SCHC-266
Determination of Manganese in Steel
Addendum
1. Dry the sodium oxalate standard, Na2C2O4 at 100oC for 1 hour. Let cool in your
dessicator.
2. Dissolve the steel unknown in dilute HNO3
3. Standardize the prepared 0.10 N KMnO4 solution.
unbalanced
C2O42- + MnO4- 
CO2 + Mn2+ (HNO3 solution)
4. Calculate an approximate volume of KMnO4 required to reach the endpoint in each
titration. Molecular weight of sodium oxalate is 134.00. In this experiment an
indicator is not necessary, Why?
5. Beer’s Law Plot, Absorbance vs. concentration. Use two burets to prepare the various
solutions. Your notebook should contain a Table with the solution number, mL of
permanganate solution, mL of water, molarity, transmittance, and absorbance. The
wavelength of measurement should be included in the table heading. Using Excel,
prepare the Beer’s Law plot and calculate the molar absorptivity. Check your value
with the instructor before proceeding.
6. The data for the plot of absorbance versus wavelength can be done with a partner.
The Beers Law plot may be done with a lab partner but each student should analyze a
steel sample.
1
Spectrometric-Determination of Mn in Steel
Spectrometric methods are perhaps the most frequently used and important methods of
quantitative analysis. They are based on the absorption of visible light or other radiant
energy by a solution. The amount of radiant energy absorbed is proportional to the
concentration of absorbing material in solution according to Beer’s Law. By measuring
the absorption of light or other radiant energy, it is possible to determine quantitatively
the amount of absorbing substance present.
This experiment is a spectrophotometric determination of manganese in steel. The
spectrometer used in this experiment is the Spectronic 20 or equivalent.
Preparation of Standard Permanganate Solution
Pipette quantitatively 10.0 mL of the Potassium Permanganate solution, “Baker
Analyzed,” available in the laboratory into a clean 1000 ml volumetric flask. Dilute to
the mark with distilled water. Mix well. Use this for the standard permanganate solution
mentioned throughout this procedure.
A. Absorption Spectrum of a Permanganate Solution
A visible spectrum of MnO4- will be prepared by you. From the plot of absorbance vs.
wavelength (nm) determine the max.
B. Calibration Curve
A plot of absorbance versus concentration (Beer’s Law plots) of various permanganate
solutions will be made. Prepare the following solutions by mixing the indicated reagents
in the following quantities using two burets or pipettes (measure volume to 0.02 ml):
Soln #
1
2
3
4
5
6
7
8
9
MnO4-
20
20
20
20
20
10
10
10
5
H2O
5
10
15
20
30
20
30
40
45
Volumes are in mL. Record and use exact figures measured.
Measure the absorbance of each of these solutions using the Spectronic 20 at the
wavelength corresponding to the highest absorbance on your graph from the first part of
this experiment (it should be about 525 nm). Re-measure the absorbance of the standard
MnO4- solution at this maximum wavelength.
Plot the absorbance of these solutions versus the permanganate concentration (Molarity)
of each solution using Excel. Be sure to correct for any dilutions. Calculate , the molar
absorptivity of MnO4-, using the Beer-Lambert Law, A = bc, where b (the cell length in
centimeters) is equal to 1.00 cm for the Spectronic 20 cells. See your textbook for an
explanation of the Beer-Lambert Law.
2
C.
Determination of Mn in Steel
The determination of manganese in steel is accomplished by dissolving the steel and
oxidizing the resulting manganese (II) to manganese (VII), (MnO4-), which is then
determined by spectrometry.
The steel is first dissolved in dilute nitric acid that results in iron (II) and manganese (II).
(Eq. 1) During the dissolution, NO2 and NO are also produced. These may interfere later
in the experiment by reducing the periodic acid: hence, they are partially removed by
boiling. Ammonium persulfate (ammonium peroxydisulfate) is used as an auxiliary
oxidant, to oxidize the remaining oxides of nitrogen (Eq.2) and any organic material
present. Unreacted peroxydisulfate is then decomposed by reduction with boiling water
(Eq. 3).
3Mn(s) + 2NO3-(aq) + 8H+(aq)  3Mn2+(aq) + 2NO(g) + 4H2O(l)
(1)
2NO2(g) + S2O82- (aq) + 2H2O(l)  2NO3-(aq) + 2SO42-(aq) +4H+(aq)
(2)
2S2O82-(aq) + 2H2O(l)  4SO42-(aq) + O2(g) + 4H+(aq)
(3)
Theoretically, peroxydisulfate has the oxidizing potential to oxidize manganese (II) to
manganese (VII), but the reaction is kinetically too slow to be useful. Potassium
periodate oxidizes manganese (II) to manganese (VII) more rapidly and reproducibly and
is preferred for the oxidation of (Eq. 4). The oxidation is carried out at the boiling point
to insure a rapid reaction and to increase the solubility of the sparingly soluble potassium
periodate. Since periodate also decomposes slightly at these temperatures, it is a good
technique to add it in two portions, to maintain an excess.
2Mn2+(aq) + 5IO4-(aq) + 3H2O(l)  2MnO4-(aq) + 6H+(aq) + 5IO3-(aq)
D.
(4)
Spectrometric Determination
1. Turn on the Spectronic 20 and allow it to warm up while the procedures below are
carried out.
2. Weigh quantitatively three samples of the unknown steel from 0.50 to 0.75 g to
the nearest 0.1 mg. Do not dry the steel.
3. Place the samples in separate 400 ml beakers, add 50 to 60 ml of dilute (1:3) nitric
acid and heat in a hood to dissolve. Finally, cover and boil gently for two (2)
minutes to remove nitric oxide. A black residue of carbon may remain in the
solution at this point.
4. Remove the beakers from the heat and carefully sprinkle 1gram of
ammonium persulfate into each beaker. Boil gently for 10 to 15 minutes to
oxidize carbon. The excess ammonium persulfate will be destroyed by the
heat.
3
5. Dilute each solution to about 100 ml, add 15 ml of 85% phosphoric acid and 0.5 g
of potassium periodate. Boil gently for about three (3) minutes to effect
oxidation to permanganate. Remove the heat and allow the sample to cool for
about two minutes, then add an additional 0.2 g portion of potassium periodate.
Boil for another minute or two.
6. Transfer each unknown solution to a 250 ml volumetric flask, and when cool
dilute to volume.
7. Fill one cell with distilled water and a second with a solution of permanganate
from one steel sample. Determine the absorbance at  max. of the steel sample
using the Spectronic 20. Repeat this with a second aliquot of the same steel
sample.
8. Repeat step 7 using the second and third steel samples.
9. Determine the concentration of Mn in your solution using your Excel calibration.
Report this concentration. Calculate the percent Mn in your original sample and
report your result as the average of the three determinations. Report you
confidence interval from your calibration.
4