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M-7
it is critical to closely control the pH of the reaction solutions. This will be done using sodium
acetate which reacts with the hydronium ions present in the iron(II) solution to create an acetic
acid-sodium acetate buffer of approximately pH 4.5 to 5.0.
Several standard [Fe(Bipy)3]
2+
complex solutions will be prepared. The
max
for the
[Fe(Bipy)3]
2+
complex will be determined and then the absorbance for each standard solutions
will be measured. This information will be used to construct a calibration curve that will
correlate absorbance to concentration.
NOTE: The stock iron solutions will be prepared using ferrous ammoniun sulfate hexahydrate,
FeSO
4
(NH
4
)2SO
4
6 H2O. Hydroxylamine will be added to these solutions to ensure
that all iron exists as Fe
2+
The final step is to find the absorbance of the [Fe(Bipy)3]
2+
complex in an unknown water
sample that has been prepared in a manner similar to that used to make the standard solutions.
By placing this absorbance on the calibration curve, the concentration of the [Fe(Bipy)3]
2+
complex can be determined and from this, the concentration of total iron in the water sample.
NOTE: The concentration of the [Fe(Bipy)3]
2+
complex in the solution prepared from the
unknown water sample may need to be adjusted so that the absorbance of the solution
will fall within the range of the calibration curve.
REAGENTS & EQUIPMENT
solution of 0.20% bipyridine and 10% hydroxylamine hydrochloride (NH2OH
HCI)
2.5x x 10
-4
M stock iron(II) solution (prepared using FeSO
4
(NH
4
)2SO
4
6 H2O)
0.25 M sulfuric acid (H2SO
4
)
1.0 M sodium acetate (CH3COONa)
100 mL volumetric flask (2)
buret (1)
Spectronic 20 spectrophotometer
a set of matched cuvettes (3)
