48 
ESTIMATION  OF  NITRIC  AND  ACETIC  ACIDS. 
and  shows  itself  in  the  receiver  by  the  production  of  a  dense 
white  cloud,  from  its  decomposing  some  of  the  nitrate  of  baryta 
in  solution.  This  decomposition  is  of  no  moment,  as  the  nitric 
acid  thus  set  free  immediately  decomposes  in  its  turn  an  equival- 
ent of  the  carbonate  of  baryta.  The  liquid  must  be  boiled  in 
order  to  agglomerate  the  sulphate,  and  to  cause  the  deposition  of 
any  carbonate  of  baryta  that  may  be  dissolved  by  the  free  car- 
bonic acid.  After  filtering  away  the  sulphate  and  undissolved 
carbonates,  a  clear  solution  of  pure  nitrate  of  baryta  is  obtained. 
This  may  be  evaporated  to  dryness  and  weighed ;  or,  as  the  salt 
is  rather  apt  to  decrepitate  when  heated,  it  will  be  found  prefer- 
able  to  convert  the  nitrate  into  the  sulphate,  and  from  that  to 
calculate  the  amount  of  nitric  acid  which  passed  over  into  the 
receiver. 
The  points  which  require  special  notice  in  this  process  are, — 
1st,  the  purity  of  the  sulphuric  acid  employed ;  as  commercial 
sulphuric  acid  frequently  contains  a  trace  of  nitric  acid,  it  must 
be  tested  before  it  is  used ;  2nd,  the  gentle  heating  of  the  mix- 
ture in  the  retort,  when  there  is  a  good  deal  of  nitric  acid  in  it, 
in  order  to  prevent  any  chance  of  the  formation  of  nitrous 
fumes;  3rd,  the  long-continued  heating  of  the  liquid,  so  as  to 
ensure  the  distillation  of  as  nearly  the  whole  of  the  nitric  acid 
as  possible.  The  liquid  should  scarcely  be  allowed  to  boil,  lest 
some  portion  of  a  salt  should  spring  over,  which,  whatever  it  was, 
if  soluble,  would  vitiate  the  experiment.  When  these  precau- 
tions are  properly  attended  to,  a  very  good  result  is  obtained. 
Known  weights  of  pure  nitre  were  tried  ;  the  amount  of  nitric 
acid  found  was  not  1  per  cent,  less  than  the  theoretical  amount. 
Very  small  quantities  of  nitric  acid,  combined  with  other  matters, 
can  be  determined  with  considerable  accuracy  in  this  way,  as  was 
proved  when  0-2  gr.  of  nitre  was  mixed  with  phosphate  and  sul- 
phate of  copper  and  soda,  and  subjected  to  the  same  treatment. 
It  is  self-evident  that  this  process  cannot  be  employed  when 
there  is  some  other  acid  mixed  with  the  nitrate,  which  is  volatile, 
and  gives  a  soluble  baryta  salt ;  or,  rather,  the  process  would 
require  to  be  modified  to  suit  the  particular  case,  and  such  modi- 
fications can  generally  bo  easily  devised. 
It  will  also  be  evident  that  this  process  may  be  applied  equally 
well  to  a  number  of  other  volatile  acids.    Most  of  these  however 
