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Colorimetric Determination of Manganese in Steel

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Colorimetric Determination of Manganese in Steel

Aim:- The aim of this experiment was to find out the percentage of
manganese

present in a steel paper clip.

Introduction:-

Colorimetry is an analytical technique that is used to determine the
concentrations of coloured substances in solution. It relies on the
fact that a coloured substance absorbs light of a complimentary colour
to its own and that the amount of light absorbs is proportional to its
concentration. Colorimetry is particularly suited to the determination
of manganese in steel because the manganese can be converted to the
permanganate (VII) ion which is strongly coloured.

The oxidation is achieved in two stages:

1st stage oxidation

[IMAGE] Mn(s) Mn2+(aq)

This oxidation is brought about by the nitric acid which itself is
reduced to nitrogen monoxide, which forms nitrogen dioxide (brown
fumes) on contact with the air.

2nd Oxidation

[IMAGE]2 Mn2+(aq) + 5IO-4 + 3H2O 2MnO-4(aq) + 6H+ + 5IO3-(aq)

The oxidation is bought about by Potassium Periodate.

Procedure:-

Part-A Calibration Graph

Standard solutions of acidified potassium permanganate solution were
made up by diluting (2, 4, 6, 8, 10, 12, 14cm3) stock standard
solution (of 9.78 x 10-4 moll-1) to the mark in a (50cm3) standard
flask. Using a spectrophotometer the absorbance of each solution was
measured in the colorimeter using deionised water as a standard in
optically matched cuvettes. A calibration graph of absorbance against
concentration of the potassium permanganate solution was plotted.

Part-B Oxidation of Manganese to Permanganate

The steel paper clip was degreased with propanone, dried, cut into
small pieces and accurately weighed (approx 0.2g). The steel was then
placed in a separate (250ml) tall form glass beaker. Nitric acid
(approx 40ml of 2M) was added to the beaker and heated gently to
dissolve the steel. The mixture was heated cautiously, in a fume
cupboard, until the reaction began. Once the steel was dissolved
(after about 10 minutes), the solution was cooled a little. A glass
rod was placed in the beaker and then the solution was boiled until no
more brown fumes were given off (after about 10 minutes). The solution
was cooled considerably; the glass on the top was carefully removed
and the underside was washed into the beaker. Phosphoric acid (approx
5ml of 85%) was added while stirring and then potassium persulphate
(approximately 0.2g) was added. To this solution (20ml of 2M) nitric
acid containing potassium periodate (approx 1g) was added and the
mixture was gently brought to boiling. The solution started to turn
pink. Once the intensity of pink colour remained constant the heating
was stopped. The solution was allowed to cool overnight at room
temperature. The solution was then transferred to a standard flask
(100ml) and distilled water added up to the mark. The flask was
inverted several times to mix the contents well. The absorbance was
measured in a spectrophotometer at a wavelength of 525 nm. The
calibration curve was used to convert the absorbance to a permanganate
concentration and from that; the percentage of manganese in the steel
paper clip could be calculated.

Hazcon:-

* Wear eye protection and if any chemical splashes on your skin wash
it off immediately.

* The acidified potassium permanganate is harmful if ingested and
irritates the eyes and skin.

* Both 2M nitric acid and its vapour are corrosive and toxic causing
severe burns to the eyes, digestive and respiratory systems and in
contact with the skin.

* 85% phosphoric acid is corrosive, it burns

Results:-

1. Table

Volume of stock solution in 50ml

Absorbance

2

0.074

4

0.155

6

0.245

8

0.330

10

0.405

12

0.490

14

0.561

2. Calibration Curve


[IMAGE]

Mass of Clip1:- 0.280g

Absorbance of solution from Clip1:- 0.35 = 9ml of stock solution per
50ml

Mass of Clip2:- 0.265g

Absorbance of solution from Clip2:- 0.26 = 6.2ml of stock solution per
50ml

Method-1

Absorbance = 0.35 = 9ml of stock solution per 50ml

Concentration of MnO4- = (9.78 x 10-4) x 9/50

= 1.76 x 10-4 mol/l

Moles of MnO4- in sample of 100ml = 1.76 x 10-4 x 0.1

= 1.76 x 10-5 mol

Moles of MnO4- = Moles of Mn

1.76 x10-5 1.76 x 10-5

Mass of Mn = moles x gFm

= (91.76 x 10-5) x 54.9

= 9.66 x 10-4 g

% of Mn = 9.66 x 10-4 /0.280 x 100

= 0.345%

Method-2

Absorbance = 0.26 = 6.2ml of stock solution per 50ml

Concentration of MnO4- = (9.78 x 10-4) x 6.2/50

= 1.21 x 10-4 mol/l

Moles of MnO4- in sample of 100ml = 1.21 x 10-4 x 0.1

= 1.21 x 10-5 mol

Moles of MnO4- = Moles of Mn

1.21 x 10-5 1.21 x 10-5

Mass of Mn = moles x gFm

= 1.21 x 10-5 x 54.9

= 6.64 x 10-4 g

% of Mn = 6.64 x 10-4 /0.280 x 100

= 0.237%

Conclusion:-

The percentage of Mn in steel is 0.345% and 0.237 %.

Discussion:-

The wavelength was set at 525nm 1nm and it only allowed light of
that wavelength. Optically matched cuvettes were used so that any
light distortion is cancelled out. A calibration curve was prepared to
compare absorbance of a solution of unknown concentration to that of a
solution of known concentration the unknown concentration to be
determined.

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"Colorimetric Determination of Manganese in Steel." 123HelpMe.com. 15 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=148733>.




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