26  Inorganic  Chemistry  of  the  U.S. P.  (Aji°arrySm' 
citrate,  but  was  likewise  discarded,  and  a  gravimetric  method,  de- 
pending upon  the  conversion  of  lithium  sulphate,  is  introduced. 
Lithium  Carbonate  is  assayed  by  residual  titration  ;  0  5  gramme 
of  the  salt  are  dissolved  in  20  c.c.  of  normal  sulphuric  acid  V.  S., 
and  the  resulting  solution  titrated  with  normal  potassium  hydroxide 
V.  S. ;  not  more  than  6.6  c.c.  should  be  required,  methyl  orange 
being  employed  as  indicator.  In  the  old  Pharmacopoeia  the  assay 
was  a  direct  titration  with  normal  sulphuric  acid  V.  S.'  The  new 
method  is  certainly  the  better  one  for  this  as  well  as  for  all  other 
soluble  carbonates. 
Lithium  Bromide  is  assayed  in  the  same  manner  as  it  formerly 
was,  the  only  difference  being  that  instead  of  weighing  0-5  gramme 
of  the  salt  and  dissolving  it  in  20  c.c.  of  water  ;  I  gramme  is  dis- 
solved in  sufficient  distilled  water  to  make  100  c.c,  and  of  this  solu- 
tion 20  c.c.  (representing  0-2  gramme  of  the  salt)  are  taken  for 
analysis.  The  titration  is  done  with  tenth-normal  silver  nitrate 
V.  S.,  potassium  chromate  being  used  as  indicator. 
Magnesium  Salts. — Magnesii  Oxidum  and  Magnesii  Oxidum 
Ponderosum  have  been  discussed  earlier  in  this  article  ;  Magnesii 
Carbonas  is  approximately  (MgC03)4  Mg  (OH)2  -f-  5H20.  It  should 
yield  upon  ignition  not  less  than  40  per  cent,  of  residue,  of  which 
not  less  than  96  per  cent,  should  consist  of  pure  magnesium  oxide. 
Theoretically,  482-26  grammes  of  the  official  magnesii  carbonas 
should  yield  upon  ignition  200-3  grammes  of  magnesium  oxide. 
That  means  that  1  gramme  of  the  carbonate  should  yield  0-407 
gramme  of  the  oxide.  The  U.S.P.  requirement  is  that  1  gramme 
should  yield  when  ignited  0-40  gramme  of  residue.  It  is  further 
directed  in  the  newly  introduced  assay  that  0  400  gramme  of  recently 
ignited  and  cooled  magnesium  carbonate  (i.  e.}  0-400  gramme  of 
MgO)  be  dissolved  in  25  c.c.  of  normal  sulphuric  acid  V.  S.  and  the 
solution  titrated  with  normal  potassium  hydroxide  V.  S. ;  not  more 
than  5.8  c.c.  of  the  latter  should  be  required  for  neutralization, 
methyl  orange  being  used  as  the  indicator.  The  quantity  of  normal 
potassium  hydroxide  solution  used  is  deducted  from  the  25  c.c.  of 
normal  sulphuric  acid  V.  S.  added,  leaving  19  2  c  c,  which  is  the 
quantity  of  normal  sulphuric  acid  V.  S.  which  went  into  combina- 
tion with  the  0  400  gramme  magnesium  oxide  taken  for  analysis; 
each  cubic  centimetre  of  the  acid  V.  S.  representing  5  per  cent.,  or 
0-02  gramme,  of  pure  magnesium  oxide.    The  equation  is: 
