Colourimetry is an analytical technique widely used in chemistry to measure the concentration of a coloured substance in a solution. One application of this method is to determine the equilibrium constant of a reaction. In this article, we will focus on the use of colorimetry to calculate the equilibrium constant for the formation of iron thiocyanate, a common topic in HSC Chemistry courses.
The reaction of interest is the formation of iron thiocyanate complex, which is a reversible reaction:
The equilibrium constant, Keq, for this reaction can be determined by measuring the concentration of each species at equilibrium.
Colorimetry relies on the principle that the absorbance of light by a coloured solution is proportional to the concentration of the coloured substance. In our case, the coloured substance is the iron thiocyanate complex (`FeSCN^`), which has a deep red colour.
The relationship between absorbance and concentration is described by the Beer-Lambert Law:
Where A is the absorbance, ε is the molar absorptivity, c is the concentration, and l is the path length (usually 1 cm).
To use colorimetry for determining the concentration of `FeSCN^`, follow these steps:
1. Prepare a series of standard solutions: Create a range of `FeSCN^` solutions with known concentrations by mixing known volumes of `Fe^` and `SCN^-` solutions. Make sure to use the same total volume for each solution to maintain consistency.
2. Measure the absorbance: Use a colorimeter or spectrophotometer to measure the absorbance of each standard solution at a specific wavelength (typically around 447 nm for `FeSCN^`). Make sure to set the instrument to zero absorbance using a blank solution (i.e., only water without `FeSCN^`).
3. Create a calibration curve: Plot the absorbance values against the known concentrations of `FeSCN^` in your standard solutions. This curve will allow you to determine the concentration of `FeSCN^` in an unknown solution based on its absorbance.
Figure 1: Calibration curve showing the absorbance value of various standard solutions of iron(III) thiocyanate.
4. Measure the absorbance of the equilibrium mixture: Prepare an equilibrium mixture by mixing known volumes of `Fe^` and `SCN^` solutions and allowing the reaction to reach equilibrium. Measure the absorbance of this mixture using the same wavelength as before.
5. Determine the concentration of `FeSCN^`: Use the calibration curve to find the concentration of `FeSCN^` in the equilibrium mixture based on its absorbance.
Table 1: equilibrium concentrations determined from measuring the absorbance of each iron(III) thiocyanate equilibrium mixture.
Now that you have determined the concentration of `FeSCN^` at equilibrium, you can calculate the equilibrium constant, `K_`. To do this, you also need to determine the concentrations of `Fe^` and `SCN^-` at equilibrium.
Since you know the initial concentrations of `Fe^` and `SCN^-` and the amount of `FeSCN^` formed, you can calculate the concentrations of the reactants at equilibrium:
With these concentrations, you can now calculate the equilibrium constant:
Table 2: equilibrium constant is calculated using equilibrium concentrations.
