250  Volumetric  Estimation  of  Sulphates.   { Am-j^J; 
extending  its  usefulness.  Thus,  it  having  been  shown  that  HC1  is 
oxidized  on  prolonged  contact  with  Cr207,  the  strength  of  the  stand- 
ard solution  being  thereby  altered,  trichloracetic  acid  was  proposed 
to  hold  the  BaCr04  in  solution.  In  any  case  it  is  impossible  to  pre- 
cipitate BaS04  in  the  presence  of  chromate  without  coprecipitation 
of  the  latter  in  some  degree.  For  this  reason  and  in  order,  further, 
to  avoid  the  uncertainty  arising  from  the  instability  of  solutions  of 
barium  chromate,  some  authors  recommend  separate  solutions  of 
barium  and  chromate,  the,  BaS04  being  precipitated  before  the  in- 
troduction of  the  chromate.  Another  way  of  avoiding  the  instability 
of  acid  solutions  of  barium  chromate  is  to  employ  a  mechanical 
suspension  or  cream  of  the  salt.  This  idea  would  not  seem  to  be  in 
accord  with  good  volumetric  practice.  Calcium  carbonate  has  been 
used  in  place  of  ammonia,  though  with  no  very  obvious  advantages. 
Various  means  have  been  adopted  for  the  estimation  of  the  Cr207. 
The  volumetric  solutions  required  in  the  proposed  method  are : 
Normal  Potassium  Bichromate. — Dissolve  49.033  gms.3 
K2Cr207,  previously  powdered  and  dried  at  1200  C,  in  sufficient 
water  to  measure  1000  c.c. 
The  solution  is  normal  with  respect  to  oxidizing  power. 
Tenth-normal  Sodium  Thiosulphate. — Measure  10  c.c.  n/i 
KXr,C)7  into  a  100  c.c.  glass-stoppered  volumetric  flask  and  make 
up  to  the  mark.  Measure  20  c.c.  of  this  solution  into  a  500  c.c. 
Florence  flask  containing  1  gm.  KI  just  previously  dissolved  in 
20  c.c.  dilute  sulphuric  acid  (100  c.c.  cone.  H2S04  +  900  c.c.  water), 
rinse  down  the  sides  of  the  flask  with  60  c.c.  water,  mix  the  liquids, 
cover  the  flask  with  a  watch-glass  and  let  stand  six  minutes.  Dilute 
the  liquid  with  150  c.c.  water  and  titrate  the  free  I  with  a  solution 
of  sodium  thiosulphate  (25  gms.  Na2S203  +  5H20  in  1000  c.c), 
adding  starch  indicator  near  the  end.  Divide  20,000  by  the  number 
of  c.c.  required  and  dilute  each  litre  of  the  Na2S203  solution  to  the 
volume  indicated. 
The  above  is  essentially  the  method  of  Seubert  and  Henke  4  for 
the  iodometric  estimation  of  Cr207. 
Third-normal  Barium  Chloride. — The  solution  is  equivalent  to 
the  dichromate  solution  and  contains  40.72  gms.  BaCL  +  2FLO  per 
litre. 
3  The  international  atomic  weights  for  1913  were  used  throughout  the 
investigation. 
4  Zeit.  f.  ang.  C,  1900,  1147;  "  Volumetric  Analysis,"  Sutton,  1904,  184. 
