Experiment 19 - Determination of the equilibrium constant for the reaction Fe3+ (aq) + SCN (aq) = FeSCN2+ (aq)
Object: To determine the equilibrium constant by a colorimetric method
Theory:
Colorimetric methods of analysis are usually applied to the determination of small concentrations of either inorganic or organic
materials in a solution. The constituent sought must be coloured or must be capable of reacting with a reagent to produce a
substance having a suitable colour. Beers Law, which relates the degree of absorption of monochromatic light to the concentration
of an absorbing substance in a solution; is combined with Lambert's and Bouguer's relationships of absorption to length of the light
path, to form the following exponential expression.
I  I 0 e  kCT
where I = intensity of light transmitted by the absorbing layer
I 0 = intensity of light incident on the absorbing layer
C = concentration of substance in absorbing layer
T = thickness of absorbing layer
k = constant of proportionality
This combined expression is frequently called "Beers Law" and may be re-written in the form
log(
The log ratio of
I0
)  kCT
I
I0
is known as absorbance and has no unit. This term is also called optical density and is the quantity that the
I
instrument measures and displays on the meter. As the thickness of the absorbing layer is maintained constant by examining the
solution in standard test tubes, the value of absorbance A is proportional to the concentration of the solution.
I0
)
I
A  kCT (1)
A  log(
If Beers Law holds for a range of concentrations of a solution, as A is directly proportional to the concentration.
AC
Beer’s Law
Hence
A1 C1

A2 C2
(2)
where C1 = concentration of unknown
C2 = concentration of standard
A1 = absorbance of unknown
A2 = absorbance of standard
It is usually to construct a calibration curve of absorbance against
concentration. If Beers Law holds for the solutions investigated the
curve will be a straight line passing through the origin.
The reaction under consideration here is the formation of complex
ions:
Fe3+ (aq) + SCN (aq) ====== FeSCN2+ (aq)
For colour determination, one standard solution of FeSCN2+ can be
prepared by adding a small concentration of SCN (aq) to a large
excess of Fe3+ (aq) so that essentially all the SCN (aq) is converted
to FeSCN2+ (aq). Under this conditions, the final concentration of FeSCN2+ (aq) in the standard solution is equal to the
concentration of SCN (aq) solutions used.
The equilibrium concentration of FeSCN2+ (aq) in each mixture is determined by comparison with the above standard solution.
The equilibrium concentrations of Fe3+ (aq) and SCN (aq) are obtained by subtracting [FeSCN2+] formed from the initial [Fe3+]
and [SCN].
At equilibrium:
K=
[ FeSCN 2 ]
[ Fe3 ][SCN  ]
Chemicals:
0.2 M iron (III) nitrate, 0.002 M potassium thiocyanate
Apparatus:
colorimeter, burette, test-tubes
Procedure:
1. Line up five clean test tubes, all of the same size, and label them A, B, C, D, E, F and G.
2. Run the liquids, according to the following given table into the conical flask. Shake well and then allow the flask to stand at room
temperature for a few minutes, to allow the mixture to reach equilibrium. The total volume for each experiment is 10 cm 3.
Experiment
Fe3+ / cm3
water/ cm3
SCN / cm3
A
5
0
5
B
5
5
0
C
0.5
5
4.5
D
0.4
5
4.6
E
0.3
5
4.7
F
0.2
5
4.8
G
0.1
5
4.9
3. Insert a test tube A in the colorimeter, and close the light cover over the tube.
4. Turn on the instrument and adjust the rotary sensitivity control to bring the meter to full scale deflection (i.e. 100%
Transmission). It is advisable to check this setting frequently as drift may occur. If the control requires serious alteration,
previous measurements must be repeated.
5. Turn off the instrument, remove the tube A and replace with the tube B. Re-close the cover.
6. Turn on the instrument again and note the new reading in absorbance on the meter scale.
7. Repeat step 5 - 6 for the tube C, D, E, F and G.
8. Record the room temperature.
Result:
Room temperature = _________ C
Absorbance of standard (Tube B) = _______________
[FeSCN2+] of standard (Tube B) = _______________
Absorbance
[FeSCN2+]eq
[Fe3+]eq
[SCN]eq
Tube C
Tube D
Tube E
Tube F
Tube G
Calculation:
1. Consider the tube B as standard. Calculate [FeSCN2+] of the standard.
2. Calculate the equilibrium concentration of FeSCN2+ (aq) for tube C to F by using the equation (2).
3. Calculate the equilibrium concentration of Fe3+ (aq) ion, by subtracting the equilibrium concentration of FeSCN2+ (aq) ion from
the initial concentration of the Fe3+ (aq) ion.
4. Calculate the equilibrium concentration of SCN (aq) ion in the same manner as for the Fe3+ (aq) ion. Subtract the equilibrium
concentration of the FeSCN2+ (aq) ion from the initial concentration of the SCN (aq) ion.
5. Find the equilibrium constant for each test-tube.
Question:
1. What is colorimetry? What is its limitation in application?
2. Suggest a method to prepare the reaction mixture of tube C, D, E, F and G.
3. State the approximation in this experiment.
4. Plot a graph of [FeSCN2+] against [Fe3+][ SCN] and find the equilibrium constant from the graph.
5. Discuss the deviation of Beers Law.