KEY to PRELAB 359: Spectrophotometric Analysis of Permanganate Solutions
1. The substances are potassium permanganate and sulfuric acid.
Potassium permanganate is a strong oxidizing agent and a skin irritant.
Sulfuric acid is a corrosive, toxic substance that can easily cause skin burns.
2. A Beer’s Law plot is linear only for a range of concentrations. If the solution is too dilute, too much light passes through the solution; if the solution is too concentrated, not enough light passes through the solution. When this happens, the detector’s response is no longer linear, which means that the change in the absorbance is no longer the same for the same change in the concentration of the solution. This is the case for highly concentrated solutions of potassium permanganate, such as 5.0 M KMnO4.
3. The amount of complex formed is dependant on the amount of iron (II) present in the solution. The other substances that are added to the solutions are added in the same quantities so that the solution matrix is the same. When the solution matrix stays the same, the change in absorbance is due to the change in the concentration of the complex.
Stock Solution, mL
1.00
2.00
5.00
7.00
9.00
10.00
a.

Conc. [Fe(ph)3]2+, M
8.60 x 10-6
1.72 x 10-5
4.30 x 10-5
6.02 x 10-5
7.74 x 10-5
8.60 x 10-5

Transmittance, % T
80.35
64.71
33.65
21.78
14.09
11.32

Absorbance
0.095
0.189
0.473
0.662
0.851
0.946

1.00 mL = 1.00 x 10-3 L stock solution
(1.00 x 10-3 L)(4.300 x 10-4 mol/L) = 4.30 x 10-7 mol
4.30 x 10-7 mol