Experiment 22: Colorimetric
determination
of an equilibrium constant
PURPOSE
To determine the value of the equilibrium constant
for the equilibrium system involving Fe3+(aq), SCN–
(aq) and FeSCN2+(aq) using colorimetric analysis.
THEORY
• In aqueous solution, Fe3+ ions react with SCN– ions to form the blood red
coloured FeSCN2+ ion:
•
• Fe3+(aq) (pale yellow) + SCN–(aq) (colourless)  FeSCN2+(aq) (red)
•
• The colour of the solution is directly proportional to the concentration of
FeSCN2+ ions present. The concentration of these ions can be determined
by measuring the absorbance of the solution and comparing it with the
absorbance of a solution of known concentration. If the initial
concentrations of Fe3+ and SCN– are known, a value for the equilibrium
constant, K, for the reaction can be calculated.
• Colorimeters measure the amount of light that is transmitted or absorbed
by a solution. A description of how they work can be found in your text
book
SAFETY PROCEDURES
• Follow all instructions for using the equipment
in this activity.
• 2. Wear safety glasses and a laboratory coat
for this experiment.
• 3. Potassium thiocyanate is irritating to the
skin and eyes. Avoid contact.
• 4. The iron(III) nitrate solution used in this
experiment contains nitric acid. The solution is
irritating to skin and body tissues.
Part 1. Preparation of Equilibrium
Solutions
• Label six 50.00 ml flasks 1-6
• Pipet 10.00 ml of .2M Fe(NO3)3 into each flask
• Then pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml
of .002M (NaSCN)- into flasks 2 – 6
respectively
• Add .1M nitric acid to each flask to make a
total of 50.00 ml per flask and stopper each
flask
Part 1. Preparation of Equilibrium
Solutions
• Calculate the final Fe(NO3)3 concentration and
report them in part A
• Calculate the final Fe(NCS)+2 concentration in
each flask
NaSCN ml soln
1
2
Initial [SCN-] M
0
4.0x 10-5M
Equil FeNCS+2M 0
4.0x 10-5M
Percent T
100
78.5
Absorbacnce
0
0.105
3
4
5
6
Part 1. Preparation of Equilibrium
Solutions
• Obtain 2 cuvettes, rinse one cuvette with soln 1,
and discard in waste jar
• Fill the rinsed cuvette with soln 1, insert cuvette
into spectrophotometer and adjust 10 0
absorbance and 100 transmittance.
• Rinse the second cuvette with soln 2 and discard
in waste jar and measure the absorbance and
transmittance of soln 2 at 447nm
• Repeat procedures for soln 3 -6
calibration curve
Absorbance
• Prepare calibration curve by plotting
absorbance vs concentration
y = 0.05x
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Y-Values
Linear (Y-Values)
0
5
10
FeNCS+2 1x10-5M
Part B: Determination of the
calibration curve
• Label six clean dry test tubes 1- 6
• Pipet 5.00 ml of .002M Fe(NO3)3 into each
test tube
• Then pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml
of .002M (NaSCN)- into flasks 2 – 6
respectively
• Add .1M nitric acid to each flask to make a
total of 10.00 ml per flask and stopper each
flask
Part B: Determination of the
calibration curve
• Measure and record the absorbance and
transmittance of these solns at 447nm
• From your calibration curve determine the
equilibrium concentration of Fe(NCS)+2
Part B: Determination of the
calibration curve
NaSCN ml soln
1
2
3
Volume of (FeNO3)3 ml
5.00 ml
5.00 m
5.00 m
Volume of NaSCN ml
0
1.00
2.00
Initial Fe +3 M
1.0 x 10-3M
1.0 x 10-3M 1.0 x
10-3M
Initial SCN -1M
0
2.0 x 10-4M
Absorbance
0
.164
4
5
6
Part B: Determination of the
calibration curve
NaSCN ml soln
1
2
Equil [FeNCS+2] M
0
3.28 x 10-5
Equil [Fe+3] M
1.0 x 10-3M
9.67 x 10-4
Equil [SCN-] M
0
1.6 x 10-4M
Keq
-
211
3
4
5
6