Colorimetric Determination of Manganese in Steel

Colorimetric Determination of Manganese in Steel
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 colouredThe 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 × 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 × 10-4) x 9/50 = 1.76 × 10-4 mol/l Moles of MnO4- in sample of 100ml = 1.76 × 10-4 × 0.1 = 1.76 × 10-5 mol Moles of MnO4- = Moles of Mn 1.76 x10-5 1.76 × 10-5 Mass of Mn = moles x gFm = (91.76 × 10-5) x 54.9 = 9.66 × 10-4 g % of Mn = 9.66 × 10-4 /0.280 × 100 = 0.345% Method-2 Absorbance = 0.26 = 6.2ml of stock solution per 50ml Concentration of MnO4- = (9.78 × 10-4) x 6.2/50 = 1.21 × 10-4 mol/l Moles of MnO4- in sample of 100ml = 1.21 × 10-4 × 0.1 = 1.21 × 10-5 mol Moles of MnO4- = Moles of Mn 1.21 × 10-5 1.21 × 10-5 Mass of Mn = moles x gFm = 1.21 × 10-5 × 54.9 = 6.64 × 10-4 g % of Mn = 6.64 × 10-4 /0.280 × 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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