^ie'iS^}  •     Ferric  Salts  and  Thiocyanates.  293 
were  too  weak  in  tint  for  colorimetric  examination.  The  decrease 
in  ferric  thiocyanate  coloration,  resulting  from  the  dilution  of  the 
solutions  from  8-  to  120-fold,  in  the  strongest  solution,  was  27  per 
cent.,  in  the  30:  I  solution  over  60  per  cent.,  in  the  18  :  I  solution 
73  per  cent.,  whilst  diluting  from  2-  to  32-fold  in  the  case  of  the 
1  :  1  solution  resulted  in  a  reduction  of  the  coloration,  amounting 
to  94  per  cent.;  the  stability  of  the  ferric  thiocyanate,  therefore, 
appears  to  vary  regularly  with  the  amount  of  ferric  chloride  present ; 
moreover,  the  amount  of  ferric  thiocyanate  formed  in  these  solutions 
varies  with  the  dilution  in  accordance  with  the  law  of  mass.  The 
action  of  ferric  chloride  is  twofold  ;  in  most  cases  it  forms  a  solution 
in  which  the  ferric  thiocyanate  is  more  stable  than  in  pure  water, 
but  in  very  strong  solutions,  it  exerts  a  decolorizing  and  presumably 
a  destructive  action  on  the  deep-colored  thiocyanate. 
The  examination  of  a  similar  set  of  solutions,  in  which  the  pro- 
portion of  ferric  chloride  was  kept  constant,  and  that  of  the  potassium 
thiocyanate  varied,  indicated  that,  although  the  ferric  thiocyanate 
was  more  stable  in  a  solution  of  potassium  thiocyanate  than  in 
water,  yet  the  decrease  in  color  on  dilution  did  not,  in  this  case, 
follow  the  law  of  mass,  probably  owing  to  the  presence  of  im- 
purities. 
Numerous  other  experiments  with  various  ferric  salts  and  thio- 
cyanates led  to  the  following  conclusions  :  That  the  color  reactions 
of  ferric  chloride,  nitrate,  sulphate,  tartrate,  citrate,  and  acetate  with 
potassium,  ammonium,  sodium,  lithium,  calcium,  and  barium  thio- 
cyanates, indicate  that  the  formation  of  ferric  thiocyanate  in  these 
cases  is  dependent  on  two  factors,  one  being  the  nature  of  the  acid 
of  the  ferric  salt,  the  other  the  nature  of  the  base  of  the  thiocyanate  ; 
the  former  exerting  an  action  somewhat  in  accord  with  the  relative 
affinities  of  the  acids,  whilst  the  latter  shows  no  such  relationship. 
In  all  these  cases,  except  with  the  acetate,  the  color  of  the  ferric  salt 
caused  no  inconvenience. 
Heating  the  solutions  increases  the  activity  of  the  various  reac- 
tions, and  when  those  that  favor  the  formation  of  ferric  thiocyanate 
predominate,  an  increase  of  color  is  observed,  and  vice  versa.  There- 
fore, by  heating  at  20°,  300,  400,  50°  and  6o°,  or  when  decomposi- 
tion ensued  at  some  intermediate  temperature,  an  increase  of  color 
is  observed  with  thiocyanates  and  ferric  salts  of  monobasic  acids, 
and  a  decrease  with  ferric  salts  of  polybasic  acids. 
