AmMa°\U1i 9^18 arm"}  Preparation  of  Permanganate  Solutions.  361 
comes  clogged  with  oxides  these  may  be  dissolved  out  with  hot  con- 
centrated hydrochloric  acid,  followed  by  washing  with  redistilled 
water  without  disturbance  of  the  pad. 
After  standing  two  or  three  days  this  permanganate  solution 
may  be  conveniently  standardized  against  iV/50  oxalic  acid  (0.1261 
Gm.  pure  crystals  to  100  Cc.)  or  sodium  oxalate  of  similar  strength. 
To  10  Cc.  of  the  oxalic  acid  solution  add  10  Cc.  of  10  per  cent, 
sulphuric  acid  which  has  been  treated  with  just  sufficient  permanga- 
nate solution  to  give  it  a  faint  pink  color.  Place  in  a  water  bath 
at  65 0  C.  for  a  few  minutes.  Then  titrate  at  once  to  a  definite  pink 
color  which  persists  for  at  least  a  minute.  Correct  for  the  blank  ob- 
tained by  titrating  10  Cc.  of  the  sulphuric  acid  and  the  same  volume 
of  water  to  the  same  end-point. 
If  kept  in  a  dark  place,  the  oxalic  acid  solution  used  in  standardi- 
ization  does  not  lose  appreciably  in  strength  in  from  ten  days  to 
two  weeks.  Ordinarily  the  permanganate  solutions  after  they  have 
stood  several  days  will  not  vary  over  0.1  per  cent,  per  week  (see 
Table  I).  On  account  of  the  sensitivity  of  the  reagent  it  is,  never- 
theless, desirable  to  check  it  up  rather  frequently.  This  also  serves 
as  a  control  on  technique. 
The  solutions  on  which  the  above  data  were  obtained  were  kept 
away  from  the  light  except  while  in  use.  Solutions  1  and  3  were 
used  almost  every  day.  They  were  kept  at  room  temperature 
throughout  one  summer  and  in  the  case  of  Solution  1  two  summers. 
In  the  dilution  tests  redistilled  water  was  used. 
Conclusions 
The  preparation  of  weak  permanganate  solutions  by  direct  dilu- 
tion is  inaccurate  and  inconvenient.  By  means  of  the  procedure 
outlined  in  this  paper  N/100  potassium  permanganate  solutions  may 
be  prepared  which  will  retain  their  strength  and  usefulness  for  an 
indefinite  period. 
