Experiment 6: Determination Of Manganese In Steel
By Absorption Spectrophotometry
Manganese is a minor constituent found in many steels. In this experiment you will be
analyzing a steel sample to get the weight percentage of manganese. You will first dissolve the
sample in an acidic solution and then oxidize the colorless Mn2+ to form the purple
permanganate ion (MnO4-). Once this has been done, it is possible to measure the absorbance
of this solution and use Beer=s Law (A = εbc) to determine the concentration of the
permanganate ion. This can easily be related to weight percentage of the steel sample. You
will be calibrating your experiment against a standard steel sample with a known percentage of
manganese.
You will be using ammonium peroxydisulfate and potassium periodate to oxidize the Mn2+. It is
important that you add the reagents in the correct order outlined below. Deviation will ruin the
experiment. The reaction equations are as follows:
1.
2 Mn2 + + 5 S2O82 G + 8 H2O 6 2 MnO4G + 10 SO42 G + 16 H +
2.
2 Mn2 + + 5 IO4G + 3 H2O 6 2 MnO4G + 5 IO3G + 6 H +
CAUTION: You will be using concentrated acids to dissolve your sample. These are very
corrosive to your skin and your clothing. Be extremely careful in handling these substances.
Report any spills to the TA and rinse any exposed skin under water immediately.
PROCEDURE
You will treat your unknown sample and your standard in the exact same way throughout the
experiment. After the oxidation process, your standard and unknowns will be diluted to 250 mL.
It is important to keep these volumes in mind; if your solution exceeds these volumes before the
dilution step, major problems will result.
1.
Unknown Sample
Receive an unknown steel sample and record the number in your lab book.
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You should provide a dry, labeled weighing bottle to the TA for your unknown one
week prior to this experiment.
2. Dissolving the Steel Samples
You will need 4 beakers, either 400 mL or 600 mL. Weigh out accurately two samples of your
unknown and two samples of the standard into separate, labeled beakers. It is important that
these weights fall between 0.45 g and 0.55 g. Samples outside this range will give absorbance
readings outside the scale of the spectrophotometer.
In the hood, add 35 mL of distilled water and 15 mL of concentrated nitric acid to each
beaker. Cover each beaker with a watch glass and heat on a hot plate. Boil each sample
gently until brown fumes stop being evolved. This will take two minutes or longer.
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3. Oxidizing the Mn2+
In the hood, carefully add about 1 gram of ammonium peroxydisulfate to each beaker. Boil
gently for 10-15 minutes to oxidize any carbon that may be in the steel and destroy any excess
ammonium peroxydisulfate. At this point, small bubbles of released oxygen will be replaced by
larger bubbles of water vapor being released.
Next, dilute each solution to about 70 mL with deionized water. (Note: DO NOT just add 70 mL
of water to your solutions.) Add 10 mL of concentrated phosphoric acid, followed by about 1
gram of potassium periodate. Boil gently for 3 minutes and keep hot for 10 minutes. At this
point your solution should have the purple permanganate color.
4. Dilution
Allow the solutions to cool to room temperature. Quantitatively transfer the unknown solutions
to separate 250-mL volumetric flasks. Be sure and rinse your beaker several times and add the
rinse water to the flask. Dilute to mark. Use the same procedure to transfer your standard
solutions to two 250-mL flasks.
5. Spectrophotometry
The teaching assistant will demonstrate the proper use of the Spec 20. You will measure the
absorbance of all four of your samples using water as a reference. The wavelength of
maximum absorption for permanganate is 525 nm. Be sure your instrument is set to this
wavelength. For each solution record both the absorbance value and the transmittance value.
Beer=s Law:
A = εbc
A = absorbance
ε = molar absorptivity
b = cell path length
c = concentration
The calculations will have been discussed in the pre-lab session. Be sure to report your results
as a weight percentage of Mn in your unknown.
RESULTS CARD
name
date
unknown number
individual ε values and ave. ε value for standard solutions
% Mn in steel (individual & ave. values)
both important reactions for this experiment
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