  favorably  with  my 
experience  with  the  preceding  method. 
The  precipitation  of  magnesium  pyrophosphate  by  boiling  its 
