AmsStr,'i887arm'}     Barium  Phosphates  in  Acidimetry.  469 
dium  phosphate  the  solution  becomes  acid,  but  the  author  gives  a 
somewhat  different  explanation  of  the  phenomenon. 
If  the  acid  solution  produced  by  mixing  solutions  of  disodium  phos- 
phate and  barium  chloride  is  mixed  with  phenolphthalein,  and  then 
with  baryta  solution,  the  addition  of  the  first  few  drops  of  the  alkali 
causes  a  somewhat  rapid  appearance  of  the  red  color,  but  after  five  or 
six  minutes  this  disappears,  and  a  somewhat  considerable  quantity  of 
baryta  is  necessary  for  the  production  of  a  persistent  coloration.  The 
volume  of  baryta  solution  required  to  produce  the  second  end  reaction 
varies  with  the  proportions  of  barium  chloride  and  disodium  phos- 
phate. 
In  presence  of  a  large  excess  of  disodium  phosphate,  the  precipitate 
after  washing  and  drying  at  120-130°  is  constant  in  composition;  it 
is  barium  sodium  phosphate,  Ba]STaP04.  Its  formation  takes  place  in 
two  stages,  the  first  of  which  is  represented  by  the  equation  Na2HP04 
4-  BaCl2  =  BaHP04  +  2NaCl.  The  acidity  of  the  liquid  at  this 
stage  is  due  to  the  formation  of  a  small  quantity  of  tribarium  phos- 
phate, and  the  consequent  liberation  of  free  acid.  In  the  second  stage, 
the  barium  hydroxide  acts  on  the  barium,  hydrogen  phosphate  in 
presence  of  sodium  chloride,  with  formation  of  barium  chloride,  water, 
and  barium  sodium  phosphate. 
Barium  sodium  phosphate  has  previously  been  obtained  in  hydrated 
crystals  by  Schulten  (1883)  by  the  action  of  sodium  phosphate  on  so- 
dium silicate  and  barium  hydroxide.  It  can  also  be  prepared  in  an 
amorphous  and  less  pure  condition  by  the  action  of  barium  hydroxide 
on  a  solution  of  disodium  phosphate,  and  in  small  quantity  by  adding 
barium  chloride  to  a  solution  of  trisodium  phosphate,  but  it  can  only 
be  obtained  in  a  state  of  purity  by  the  method  described  above. 
Tribarium  phosphate  is  obtained  only  by  pouring  sodium  phosphate 
into  a  solution  containing  a  large  excess  of  baryta. 
Barium  hydrogen  phosphate  which  has  been  precipitated  for  some 
time,  and  has  become  crystalline,  is  not  readily  converted  into  the 
double  phosphate,  a  proof  of  the  alteration  which  the  precipitate  un- 
dergoes in  course  of  time. 
From  these  facts  it  is  evident  that  free  acids  cannot  be  accurately  ti- 
trated by  means  of  baryta  in  presence  of  disodium  phosphate,  and  this 
is  true  also  of  the  titration  of  phosphoric  acid  in  presence  of  alkaline 
salts.  Accurate  estimations  can,  however,  be  made  in  either  case  by 
means  of  potassium  hydroxide  solution. 
