Mtlch^%^]     De-Arsenication  of  Sulphuric  Acid.  i8i 
the  arsenic  content  had  risen  only  to  i .  5  parts  AS2O3  per  million. 
Two  days  after  the  acid  has  been  filtered  it  still  smells  of  hydrogen 
sulphide,  which  shows  that  the  rate  of  reaction  between  hydrogen 
sulphide  and  80  per  cent,  sulphuric  acid  is  small. 
The  behavior  of  70  per  cent,  acid  resembles  that  of  80  per  cent., 
the  chief  differences  being  that  (i)  de-arsenication  is  slightly  more 
rapid,  (2)  filtration  of  the  arsenic  trisulphide  may  be  delayed  several 
days  without  any  serious  increase  of  the  arsenic  content,  and  (3) 
there  is  even  less  reduction  of  sulphuric  acid. 
Finally,  an  experiment  with  Glover  tower  acid  of  unusually  high 
arsenic  content  was  carried  out,  in  which  hydrogen  sulphide  in  fine 
bubbles  was  passed  through  500  Gms.  of  the  acid  contained  in  a 
bottle  fixed  on  a  shaking  machine.  One  hour's  treatment  with 
hydrogen  sulphide  proved  insufficient  for  this  acid,  and  in  a  second 
experiment  treatment  was  continued  for  1V4  hours.  Part  of  the 
product  was  filtered  immediately  and  part  after  two  days.  The 
results  of  analysis  were: 
Before  After  Immediate  After  Standing 
Treatment.  Filtration.  Two  Days. 
%.                              %•  %.  " 
H2SO4                             77  I                      77  I  77.1 
N2O3                                0.18                    None  None 
AS2O3   0.29  Between  o . 6  and 
I  part  in  10^ 
AS2O5                               I  .58                    None  None 
These  figures  need  some  comment,  for  as  they  stand  they  give 
the  impression  that  there  is  no  loss  of  sulphuric  acid.  The  follow- 
ing calculation  shows  that  de-arsenication  should  actually  cause  an 
increase  of  the  sulphuric  acid  content.  The  reactions  proceeding 
are: 
N2O3  +  H2S  =  H2O  +  2NO  +  S; 
AS2O3  +  3H2S  =  AS2S3  +  3H2O; 
AS2O5  +  5H2S  =  AS2S3  +  5H2O  +  2S. 
Thus,  apart  from  absorption  of  water  from  the  atmosphere  and  from 
reduction  of  sulphuric  acid,  100  Gms.  of  the  original  acid  contain- 
ing: 
771    Gms.  H2SO4  77  . 1  Gms.  H2SO4 
0.18  Gm.  N2O3  should  give  0.05  Gm.  H2O 
0.29  Gm.  AS2O3  with  HavS  0.08  Gm.  HoO 
1 .58  Gms.  AS2O  0.63  Gm.  H.O 
20.85  Gms.  H2O  20.85  Gms.  H2O 
