348 
ON  THE  VALUATION  OF  CHLORIDE  OF  LIME. 
convert  the  latter  into  ferridcyanide.  The  available  chlorine 
of  the  chloride  reacting  on  the  yellow  prussiate  in  the  usual 
manner : — 
2(K,EeCy3)+Cl  =(K3,Fe,Cye)+KCI. 
The  exact  point  at  which  complete  conversion  had  taken  place 
was  ascertained  by  taking  out  a  drop  of  the  liquid  and  testing 
it  with  a  persalt  of  iron.  Mr.  Mercer's  process  does  not  appear 
to  have  obtained  much  attention  from  chemists  ;  quite  recently, 
however,  Dr.  Davy,  of  Dublin,  has  been  engaged  in  some  ex- 
periments on  the  application  of  ferrocyanide  of  potassium  to 
several  analytical  processes,  and  has,  therefore,  been  led  to 
examine  this  method  and  suggest  several  improvements  in  it. 
Dr.  Davy  finds  that  Mr.  Mercer's  process  gives  no  better 
results  than  the  methods  commonly  in  use,  and  that  a  certain 
loss  of  chlorine  always  occurs.  He  introduces,  therefore,  an 
important  modification  of  the  process  by  which  this  loss  may  be 
avoided.  It  depends  on  the  action  which  ferrocyanide  of  pot- 
assium exerts  on  bichromate  of  potash.  When  these  two  salts 
are  mixed  in  solution  in  presence  of  hydrochloric  or  sulphuric 
acid,  the  ferrocyanide  is  immediately  converted  into  ferridcyan. 
ide.  The  following  appears  to  be  the  reaction  : — 
6(K,FeCy3,)+KO,2Cr03+7HCl  =  3(K3,Fe.,Oyg)  +  4KC1+Cr, 
CI3+7HO. 
Dr.  Davy  applies  this  reaction  thus  : — He  mixes  together  a 
certain  quantity  of  a  standard  solution  of  ferrocyanide  with  a 
given  amount  of  a  graduated  solution  of  the  chloride  of  lime, 
using  more  of  the  former  than  the  latter  can  convert,  then 
adding  hydrochloric  acid  to  dissolve  the  precipitate  formed  and 
render  the  mixture  strongly  acid,  and  finally  ascertain  how 
much  of  the  ferrocyanide  remains  unconverted  by  dropping  in 
from  a  burette  a  standard  solution  of  bichromate,  till  a  minute 
drop  taken  from  the  mixture  ceases  to  give  a  blue  or  greenish 
color  with  a  persalt  of  iron,  but  produces  instead  a  yellowish, 
brown.* 
*The  yellowish  brown  coloration  which  is  at  first  produced  when 
enough  of  the  bichromate  has  been  added,  quickly  changes  to  a  greenish 
color  by  some  secondary  reactions  which  take  place  when  the  persalt  of 
iron  is  left  in  contact  with  the  mixture.  But  this  does  not  interfere  with 
the  test ;  for  it  is  the  first  efi'ect  which  is  produced  which  indicates  the 
completion  of  the  reaction,  and  not  the  after-changes  which  may  result. 
