Chapter 9: Transition Metals
DEMONSTRATION 9.13
REVERSIBLE FORMATION OF A COBALT(II) ISOTHIOCYANATE COMPLEX
Addition of a concentrated solution of colourless ammonium thiocyanate, to a
cobalt(II) solution [Co(H2O)6]2+ (pink), results in a blue color due to formation of
a complex ion [Co(NCS)4]2- (blue). Water competes with the SCN- ion for the
coordination to cobalt(II). Addition of acetone reduces the concentration of
water and favours the coordination of SCN-, resulting in a blue [Co(NCS)4]2
solution. Addition of more water returns the solution to a pink colour as more
[Co(H2O)6]2+ is formed.
EQUIPMENT





REAGENTS




PREPARATION



PROCEDURE
RESULTS
Measuring cylinders, glass, 1 x 100 mL and 1 x 250 mL
Conical flask 1 x 2 L
Conical flask 1 x 100 mL
Stirring rod
Light box
Cobalt(II) chloride hexahydrate, CoCl2.6H2O (4 g)
Ammonium thiocyanate, NH4SCN, (20 g)
Distilled water (1000 mL)
Acetone (300 mL)
Dissolve cobalt(II) chloride hexahydrate (4 g) in water (80 mL).
Dissolve ammonium thiocyanate (20 g) in water (80 mL) and provide in a
100 mL conical flask.
Provide the acetone and distilled water in reagent bottles. Also provide
the two glass measuring cylinders, stirring rod and light box.

Add the conc. NH4SCN to the pink CoCl2 solution to give blue colour.

Add 240 mL aliquot of water. Results in pink solution.

Add 80 mL aliquot of acetone. Results in blue solution.

Alternate between addition of water and acetone.
The colour of the cobalt(II) solution alternates between pink and blue as the
concentration of water is increased and reduced against the concentration of
acetone:
[Co(H2O)6]2+ + 4 SCN- → [Co(NCS)4]2- + 6 H2O
DISPOSAL
Dispose of used solutions in the heavy metal waste drum.