Then the manganese ions were oxidised into permanganate ions using the oxidising agent, potassium periodate:
14 Mn2+ (aq) + 16 H20 (l) + 10 I04- (aq) → 14 MnO4- (aq) + 5 I2 (g) + 32 H+ (aq)
Procedure
The apparatus used for the experiment were:
50cm3 and 100cm3 standard flasks
50cm3 burette
spectrophotometer
optically matched cuvettes
balance
50cm3 and 250cm3 beakers
bunsen burner
10cm3 and 50cm3 measuring cylinders
clock glass
filter funnel
tweezers
wash bottle
dropper
wire cutters
steel paper clips
0.0010 mol l-1 acidified potassium permanganate
2 mol l-1 nitric acid
85% phosphoric acid
acidified potassium periodate solution
potassium persulphate
propanone
deionised water
anti-bumping granules.
Part A
The burette was rinsed and filled with 0.0010 mol l-1 potassium permanganate, 2cm3 of which was run into a 50cm3 flask which was filled up with deionised water. This solution was then used to fill a cuvette. The spectrophotometer was set to 520nm wavelength and zeroed with deionised water before the absorbance of the solution in the cuvette was measured. These stages were repeated for different volumes of potassium permanganate, 4cm3, 6cm3, 8cm3, 10cm3, 12cm3 and14cm3, which allowed enough results to be gathered to plot a calibration graph of absorbance against concentration of potassium permanganate.
Part B
A paper clip was degreased by swirling it in some propanone. Tweezers were used to remove the paper clip which was dried using a paper towel. The paper clip was cut it into small pieces and approximately 0.2g of the paper clip was put into a 250cm3 beaker. 40 cm3 of 2 mol l-1 nitric acid was added to the beaker which was covered with a clock glass. The mixture in the beaker was heated in a fume cupboard until the steel dissolved. Anti-bumping granules were added and the solution was boiled until no more brown fumes were given off. 5cm3 of 85% phosphoric acid and 0.2g of potassium persulphate were added to the solution when it had cooled. The solution was boiled further for 5 minutes. 15cm3 of acidified potassium periodate and more anti-bumping granules were added and the newly pink solution was transferred to a 100cm3 flask after the intensity of the pink colour had remained constant.The beaker was rinsed with deionised water and the rinsings were added to the flask. A dropper was used to fill up to the graduation mark with deioinised water. The flask was stoppered and inverted several times to make sure the solution was completely mixed. The spectrophotometer was set at a 520nm wavelength and zeroed with deionised water before being used to measure the absorbance of the solution in the flask. The calibration graph was then used to find the concentration of the solution.
Safety
Safety glasses were worn throughout the experiment.
Results
Results for Calibration Graph
Specimen Calculation
Concentration = 2/50 * 0.001 = 0.00004 mol l-1
Results for Manganese Percentage Calculation
Specimen Calculation
Mass of paper clip = 0.205g
Concentration of potassium permanganate = 0.000120 mol l-1
Volume of potassium permanganate = 0.1 l
n = c * v
= 0.000120 * 0.1
= 0.0000120 moles
m = n * GFM
= 0.0000120 * 54.9
= 0.0006588g
% = 0.0006588/0.205 * 100
= 0.321 %
0.321 % manganese in steel.
Discussion
The results indicate that the average percentage of manganese in steel is approximately between 0.30% and 0.31%. The results are quite precise are they are within 0.035% of each other. A 520nm wavelength was set in the spectrophotometer because this wavelength is in the green region of the visible spectrum, which is the complementary colour of the colour of the permanganate ions. This meant that the spectrophotometer was able to measure the absorbance of this complementary colour. Optically matched cuvettes were used so that any light distortion was cancelled out. It is necessary to produce a calibration graph because it is used to work out the concentration of permanganate that gives a specific absorbance value so that the calculation can be worked out to give the percentage of manganese in steel. The need to wait until the intensity of the pink colour in the permanganate solution remained constant was so that the reaction had completely taken place and all the permanganate was measured for its absorbance.
