46 
Colloidal  Dey^ivatives  of  ferric  Hydroxide. 
(  Am.  Jour.  Pharni. 
1      Jan.,  1885. 
hydroxide,  and  potash.  Similar  solutions  are  obtained  by  .substituting 
either  ammonia  or  soda,  or  sodium  or  potassium  carbonate  for  potash. 
They  seem  to  be  compounds  of  glycerol,  ferric  hydroxide,  and  alkalis, 
and  are  easily  dissociated  by  water  into  glycerol  and  insoluble  com- 
pounds richer  in  ferric  hydroxide.  With  acetic  acid,  they  yield  a  pre- 
cipitate soluble  in  excess,  and  this  solution  gives  a  greenish  precipitate 
with  potassium  ferrocyanide,  converted  into  Prussian  blue  by  the  action 
of  a  mineral  acid. 
Mannitol,  erythrol,  and  sugar  yield  solutions  having  precisely  similar 
properties. 
Ferropotassium  tartrate  also  forms  colloidal  solutions  with  similar 
properties,  but  they  are  not  precipitated  by  jDotash  in  the  cold,  and  the 
precipitate  with  carbonic  anhydride  is  ochreous,  and  resembles  ferric 
hydroxide.  Potassium  ferrocyanide  produces  a  violet  coloration,  and 
Prussian  blue  is  precipitated  from  this  solution  on  adding  an  acid. 
When  a  solution  of  ferric  chloride  is  added  to  a  solution  of  sodium 
arsenate,  a  precipitate  of  ferric  arsenate  is  formed,  but  redissolves  in 
excess  of  ferric  chloride,  forming  a  colloidal  solution,  which  yields  an 
opaque  yellowish-white  coagulum  of  the  composition  Fe2(As04)2.  In 
preparing  this  solution,  excess  of  ferric  chloride  must  be  avoided,  since 
this  substance  prevents  coagulation  by  heat.  If  the  solution  is  dialysed, 
it  loses  sodium  chloride,  and  the  jelly  which  forms  is  perfectly  trans- 
parent and  of  a  pale  yellow  color,  but  by  prolonged  dialysis  it  becomes 
reddish-yellow. 
Potassium  arsenite  and  ferric  chloride  yield  a  similar  solution,  which 
is  very  unstable,  and  coagulates  spontaneously.  With  potash,  it  forms 
a  precipitate  soluble  in  excess,  and  when  this  solution  is  dialysed  it 
yields  a  colloidal  solution  of  ferropotassium  arsenite,  which  does  not 
gelatinise  on  heating  unless  mixed  with  potash  solution,  when  a  thick 
brown  jelly  resembling  ferropotassium  tartrate  is  formed.  Arsenious 
acid  therefore  resembles  tartaric;  acid  in  its  behavior  with  ferric  salts. 
Boric  acid  and  sodium  phosphate  behave  like  arsenic  acid,  but  in  the 
case  of  the  phosphate  the  solution  shows  less  tendency  to  coagulate. 
A  2*26  per  cent,  solution  of  silica,  obtained  by  the  decomposition 
of  methyl  silicate,  forms  with  ferric  chloride  and  excess  of  potash  a 
limpid  solution,  which  becomes  turbid  after  a  few  minutes. — Compt. 
rend.,  xcviii.,  pp.  1485-1488  and  1540-1542  ;  Jour.  Chem.  Soc,  p.  966, 
October,  1884. 
