SPECTROPHOTOMETRIC DETERMINATION OF Fe IN DRINKING WATER
Purpose
To apply the principles of calorimetric analysis in determination of the iron content of an
unknown water sample.
The safety of drinking water is a very important public health issue. The precipitation of
excessive iron causes a reddish brown colour in the water. High concentration of iron often
promotes iron bacterial growth. This causes a slimy coating in pipes as well as “rotten egg”
odour by creating conditions for sulfide formation. At levels above 0.3 mg/L, iron stains laundry
and plumbing fixtures and causes an undesirable taste. Health Canada limits the amount of iron
to less than 0.3 ppm (0.3 mg/L) in municipal drinking water
In this experiment, we will be using an instrument known as a spectrophotometer to determine
the iron content of various water samples. You will analyze the iron content by reacting Fe2+
with 1,10-phenanthroline to form a bright orange-red complex ion according to the following
equation:
Fe2+ + 3 Phen→ [Fe(Phen)3]2+
This highly colored complex absorbs visible light, and thus can be subjected to
spectrophotometric analysis to determine its concentration
In most water samples iron exists in its oxidized form, (Fe3+), due to the presence of oxygen.
Since it is the Fe2+ ion that forms the complex with 1,10-phenanthroline, a reduction must first be
carried out to ensure all iron is in the form of Fe2+. This can be accomplished by the addition of a
reducing reagent such as hydroxylamine.
4 Fe3+ + 2 NH2OH·HCl
→ 4 Fe2+ + N2O + 4 H+ + H2O
Procedure:
The determination of the iron-phen complex is performed with a spectrophotometer at a fixed
wavelength of 508 nm using external calibration based on iron standard solutions.
(For RWL delivery, 3 standard solutions + 1 unknown solution are made by the lab assistant; 4
cuvettes are filled up with these solutions for students to obtain the % T or A at 508 n.m.)
This procedure is a guideline for the Lab Assistant; however, the students need to know the
appropriate concentrations and how the solutions were prepared in order to be able to do the final
calculations.
•
The iron calibration solutions are prepared by pipetting the indicated amounts of the iron
stock solution into labeled 50 mL volumetric flasks. The first flask is a blank containing no iron.
With a clean and dry tip, the following volumes of the iron stock solution were added to each of
the flasks. Flask 1: 0.00 mL, Flask 2:4.00 mL, Flask 3: 8.00 mL, Flask 4: 12.00 ml, Flask 5:
16.00 mL, and Flask 6: 20.00 mL; the iron stock solution has a concentration of 0.0125 g/L, and
the standards have concentrations of 0.00 mg, 0.05 mg, 0.10 mg, 0.15 mg, 0.20 mg and 0.25 mg
in 50 mL of solution, respectively
•
10.00 mL of an unknown sample solution was pipetted into a 250 mL volumetric flask and
diluted to the mark with distilled water. The flask was inverted and shook several times to
mix the solution.
•
25.00 mL aliquot of this solution was pipetted into a 50 mL volumetric flasks labeled
unknown.
•
Using a 10 mL graduated cylinder, the following solutions were added to each flask.
a. 4.0 mL of 10% hydroxylamine hydrochloride
b. 4.0 mL of 0.3% 1,10-phenanthroline
The flasks were filled 3/4 of the way full with purified water, swirled and reagents were allowed
to mix.
•
The volumetric flasks were diluted to the mark. Stoppered and inverted multiple times to
allow thorough mixing. The mixture was allowed to react for at least 15 minutes.
•
4 cuvettes are filled up with these solutions for students to obtain the % T or A at 508 n.m.
Now…
•
Using the Spectronic 301 spectrophotometer, carefully measure the percent transmittance of
the various solutions in the cuvettes, including the unknown solutions. Record your results in
the following table.
Solution
Blank
Flask 2
Flask 3
Flask 4
Flask 5
Flask 6
Unknown
% Transmittance
100 %
Absorbance (A=-logT)
0
Calculations and Discussion
1. Prepare a plot of absorbance versus concentration of the known solutions in mg Fe per 50 mL
of solution. Draw the best fitting straight line through.
2. Place the Absorbance value of the unknown solution onto this plot and determine the
concentration.
3. Calculate the amount of iron in the unknown sample. Express this as mg of Fe per litre of the
original unknown solution (mg/L Fe).