246        Pharmacy  of  Calcium  Glycerophosphate.    {  AJul°UT^arm- 
that  the  solubilities  of  the  isomers  are  quite  different  and  where  the 
relative  proportion  of  the  two  is  unknown  one  cannot  adopt  the  usual 
method  for  determining  the  solubility  of  the  salt — that  of  shaking 
the  solvent  with  an  excess  of  solute  at  definite  temperature  and 
analyzing  the  solution — for,  as  DuBois1  has  pointed  out,  such  a 
solution  will  contain  a  larger  proportion  of  the  more  soluble  isomer 
than  the  original  mixture.  On  the  contrary,  solvent  must  be  added 
to  a  weighed  portion  of  the  salt  until  it  dissolves  in  order  that  the 
solution  may  truly  represent  the  original  compound.  To  verify  this 
the  solubility  of  the  salt  was  determined  by  each  method.  The  first 
procedure  in  which  an  excess  of  salt  was  used  gave  a  solubility  in 
water  of  1:31.59  at  250  C,  while  the  second  method  yielded  the 
result  1  :  56.95  at  the  same  temperature.  Most  of  the  solubilities 
reported  in  the  following  experiments  were  determined  by  the  latter 
method :  the  first  method  was  used  in  some  comparative  experiments. 
All  determinations  were  made  at  25  degrees  C. 
Before  entering  upon  a  discussion  of  these  determinations,  how- 
ever, we  may  profitably  review  the  present  knowledge  of  calcium 
glycerophosphate. 
The  U.  S.  P.  IX  states  that  its  solubility  in  water  at  25 0  is  about 
1  :  50 ;  DuBois1  says  commercial  calcium  glycerophosphate  should 
dissolve  in  40  to  50  parts  of  water  at  20  degrees.  In  this  work  the 
solubility  was  determined  as  1 156.95  at  25  °. 
The  commercial  product  is  a  mixture  of  a  and  f$  calcium  glycero- 
phosphates derived  from  isomeric  a  and  (3  glycerophosphoric  acids 
whose  relationship  is  shown  by  the  following  structural  formalas : 
CH2OH 
I  CH2OH  OH 
CHOH         OH  I  / 
I  /  and  CH  O— P:0 
CH2— O— P:0  I  \ 
\  CH2OH  OH 
OH 
a.  j8 
Salts  of  the  diglycerophosphoric  acids  may  also  be  present  in  the 
commercial  salt  as  impurities  but  are  excluded  by  the  alcohol- 
soluble  tests  of  the  Pharmacopoeia. 
The  solubilities  of  the  isomeric  calcium  salts  is  given1  as : 
1  "  The  Chemistry  and  Properties  of  Glycerophosphates,"  read  before  the 
pharmaceutical  division  of  the  American  Chemical  Society,  September  10,  1913. 
