Spectrophotometric Determination of an Equilibrium Constant

1. Determining the Beer's Law Curve for Fe(SCN)²⁺

1. Calibrate a visible spectrophotometer using distilled water as a blank.
2. Add 1.0 mL of 1.0 × 10^-4 M Fe(NO₃)₃ solution to a test tube.
3. To the same test tube, add 5.0 mL of 0.50 M KSCN solution.
4. To the same test tube, add 4.0 mL of 0.10 M HNO₃ solution. Cover the tube with a gloved finger and gently shake to mix.
5. Use a Pasteur pipette to transfer a small quantity of the solution to a cuvette. Ensure that the liquid level is above the path of the light beam in the spectrophotometer.
6. Place the cuvette in the spectrophotometer, so the light passes through the transparent sides.
7. Acquire a spectrum and record the λ_max value and the absorbance at λ_max.
8. In order to construct a Beer's law curve, additional solutions with known concentrations of Fe(SCN)²⁺ must be prepared and measured. Repeat steps 2 – 7 using the volumes of Fe(NO₃)₃, KSCN, and HNO₃ solutions in Table 2. Make sure to use the same cuvette for all measurements, rinsing 3 times with distilled water in between each sample.
9. Plot the measured absorbance versus the concentrations of Fe(SCN)²⁺ in each test tube and determine the line of best fit for the data. The slope of this line is the molar absorptivity and the path length is 1 cm.

2. Measuring K for the Iron (III) Thiocyanate System

1. Prepare 4 medium test tubes containing the indicated volumes of 0.0025 M Fe(NO₃)₃, 0.0025 M KSCN, and 0.10 M HNO₃ solutions in Table 3.
2. Cover each tube with a finger and gently shake to mix. Allow them to stand for at least 10 min. This resting period ensures that the solutions are at chemical equilibrium.
3. Use a Pasteur pipette to transfer a small quantity of solution 6 to a cuvette. Ensure that the liquid level is above the path of the light beam in the spectrophotometer.
4. Acquire a spectrum and record the absorbance at λ_max.
5. Multiple reactions with different initial concentrations of reactants can be studied to illustrate that K does not depend on concentration. To determine K for different initial conditions, repeat steps 3 and 4 for solutions 7 – 9.

Tube Number	Volume 1.0 x 10–4 M Fe(NO3)3 (mL)	Volume 0.50 M KSCN (mL)	Volume 0.10 M HNO3 (mL)
1	1.0	5.0	4.0
2	2.0	5.0	3.0
3	3.0	5.0	2.0
4	4.0	5.0	1.0
5	5.0	5.0	0.0

Table 2. Appropriate volumes of Fe(NO₃)₃, KSCN, and HNO₃ solutions to be placed in tubes 2 - 5.

Tube Number	Volume 0.0025 M Fe(NO3)3 (mL)	Volume 0.0025 M KSCN (mL)	Volume 0.10 M HNO3 (mL)
6	1.0	1.0	5.0
7	1.0	2.0	4.0
8	2.0	2.0	3.0
9	2.0	3.0	2.0

Table 3. Appropriate volumes of 0.0025 M Fe(NO₃)₃, 0.0025 M KSCN, and 0.10 M HNO₃ solutions.