252  Volumetric  Estimation  of  Sulphates.  {Am'jun7i9urm' 
(2)  The  same  error  in  less  degree  occurs  when  the  BaCl2  solu- 
tion is  added  to  the  diluted  and  boiling  K2Cr207  solution  and  the 
neutralization  effected  at  the  boiling  point. 
(3)  The  error  is  eliminated  only  when  the  K2Cr207  and  BaCl2 
solutions  are  mixed  in  the  presence  of  sufficient  HQ  to  prevent  the 
precipitation  of  BaCr04,  and  then  ammonia  water  is  added  drop 
by  drop  to  the  hot  liquid ;  further,  the  ammonia  water  must  be  fairly 
dilute  so  that  not  too  much  BaCr04  is  thrown  out  at  once  as  the 
liquid  approaches  neutrality. 
Acetic  acid  is  unaffected  by  dichromate  under  the  conditions  of 
the  procedure.  10  c.c.  n/i  K2Cr207  were  measured  into  a  100  c.c, 
volumetric  flask,  40  c.c.  water  and  1  c.c.  36  per  cent,  acetic  acid 
were  added,  the  liquid  was  boiled  1  minute,  cooled  and  made  up  to 
the  mark.  20  c.c.  required  19.95  c.c.  n/io  Na2S203.  An  inde- 
pendent experiment  done  in  the  same  manner  required  20  c.c.  n/io 
Na2S203. 
The  use  of  acetic  acid  widens  the  application  of  the  method 
greatly.  Numerous  chromates  insoluble  in  the  presence  of  am- 
monia readily  dissolve  in  acetic  acid.  The  latter  also  permits  of  the 
application  of  the  method  in  the  presence  of  borates  and,  especially, 
of  phosphates  in  limited  amounts.  It  is  highly  important  that  the 
excess  of  acetic  acid  be  not  much  greater  than  0.5  c.c,  which  is  nearly 
equivalent  to  10  c.c.  n/i  K2Cr207.  An  undue  excess  would  exert 
a  marked  influence  on  the  solubility  of  the  BaCr04  .and  lead  to  too 
high  results.5  The  quantities  of  reagents  specified  are  such  that 
under  the  actual  working  conditions  of  the  procedure  the  excess  of 
acetic  acid  finally  present  is  close  to  0.5  c.c. 
The  method  being  designed  for  technical  use,  the  volumes  re- 
quired to  be  handled  are  kept  within  moderate  limits. 
In  the  volumetric  standardization  of  the  BaCL  under  the  con- 
ditions specified  various  errors  compensate  each  other  so  perfectly 
that  gravimetric  assays  show  very  nearly  theoretical  results.  Ob- 
viously this  fact  is  of  the  highest  importance  in  the  practical  opera- 
tion of  the  method.  If  this  were  not  so  it  would  be  necessary  to 
standardize  the  BaCL  gravimetrically  and  apply  a  correction  to  the 
titration.  In  the  course  of  the  work  two  independent  sets  of  reagents 
were  prepared.   First  lot  of  BaCL  :  10  c.c.  yielded  0.3885  gm.  BaS04, 
5  Cf.  Bray,  "  A  System  of  Qualitative  Analysis  for  the  Common  Ele- 
ments," /.  Am.  Ch.  Soc,  1909,  611-637. 
