Am.  Jour.  Pharm. 
June,  1920. 
Calcium. 
409 
ratus;  the  entrance  and  exit  of  the  apparatus  being  properly  guarded 
by  bulbs  containing  concentrated  sulphuric  acid. 
No.  10.    Residual  titration  method. 
A  0.5  Gm.  sample  of  the  calcite  was  placed  in  a  flask  connected 
with  a  straight  tube  reflux  condenser.  A  measured  excess  of  0.2  A/" 
H2SO4  was  run  into  the  flask.  The  flask  was  connected  with  the 
reflux  condenser  and  heated  by  a  flame  until  the  reaction  was  com- 
plete. The  condenser  was  then  washed  with  water,  allowing  the 
washings  to  flow  down  into  the  flask  and  the  excess  of  0.2  N  H2SO4 
was  titrated  with  0.1  N  Na2C03,  using  methyl  orange  as  indicator. 
The  0.1  N  Na2C03  was  standardized  by  accurately  weighing  out  the 
proper  quantity  of  freshly  ignited,  chemically  pure  sodium  carbonate 
and  the  0.2      H2SO4  was  standardized  against  the  0.1  N  Na2C03. 
Results   99 . 66 
99.83  Av.,  99.75  percent  CaCOa 
For  convenient  comparison  the  results  are  herewith  presented  in 
tabular  form: 
Theoretical  CaCOs  content  of  standard  material,  99.959  per 
cent. 
Results 
99.68 
99.93  Av.,  99.80  per  cent.  CaCOs 
Method. 
CaCOs  Found. 
No.  I  — Precipitating  as  calcium  oxalate  and 
weighing  as  calcium  carbonate. . .    99 . 64 
99-49 
Av.,   99.57  per  cent. 
No.   2 — Precipitating  as  calcium  carbonate 
and  weighing  as  such   100.05 
00. 10 
Av.,  100.07  per  cent. 
No.   3 — Liberating  CO2  with  HCl  and  weigh- 
ing liberated  CO2  in  KOH  solution   99 . 57 
100.50 
No.  4 — Precipitating  as  calcium  oxalate  and 
weighing  as  calcium  oxide   99-75 
99  98 
No.   5 — Precipitating  as  calcium  sulphate 
and  weighing  as  such   99  •  64 
Av.,  100.03  per  cent. 
Av.,  99 . 86  per  cent. 
99-73 
Av.,   99.68  per  cent. 
No.  6 — Precipitating  as  calcium  oxalate  and 
weighing  as  calcium  sulphate         100. 15 
100.64 
Av.,  100.39  per  cent. 
