Introduction

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

Diluting solutions

*There is no prep box for this demo in the demo room. Chemicals are on the shelves and  the plastic sheet is on the back of demo manual.  

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.

Supplies

Short List

  •  Stirring rod
  •  Paper towels
  •  Goggles and gloves

Chemicals

  •  0.1M Fe(NO3)3 (ferric nitrate)
  •  0.1M KSCN (potassium thiocyanate)
  •  0.1M AgNO3  (silver nitrate)    
  •  1.0M NaOH (sodium hydroxide)    

Prior to Lecture  (prep time ~ 1 min.)

1.     Prepare the following solutions:

  •  0.1M Fe(NO3)3 (ferric nitrate)   (MW = 404g/mol, 40.4g Fe(NO3)3.9H2O/L)
  • 0.1M KSCN (potassium thiocyanate)  (MW = 97.18g/mol, 9.7g KSCN/L)
  • 0.1M AgNO3 (silver nitrate)   (MW = 169.88g/mol, 17g AgNO3/L)
  • 1.0M NaOH (sodium hydroxide)    

       2. Mix 2.5mL each of 0.1M Fe(NO3)3  and 0.1M KSCN, and dilute to 250mL.  Place 50mL in each of five 150mL beakers.

To Conduct Demonstration:  

  1. Display five 150mL 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 and Disposal

Silver solutions will stain skin black on contact; gloves should be worn.  Materials may be disposed of with 1131 Qual lab.