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 and irritates the skin and eyes. A systematic irritant if inhaled and if swallowed, causes serious internal injuries.
- Acidified potassium periodate solution is harmful if swallowed and is an irritant to the eyes, skin and respiratory system. It is also corrosive.
- Potassium Persulphate is harmful if swallowed or inhaled as a dust. It irritates the eyes, skin and respiratory system causing dermatitis and possible allergic reactions.
- Propanone is volatile and highly flammable and is harmful if swallowed the vapour irritates the eyes, skin and lungs and is narcotic in high concentrations.
Results:-
- Table
- Mass of paper clips:-
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. When I compared my answers with the other group’s results, the answers were around 0.345% and therefore I think my answer is reasonably valid.