2l6 
Anhydrous  Sodium  Sulphite. 
J  Am.  Jour.  Phariu. 
\      May,  1910. 
normal  salt  probably  could  replace  it  in  most  cases ;  and  where 
it  could  not  replace  it  directly  it  might  replace  it  indirectly  by  using 
it  in  conjunction  with  an  equivalent  amount  of  a  suitable  acid,  as 
hydrochloric,  sulphuric,  or  sulphurous  acid.  Of  course  when  we 
compare  the  concentration  of  the  available  S02  (25.4  per  cent.) 
in  the  sulphite  (Na2S03.7H20)  with  that  in  the  bisulphite  (61.5 
per  cent.),  the  difference  in  favor  of  the  latter  is  quite  large,  thus 
rendering  it  of  especial  value  in  all  cases  where  a  high  concentration 
of  available  S02  is  required ;  but  when  we  remember  that  by  sub- 
stituting the  anhydrous  sodium  sulphite  for  the  hydrated  sulphite 
of  the  U.S. P.  we  also  double  the  concentration  of  available  S02, 
the  difference  in  favor  of  the  bisulphite  becomes  practically  negli- 
gible. Thus  the  bisulphite  of  the  U.S. P.  (90  per  cent,  purity)  would 
have  a  concentration  of  available  S02  equivalent  to  about  55  per 
cent.,  whereas  the  anhydrous  sodium  sulphite  (over  96  per  cent, 
purity  in  many  commercial  samples)  would  have  a  concentration  of 
available  S02  equivalent  to  about  49  per  cent. ;  the  difference  in  the 
concentration  of  available  S02  in  the  latter  case,  therefore,  could 
hardly  affect  the  reaction  in  any  given  case  where  the  bisulphite 
might  be  used  as  a  reagent. 
On  the  other  hand,  it  seemed  that,  having  the  anhydrous  sodium 
sulphite  as  a  ready  source  of  supply  of  S02,  we  might  also  be  able 
to  readily  prepare  and  isolate  the  bisulphite  whenever  wanted  by 
taking  advantage  of  the  comparatively  sparing  solubility  of  the 
latter  in  strong  alcohol.  In  order  to  test  this  plan  the  following 
experiment  was  tried  : 
Since  1  Gm.  of  water  will  absorb  9  0.168  Gm.  of  S02  (at  8°  C. 
and  a  barometric  pressure  of  760  mm.),  to  saturate  40  Gm.  of  water 
would  require  6.72  Gm.  S02.  To  obtain  this  quantity  of  S02  from 
Na2SO,,  13.2  Gm.  of  the  latter  will  be  required.  Since,  however, 
the  actual  amount  of  Na2S03  in  the  sample  of  sulphite  used  was 
only  about  95  per  cent.,  it  would,  therefore,  theoretically  require 
13.9  Gm.  of  the  sulphite.  This,  therefore,  gives  us  an  idea  as  to 
the  approximate  amount  of  sulphite  required  under  the  conditions 
of  this  experiment;  hence  15  Gm.  of  the  sulphite  were  used.  This 
was  placed  in  a  distilling  flask  of  about  250  c.c.  capacity,  the  mouth 
of  which  was  fitted  with  a  doubly  perforated  stopper;  one  of  these 
perforations  being-  used  for  connecting  with  the  separatory  funnel, 
9  Watts'  Dictionary  of  Chemistry,  vol.  4,  p.  614  (1902). 
