Estimation of Iron (II) and Iron (III) concentration

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The Estimation of Iron(II) and Iron(III) in a Mixture Containing Both

Introduction – theory behind two methods and why I am choosing one of them

There are several possible methods that could be employed to determine the concentration of Iron(II) and Iron(III) ions in solution: one of these is colorimetry. Colorimetry is the technique of using the depth of colour of a substance to measure its concentration. We use a colorimeter (see fig. 1 and 2) to measure the depth of colour. The machine is calibrated by checking a series of different known concentrations of solution; from the readings we construct a graph of absorbance (the percentage of light absorbed by the sample) against concentration - this is known as a calibration curve. We can then read off the graph the concentration value that goes with the absorbance value for our unknown solution.

Figure 1: A colorimetry set up

Figure 2: A colorimeter

The machine may be set by the wavelength of the light involved. Visible light wavelengths vary from around 400 nm (blue) to 700 nm (red).

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Colorimetry is relevant to our investigation as Fe(II) will go from pale green (almost colourless) to form a violet complex ion with the bidentate ligand ferrozine (see fig.3). The formula of this complex ion is [Fe(ferrozine)3]2+. A solution containing ferrozine would appear coloured to us because white light enters and violet light is not absorbed (otherwise there would be no violet to see). It is the other colours that are absorbed. So if we were running some samples of ferrozine in a colorimeter we might put in a green filter (as green is violet’s complementary colour) and measure how much ...

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