39^ 
Chromic  Acid  and  the  CJiromates. 
( Am.  Jour.  Pharai. 
I      August,  1901. 
heat  {e.g.,  Cr03,  PbO,2,  Mn02,  As205,  FeC)3,  etc.).  The  details  of 
the  various  methods  must  be  worked  out  for  each  substance  to  be 
estimated.  If  a  chromate  is  boiled  with  an  excess  of  strong  hydro- 
chloric acid,  chlorine  is  liberated,  which  can  be  distilled  and  con- 
ducted  into  a  solution  of  potassium  iodide,  contained  in  a  suitable 
apparatus.  The  distillation  may  be  avoided  by  mixing  the  chro- 
mate, a  saturated  solution  of  potassium  iodide  and  the  hydrochloric 
acid  in  a  strong  bottle,  provided  with  an  accurately  ground  stopple. 
The  stopple  is  firmly  tied  in,  the  bottle  with  its  contents  immersed 
in  water  and  the  temperature  raised  to  boiling,  where  it  is  kept  for 
one  hour.  The  bottle  is  then  removed,  cooled  and  the  amount  of 
liberated  iodine  estimated  by  means  of  N  io  sodium  thiosulphate. 
Both  of  the  above  methods  are  more  or  less  tedious  and  are 
liable  to  give  abnormally  high  results,  on  account  of  the  proneness 
of  the  hydriodic  acid  formed  to  decompose.  A  blank  should  always 
be  carried. 
The  writer  has  used  the  following  method  with  considerable  sat- 
isfaction :  Dissolve  about  I  gramme  (accurately  weighed)  in  enough 
distilled  water  to  make  exactly  100  c.c.  Of  this  solution  transfer 
20  c.c.  into  a  porcelain  evaporating  dish  containing  75  c.c.  of  water, 
add  2  grammes  of  potassium  iodide,  15  c.c.  of  10  per  cent,  sulphuric 
acid  and  mix  well.  Then  add,  from  a  burette,  N/10  sodium  thiosul- 
phate until  a  distinct  blue  color,  without  yellowish  cast,  results,  or 
the  end  may  be  determined  by  means  of  a  starch  solution. 
It  was  at  first  thought  that  some  time  must  be  allowed  for  com- 
plete reaction  of  the  above  mixture,  before  the  liberated  iodine  can 
be  estimated,  but  the  writer  soon  found  that  the  reactions  were 
almost  instantaneous. 
The  reactions  involved  are  represented  by  the  following  equa- 
tions : 
(1)  2  Cr  03  +  6  KI  +  6  H2S04  ==  3  I2  -j-  Cr2  (S04)3  +  3  K2S04 
+  6  H20. 
(2)  3  I2  6  Na,S£>8      3  Na2S406  +  6  Nal. 
(3)  K0CrX>7  4-  6  KI  4-  7  H.7S04  =  3  I2  4  K,S04  4-  Cr„  (S04)3 
+  7  H20. 
(4)  3  I2  4-  6  Na2S203  =  3  Na2S406  4-  6  Nal. 
According  to  equations  (1)  and  (2),  one  equivalent  of  Cr03 
requires  three  equivalents  of  Na2S203,  or  the  decinormal  factor  of 
CrOs  is  one-third  of  0-009988  or  0-003329.    In  the  same  way  the 
