524  Action  of  Acids  on  Thiosulphates.      {Am  oc°cu,ril9arm' 
ACTION  OF  ACIDS  ON  THIOSULPHATES.1 
By  Berthelot. 
Thiosulphates,  as  is  well  known,  are  decomposed  by  acids  with 
liberation  of  sulphurous  anhydride  and  precipitation  of  sulphur.  The 
sulphur,  however,  exists  in  different  allotropic  modifications,  the  sul- 
phurous anhydride  reacts  with  the  liberated  thiosulphuric  acid  before 
it  has  time  to  decompose,  and  forms  thionic  acids,  and  the  latter  are 
partially  dissociated  by  the  water.  It  follows  that  the  final  state  of 
the  system  is  a  condition  of  equilibrium  between  many  reactions. 
When  a  solution  of  sodium  thiosulphate  is  mixed  with  boric  acid 
there  is  no  sensible  thermal  disturbance,  and  the  liquid  remains  color- 
less and  transparent.  ( Acetic  acid  likewise  produces  no  sensible  varia- 
tion of  temperature,  but  a  precipitate  gradually  forms  and  increases 
in  quantity.  The  quantity  of  iodine  absorbed  is  also  much  less  than 
would  be  required  if  all  the  thiosulphurous  acid  were  decomposed 
into  sulphurous  acid  and  sulphur.  The  thiosulphate  is  only  decom- 
posed to  a  slight  extent,  and  it  follows  that  the  heat  of  neutralization 
of  thiosulphuric  acid  is  greater  than  that  of  acetic  acid,  -j- 13*3  CaL 
It  is  possible  that  an  acid  sodium  acetate  is  formed,  and  the  sulphur- 
ous anhydride  which  is  liberated  produces  thionic  acids  by  interaction 
with  the  thiosulphuric  acid,  and  hence  the  great  length  of  time  required 
before  equilibrium  is  established. 
With  sodium  thiosulphate  and  very  dilute  sulphuric  acid  there  is 
an  immediate  slight  rise  of  temperature,  followed  very  rapidly  by  the 
formation  of  a  precipitate  and  a  slight  fall  of  temperature.  The  heat 
of  neutralization  of  thiosulphuric  acid  is  therefore  lower  than  that  of 
sulphuric  acid. 
Hydrochloric  acid  of  certain  concentrations  produces  a  slight  devel- 
opment of  heat,  whilst  with  other  concentrations  there  is  a  slight 
absorption  of  heat.  The  liquid  becomes  turbid  almost  immediately,, 
owing  to  precipitation  of  sulphur.  It  is  evident  that  the  reaction  is 
somewhat  complicated,  but  it  is  also  evident  that  the  heat  of  neutrali- 
zation of  thiosulphuric  acid  is  practically  the  same  as  that  of  hydro- 
chloric acid,  and  it  may  be  taken  as  -f  13*8  Cal.  without  sensible  error. 
The  absorption  of  heat  observed  in  some  cases  is  due  to  the  decompo- 
sition of  the  thiosulphuric  acid,  and  is  exactly  equal  to  the  difference 
between  the  heats  of  formation  of  thiosulphuric  and  sulphurous  acids 
1  Compt.  rend.,  cviii,  971-978 ;  reprinted  from  Jour.  Chem.  Soc,  Sept.,  1889,. 
p.  824. 
