1 8o  De-Arsenication  of  Sulphuric  Acid.      j^*"'  ^mZcK\^To. 
periments  described  in  this  paper  were  undertaken  at  a  time  when 
the  demand  for  arsenic-free  C.  O.  V.  was  very  large  and  when  all 
unnecessary  concentration  had  to  be  avoided  on  account  of  shortage 
of  plant,  to  find  out  whether  more  concentrated  acid,  particularly 
Glover  tower  acid  of  about  174°  Tw.  (80  per  cent.  H2SO4)  could  be 
de-arsenicated  without  prohibitive  loss  of  sulphuric  acid  by  reduc- 
tion. 
The  experiments  were  carried  out  at  the  temperature  of  the 
laboratory.  Hydrogen  sulphide  at  the  rate  of  one  bubble  a  second 
was  passed  through  200  Gms.  of  the  acid  under  examination,  and 
the  product  filtered  through  asbestos  in  three  separate  portions: 
(i)  Without  any  special  precaution,  (2)  with  hydrogen  sulphide 
passing  through  the  liquid  while  filtering,  and  (3)  without  precau- 
tion, after  the  liquid  and  precipitated  arsenious  sulphide  had  re- 
mained together  for  one  or  two  days. 
In  de-arsenicating  arsenical  C.  O.  V.  (95  per  cent.  H2SO4,  0.15 
per  cent.  AS2O3,  o.ii  per  cent.  AS2O5)  it  was  found  (i)  that  the 
process  was  slow,  taking  2^/2  hours  in  laboratory  apparatus;  (2) 
that  excessive  reduction  of  sulphuric  acid  occurred,  the  strength  of 
the  resulting  acid  being  91 .8  per  cent.;  (3)  that  if  filtration  from  the 
arsenic  trisulphide  is  carried  out  immediately  after  the  precipita- 
tion the  arsenic  content  is  2  to  3  parts  AS2O3  per  million,  but  that 
the  arsenic  content  is  higher  the  longer  the  filtration  is  delayed; 
and  (4)  that  if  hydrogen  sulphide  is  passed  through  the  liquid  dur- 
ing filtration  the  arsenic  content  falls  to  less  than  i  part  AS2O3  per 
million.  Further,  under  the  conditions  last  mentioned,  the  acid 
coming  through  the  filter  is  perfectly  clear,  but  soon  becomes  cloudy 
owing  to  the  precipitation  of  sulphur,  which  shows  that  the  rate 
of  oxidation  of  hydrogen  sulphide  by  strong  sulphuric  acid  is  not 
large  enough  to  prevent  the  accumulation  in  solution  of  an  apprecia- 
ble quantity  of  the  gas. 
With  arsenical  C.  O.  V.  diluted  to  contain  approximately  80 
per  cent,  of  sulphuric  acid  it  was  found:  (i)  That  the  rate  of  re- 
duction of  the  sulphuric  acid  is  quite  small  (original  acid  contained 
79.7  per  cent  H2SO4.  After  one  hour's  treatment  with  hydrogen 
sulphide,  the  acid  contained  79 . 4  per  cent.  H2SO4.  After  two  hours' 
treatment  it  contained  77.8%  H2SO4);  (2)  that  acid  filtered  imme- 
diately after  one  hour's  treatment  contained  i  part  AS2O3  per  mil- 
lion, and  the  arsenic  content  was  not  diminished  by  passing  hydro- 
gen sulphide  during  filtration;  (3)  that  when  filtration  was  delayed 
