5i8 
Inorganic  Chemistry  of  the  U.S.P. 
(  Am.  Jour.  Pharm. 
I  November,  1905. 
retained  2  molecules  of  water  of  crystallization,  or  about  26  per 
cent.,  and  is  very  hygroscopic,  and  therefore  like  the  crystalline  salt 
of  uncertain  and  varying  strength.  Thus  the  introduction  of  sodii 
carbonas  monohydratus  furnishes  a  salt  the  composition  of  which  is 
definite,  and,  being  only  very  slightly  efflorescent  and  hygroscopic, 
it  may  be  depended  upon  as  being  more  uniform  than  the  two  car- 
bonates of  sodium  formerly  official. 
The  U.S.P.  gives  three  tests  for  identity,  namely,  the  alkalinity 
test  with  litmus,  the  effervescence  test,  and  the  sodium-flame  test. 
Impurities  are  detected  by  the  time-limit  test  for  heavy  metals.  It 
is  titrated  with  half-normal  sulphuric  acid  v.s.,  using  methyl  orange 
as  indicator. 
The  1890  Pharmacopoeia  gave  tests  for  the  following  impurities: 
Insoluble  matter,  sulphocyanate,  calcium,  potassium,  chloride,  sul- 
phate, sulphite  hyposulphites,  ammonia,  arsenic,  lead  and  aluminum 
and  iron,  the  four  latter  by  the  hydrogen  sulphide  test.  It  is  aston- 
ishing that  none  of  these  impurities  are  directed  to  be  sought  in  the 
new  official  salt,  especially  sulphate,  chloride  and  calcium.  The 
time-limit  test  may  detect  arsenic,  lead  and  iron,  however.  The 
Pharmacopoeia  requires  99- 5  per  cent  purity  for  the  salt  by  volu- 
metric analysis. 
Sodii  Citras,  Sodium  Citrate  (2Na3C6H507  +  nH20). — ''It  should 
contain  not  less  than  97  per  cent,  of  pure  sodium  citrate  (2C3H4(OH) 
(COONa)g  -f-  nH20)  and  should  be  kept  in  well-stoppered  bottles." 
It  is  slowly  efflorescent,  but  not  as  deliquescent  as  potassium  citrate. 
It  is  assayed  by  neutralization  after  ignition  in  the  same  manner  as 
other  organic  salts  of  the  alkalies. 
Sodii  Phosphas  Effervescens,  Effervescent  Sodium  Phosphate. — 
An  effervescent  sodium  phosphate  has  long  been  in  use,  and  a  simi- 
lar preparation  is  official  in  the  British  Pharmacopoeia. 
Sodii  Phosphas  Exsiccatus,  Exsiccated  Sodium  Phosphate 
(Na2HP04). — "  It  should  contain  not  less  than  99  per  cent,  of  pure 
anhydrous  sodium  phosphate  (PO(OH)(ONa)2)  and  should  be  kept 
in  well-stoppered  bottles."  This  is  a  very  deliquescent  salt.  The 
crystallized  sodium  phosphate  (Na2HP04  -f  I2H20)  contains  60  3  per 
cent,  of  water;  355-61  parts  contain  214-56  parts  of  water.  The 
same  quantity  of  the  exsiccated  salt  contains  no  water,  therefore 
141-05  parts  of  the  exsiccated  salt  are  equal  in  strength  to  355  61 
parts  of  the  crystallized  salts,  which  means  it  is  about  2^  times  as 
strong. 
