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M-12
Part IV:
Absorbance of Unknown Water Solution
1.
The water sample containing the unknown iron concentration is available for common use in
a buret on the side bench. Using the same procedure as in Part I, dispense approximately
10.xx mL of solution, accurately measured from the buret, into a clean
100 mL volumetric flask. Record on your Observations Sheet, the buret readings for the
volume of unknown taken.
2.
In the order given, add the following reagents to the water sample in the volumetric flask,
mixing the solution after each addition::
2.0 mL of 0.25 M H2SO
4
1.0 mL of 10 % hydroxylamine solution
5.0 mL of 0.20 % bipyridine solution
5.0 mL of 1.0 M sodium acetate solution
Allow 10 minutes for the reduction of iron(III) to occur and then fill the volumetric flask to
100.0 mL mark with distilled water.
3.
Using the correct procedure, fill a clean cuvette with the above [Fe(Bipy)3]
2+
complex
solution of the unknown water sample. Measure the absorbance of this solution. If the
absorbance of this solution is within the range of the calibration curve established by the
standard solutions in Part II, then NO dilution of the unknown is required.
If the absorbance of this solution is HIGHER than the range of the calibration curve, then
make appropriate dilutions of this solution by taking a smaller volume of the unknown water
sample to make the [Fe(Bipy)3]
2+
complex solution. Record the absorbance of the final
diluted solution.
If the absorbance is LESS than the lower limit of the calibration curve, then prepare a more
concentrated solution of the unknown by taking a larger volume of the unknown water
sample to make the [Fe(Bipy)3]
2+
complex solution. Record the absorbance of the final
solution.
Note: Record all dilutions or volume changes made on your Observations Sheet.
4.
Recalibrate the spectrophotmeter
max
and then check the absorbances of each standard
solution and the unknown solution at
max
to be sure the values are consistent.
Before leaving the laboratory, check to make sure that all of your equipment is clean, and in your
drawer. As well, all communal equipment must be clean and returned to the designated location in
the laboratory.
