Am.  Jour.  Pharm. 
Oct.,  1890. 
Monocalcium  Phosphates. 
513 
normal  potash  in  the  presence  of  methyl-orange.  Alcohol  contain- 
ing only  small  quantities  of  water  decomposes  monocalcium  phos- 
phate. The  author  finds  that  even  the  purest  preparations  decom- 
pose in  closed  vessels  spontaneously,  so  that  about  0-05  of  free  acid 
is  formed  in  a  month. 
Monocalcium  phosphate  is  not  hygroscopic,  as  usually  stated, 
especially  by  Birnbaum  and  Packard.  It  attracts  some  moisture 
in  an  atmosphere  saturated  with  aqueous  vapor,  but  loses  it  again 
in  dry  air.  The  hygroscopic  nature  of  the  preparation  observed 
by  the  authors  quoted  above  is,  as  the  author  shows,  due  to  the 
presence  of  free  phosphoric  acid,  of  which  at  least  0-2  per  cent, 
must  have  been  present  in  Birnbaum's  specimen.  Steam  at  8o° 
decomposes  monocalcium  phosphate.  With  regard  to  the  very 
discrepant  statements  as  to  the  decomposition  of  monocalcium 
phosphate  by  cold  water,  the  author  has  treated  pure  preparations 
with  varying  quantities  of  water,  and  gives  his  results  in  a  tabular 
form  ;  the  decomposition  being  represented  by  the  general  formula 
aCaH4(P04)2,H20  +  H20  =  (a  -  i)CaH4(P04)2,H20  +  CaHP04  + 
2H20  -f  H3P04.  For  example,  for  1  part  of  salt  and  1  part  of 
water  a  =4,  and,  therefore,  the  quantity  of  undecomposed  salt 
going  into  the  aqueous  solution  {a  —  1)  =  3,  26  per  cent,  of  the  salt 
being  decomposed  in  the  above  manner.  With  increase  in  the 
quantity  of  water,  more  salt  goes  into  solution,  and  when  1  part 
of  salt  is  treated  with  200  parts  of  water,  the  decomposition  is 
very  small,  whereas  the  solution  is  complete;  here  a  —  1024,  and 
from  this  only  1  part  is  decomposed  =o-i  per  cent.  The  decom- 
position is  regular,  for,  whereas,  for  the  proportion  of  1  salt  :  I 
water,  the  quantity  of  free  phosphoric  acid  formed  ==  7-5  1 ,  for  1 
salt:  25  water,  that  quantity  becomes  one-half  this  =  3-75,  and  for 
every  increase  of  25  parts  of  water  again  one-half  of  the  previous 
amount.  The  salt  dissolves  in  200  parts  of  water  to  a  clear  solu- 
tion, and  at  this  point  it  becomes  impossible  to  prove  the  existence 
of  the  free  acid,  which  would  indicate  decomposition.  Above  this 
limit  the  monocalcium  phosphate  is  re-formed  from  its  products  of 
decomposition — dicalcium  phosphate  and  phosphoric  acid.1 
Free  phosphoric  acid,  added  to  monocalcium  phosphate  solutions, 
prevents*  their  decomposition.    If  we  add  to  monocalcium  phos- 
1  Probably  a  siate  of  unstable  equilibrium  sets  iu  between  these  three  sub- 
stances, if  Meudeleeff's  theory  of  solution  is  correct. — B.  B. 
