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M-6
solution. If a graph of absorbance versus concentration for a series of standard solutions of the
absorbing species is established, the slope will be
b. From the absorbance of an unknown
solution, the concentration of the absorbing species can be determined. Generally, the
wavelength used to construct the calibration curve is the wavelength on the absorption spectra
where the absorption is at a maximum,
max
. At this wavelength, the absorbance is the most
sensitive to changes in the concentration of the absorbing species.
EXPERIMENTAL METHOD
In order to use the absorption of visible light as a quantitative measurement tool, the species
being analyzed must absorb light in the visible region. In this experiment, the concentration of
total iron in a water sample will be determined. To do this, all of the iron must be converted to
the same oxidation state and then reacted to form a substance that strongly absorbs in the visible
region of the electromagnetic spectrum.
Iron(III) is the most common oxidation state for naturally occurring iron on earth. In this
experiment, all iron will be reduced to iron(II) ions which can then be reacted with an organic
ligand to form an intensely colored complex ion. Hydroxylamine, NH2OH, will be the reducing
agent used to convert any iron(III) in the water sample to iron(II) according to the following
reaction:
4 Fe
3+
(aq)
+ 2 NH2OH
(aq)
4 Fe
2+
(aq)
+ N2O
(g)
+ 4 H
+
(aq)
+ H2O
(l)
The iron(II) ions will then be reacted with bipyridine (Bipy) to form the highly colored complex,
[Fe(Bipy)3]
2+
(aq)
:
Fe
2+
(aq)
+ 3 Bipy
(aq)
[Fe(Bipy)3]
2+
(aq)
pink-red
Bipyridine is a weak organic base; therefore, the position of the equilibrium for this reaction will
depend on the concentration of the hydronium ion (H3O
+
) in solution. The equilibrium will not
favor the formation of the [Fe(Bipy)3]
2+
complex in strongly acidic solutions, while in strongly
basic solutions, the undesirable precipitation of iron(II) hydroxide can occur. For these reasons,
