2nd Oxidation
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 stock standard solution( 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 approximately 0.2g of potassium persulphate was added. To this solution (20ml of 2M) nitric acid containing (1g) potassium Periodate 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 100ml standard flask and the rest of the filled with distilled water up to the mark. The flask was inverted several times and left for about ten minutes for the contents to return to room temperature. 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 calculated, by mass, the percentage of manganese in the steel paper clip.
Results:-
- Table
- Calibration Curve
Method-1
Absorbance = 0.35 = 9ml
Concentration of MnO4- = (9.78*10-4) * 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*10-5 1.76*10-5
Mass of Mn = moles*gFm
= 1.76*10-5 * 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
Concentration of MnO4- = (9.78*10-4) * 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*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 reason for using a filter was that it removes any other 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.
