Introduction

Dilute solutions of Fe(NCS)2+ have their equilibrium positions shifted with the addition of Fe(NO3)3, KSCN, NaOH, and AgNO3.

Diluted solutions

Equations

The ferric thiocyanate solution is blood red in color, and the intensity of its color is proportional to the amount of complex in solution.  The complex is formed according to the reaction:

Fe3+ + SCN-   ↔ Fe(NCS)2+

Addition of Fe3+ or SCN- will push the equilibrium to the right, forming more complex and intensifying the color; while precipitating out Fe3+ (as Fe(OH)3) or SCN- (as AgSCN) will push the equilibrium to the left, consuming the complex and decreasing color intensity.

To Conduct Demonstration:  

  1. Display five 250mL beakers with 50mL of Fe(NCS)2+ solution.  Reserve one as a reference.
  2. Add 0.1M Fe(NO3)3 solution to beaker #2.  The color should become more intense as more complex is formed.
  3. Add 0.1M KSCN solution to beaker #3.  The color should again become more intense with the formation of additional complex.
  4. Add 1M NaOH dropwise to beaker #4.  The color should become less intense as Fe(OH)3 precipitates.
  5. Add 0.1M AgNO3 solution dropwise to beaker #5.  The color should become less intense as AgSCN is precipitated.

Safety 

Silver solutions will stain skin black on contact; gloves should be worn. 

Acknowledgements: 

Mary DuBois, Spring 1990.