Spectrophotometric Determination of the Equilibrium Constant of a Reaction-Methodology

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The following stock solutions were prepared: 1.0 M HCl, 0.1 M HCl, 0.1 M KSCN, 0.002 M KSCN (in 0.1 M HCl), 0.2 M FeCl3 (in 0.1 M HCl), and 0.002 M FeCl3(in 0.1 M HCl).

Using the prepared stock solutions, the standard (Table 1) and Unknown (Table 2) solutions were prepared.

Table 2. Preparation of Standard Solutions
0.10 M KSCN0.002 M FeCl30.1 M HCl
Blank2.0 ml0.0 ml8 ml
S12.0 ml0.1 ml7.9 ml
S22.0 ml0.25 ml7.75 ml
S32.0 ml0.5 ml7.5 ml
S42.0 ml1.0 ml7 ml
S52.0 ml2.0 ml6 ml

The spectrophotometer(Figure 1) was adjusted to wavelength of 446 nm which is the maximum absorption of the complex.

Table 3. Preparation of Unknown Solutions
0.002 M KSCN0.002 M FeCl30.1 M HCl
Blank5.0 ml0.0 ml5.0 ml
U15.0 ml3.0 ml2.0 ml
U25.0 ml3.5 ml1.5 ml
U35.0 ml4.0 ml1.0 ml
U45.0 ml4.5 ml0.5 ml
U55.0 ml5.0 ml0.0 ml

Using the (standard) blank solution, the autozero was performed. With the cuvette, containing the solution in the sample holder, absorbances of Standard Solutions 1 to 5 were read. This process was repeated for Unknown Solutions 1 to 5, but this time, using the (Unknown) Blank Solution for autozero.
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