Am.  Jour.  Pharm.  ) 
Mar.,  1878.  J 
Hydrobromic  Acid. 
and  entirely  unnecessary  if  the  larger  proportion  of  acid  be  used,  for 
then  almost  the  whole  of  the  salt  crystallizes  out,  rendering  the  process 
easy  when  it  would  be  otherwise  either  troublesome  or  entirely  imprac- 
ticable. Near  the  close  of  the  distillation  the  heat  must  be  kept 
moderate,  because  if  increased  much,  sulphuric  and  sulphurous  acids  are 
liable  to  be  distilled  over,  in  considerable  quantities.  The  minute 
quantities  of  both  which  do  go  over,  even  in  a  well  managed  distilla- 
tion, are  probably  thrown  over  mechanically  by  the  bursting  of  bubbles 
on  the  surface  of  the  boiling  liquid.  The  quantity  of  either  in  the 
distillate  of  a  well  managed  distillation  is  but  a  trace,  and  so  small  as 
to  be  unimportant.  But  should  it  be  desired  to  have  the  distillate 
entirely  free  from  these  acids,  a  very  small  quantity  of  barium  hydrate 
must  be  added  and  the  whole  be  re-distilled.  In  the  distillation  the 
hydrobromic  acid  comes  over  comparatively  weak  at  first,  the  strength 
gradually  increasing  to  about  47  per  cent.,  hence  the  distillate  should 
be  well  stirred  before  being  tested  or  assayed. 
In  testing  for  sulphurous  acid  it  is  only  necessary  to  put  a  small 
piece  of  pure  zinc  into  a  test  tube,  pour  upon  it  a  few  drops  of  the 
acid,  push  into  the  upper  part  a  loose  wad  of  cotton  wool,  and  lay 
the  paper  moistened  with  solution  of  plumbic  acetate  on  to  the  cotton 
wool. 
The  assays  and  adjustment  of  strength  are  conveniently  and  easily 
made  as  follows  :  The  atomic  weight  of  HBr  being  81,  a  half  of 
the  tenth  part  of  this  number,  namely,  4*05  grams,  is  weighed  off,  and 
normal  sodium  solution  is  dropped  into  it  from  a  burette  to  the  point 
of  saturation  as  ascertained  by  means  of  a  small  piece  of  litmus  paper 
kept  floating  in  the  acid  as  it  is  stirred  during  the  dropping  in  of  the 
volumetric  sodium  solution.  Usually  18  to  19  cc.  of  the  volumetric 
solution  is  required  for  saturation.  Then,  as  only  half  of  8*1  grams 
was  taken  for  the  assay,  this  reading  from  the  burette  must  be  doubled, 
and  therefore  indicates  36  to  38  per  cent,  as  the  strength.  Then 
weigh  the  distillate  again,  and  add  to  it  5  per  cent,  of  its  weight  of  its 
distilled  water  and  again  assay  it  as  before.  By  calculation  from  the 
quantity  of  water  used  to  reduce  it  to  this  new  strength,  the  additional 
quantity  of  water  necessary  to  reduce  it  to  the  34  per  cent,  required 
is  easily  found.  When  this  shall  have  been  added,  and  the  whole  well 
stirred,  a  final  assay  should  be  made  to  verify  the  result. 
This  acid,  in  common  with  all  the  others,  should  be  dispensed  by 
