Sub-topic #4: Iron-thiocyanate Equilibrium

• When potassium thiocyanate [KSCN] is mixed with

  iron(III) nitrate [Fe(NO₃)₃] in solution, an equilibrium

  mixture of Fe+3, SCN-, and the complex ion

  FeSCN+2 is formed.
• The solution also contains the spectator ions K+ and

  NO₃-
• The relative amounts of ions participating in the

  reaction can be judged from the solution color.
• In neutral to slightly acidic solutions:

  • Fe+3 is light yellow
  • SCN- is colorless
  • FeSCN+2 is red
• If the solution is initially reddish, and the equilibrium

  shifts to the right (more FeSCN+2), the solution

  becomes darker red/
• If the equilibrium shifts to the left (less FeSCN+2),

  the solution becomes lighter red or straw yellow.
• Adding various reagents to this reaction at equilibrium will shift the equilibrium position of the reaction and will be evidenced by a change in the color of the resulting solution.