194 
COMMERCIAL  GLACIAL*  PHOSPHORIC  ACID. 
On  neutralizing  an  aqueous  solution  of  glacial  phosphoric 
acid,  to  effect  which  the  generation  of  heat  has  been  carefully 
avoided  by  placing  the  dish  in  cold  water,  ammonia  will  some- 
times throw  down  no,  or  only  a  slight,  precipitate,  which  is 
increased  by  the  subsequent  application  of  heat.  The  precipi- 
tates thus  collected  and  heated  amounted  to  8.18,  4.16,  0.80 
per  cent.,  and  in  a  fourth  instance  no  precipitate  at  all  was 
obtained.  In  the  first  case  I  had  collected  from  550  grs.  of 
acid  45  grs.  of  precipitate,  and  intended  to  use  this  for  a  quanti- 
tative analysis;  but  meeting  with  an  accidental  loss  of  a  portion 
of  it,  I  am  only  able  to  give  the  quality  of  these  impurities. 
The  precipitate  was  dissolved  in  diluted  muriatic  acid,  and  the 
solution  filtered ;  a  residue  of  silicic  acid  remained  on  the  filter 
amounting  to  1.5  grs.  =  0.273  per  cent,  of  the  phosphoric  acid, 
or  3.338  per  cent,  of  the  weight  of  the  precipitate.  The  filtrate 
was  mixed  with  strong  alcohol,  and  sulphuric  acid  dropped  in  as 
long  as  a  precipitate  was  produced,  which  was  separated  by  a 
filter  and  well  washed  with  alcohol.  The  acid  filtrate,  from 
which  the  alcohol  had  been  evaporated,  yielded  a  granular 
white  precipitate  on  being  oversaturated  with  ammonia,  which 
was  insoluble  in  chloride  of  ammonium ;  carbonate  of  potassa 
produced  no  change  in  the  cold,  but,  on  boiling,  a  white  precip- 
itate, which  was  soluble  in  acetic  acid,  and  this  solution  was  not 
precipitated  by  oxalic  acid,  but  yielded  a  white  granular  precip- 
itate on  the  addition  of  chloride  of  ammonium,  ammonia  and 
phosphate  of  ammonia  in  succession.  The  filtrate,  therefore, 
contained  phosphate  of  magnesia. 
The  residue  on  the  filter  was  wholly  soluble  in  a  large  quan- 
tity of  water,  and  this  solution  was  affected  by  reagents  in  the 
following  manner  :  Phosphate  of  soda,  turbid;  after  a  while, 
white  precipitate ;  oxalate  of  ammonia,  immediately  a  white 
precipitate  insoluble  in  acetic  acid ;  ammonia,  ferro-  and  ferrid- 
cyanide  of  potassium  did  not  disturb  it.  These  reactions  prove 
the  presence  of  lime,  and  the  absence  of  alumina,  and  of  iron,  and 
other  heavy  metals. 
The  whole  precipitate  was  therefore  composed  of  silicic  acid, 
phosphate  of  lime  and  phosphate  of  magnesia.  I  must  mention 
here  that  a  portion  of  the  same  crystals  was  tested  with  chloride 
of  barium  and  caustic  potassa,  but  neither  sulphuric  acid  nor 
ammonia  was  found  in  any  of  the  specimens  examined. 
