To obtain a calibration graph. Again 2cm3 of permanganate solution was run into a 50cm3 standard flask and the flask was filled to the graduation mark with deionised water, stoppered and inverted several times. A curvette was then rinsed and filled with some of this solution. Using the colorimeter the absorbance of the solution was measured. This result was recorded. The above steps were repeated with 4, 6, 8, 10, 12 and 14cm3 of permanganate solution in the burette. The results obtained were used to plot a calibration graph of absorbance against concentration of potassium permanganate.
To convert manganese to permanganate. Firstly, a steel paper clip was degreased by swirling it in propanone in a small beaker. Then the paper clip was removed using tweezers and left to dry before being cut into small pieces. 0.2g of the paper clip was weighed out and then transferred to a 250cm3 glass beaker. About 40cm3 of 2mol/l nitric acid was added to the beaker before the beaker was covered with a clock glass and gently heated in a fume cupboard. Once the reaction occurred the beaker was continuously heated gently to maintain the reaction. After the solution had cooled 5cm3 of 85% phosphoric acid, 0.2g of potassium persulphate and two anti-bumping crystals were added to the it. The solution was then boiled for 5 minutes. To this solution was then added 15cm3 of acidified potassium periodate solution plus a couple of fresh anti-bumping granules and it was gently boiled. The solution started to turn pink. The solution was boiled until the intensity of the pink colour remained constant. The solution was allowed to cool to room temperature and then transferred to a 100cm3 standard flask leaving the anti-bumping granules in the beaker. The beaker was then rinsed with deionised water and the rinsings were added to the flask. This procedure was repeated until the flask was full to the graduation mark. The flask was stoppered and inverted. Using the colorimeter, fitted with the filter that gave maximum absorbance, the absorbance of the solution was measured and recorded. The calibration graph was then used to find the permanganate concentration of the solution and the percentage mass of manganese in steel paper clip was calculated. The above steps were repeated.
Safety precautions to be observed. Eye protection and gloves are to be worn as acidified 0.0010mol/l potassium permanganate is irritant to the skin as is phosphoric acid, acidified potassium periodate and propanone.
The Results.
Finding the filter with the best absorbance.
From the results it can be seen that the best filter had a wavelength of 520nm.
Calibration Graph.
From the results it can be concluded that the concentration of the unknown 1 solution is 1.16mol/l and the concentration of the unknown 2 solution is 1.34mol/l.
The average concentration of the unknown solutions = (1.16 + 1.34)/2 = 1.25mol/l
From this to calculate the percentage manganese in steel.
First oxidation: Mn(s) Mn2+(aq) + 2e
Second oxidation: Mn2+(aq) + 4H2O MnO4-(aq) + 8H+ + 5e
As both equations occur in a 1:1 ratio hence as the final concentration of permanganate ions is 1.25x10-4mol/l the initial concentration of manganese present in the sample was 1.25x10-4mol/l.
0.2g of paper clip was dissolved in 100cm3 standard flask; therefore there was 2.0g of paper clip in a litre.
Weight of 1 mole of manganese is 54.9g; therefore weight of 1.25x10-4mol/l of manganese is 0.0068625g.
The percentage manganese in steel paper clip = mass of manganese/mass of total paper clip x 100 = 0.0068625/2x100 = 0.34125%
It can be concluded from these results that the percentage mass of manganese present in the steel paper clip was 0.34125%.
Discussion. During the experiment variables were controlled by using the same filter for all the different concentrations, this means the results can be compared. IT is important to use an optically matched filter (520nm) as this allows for the maximum absorbance, meaning the results are clear and more reliable. It is necessary to prepare a calibration graph to enable the concentrations of acidified potassium permanganate of the unknown solutions, which in turn enables the percentage manganese in eh steel paper clip to be determined. I t is necessary to wait until the pink colour in the solution remains a constant colour before measuring absorbance as this ensures results obtained are reliable. This however is a source of error as the solution is observed until the pink colour is constant and observation allows for human error.
Eilidh L Hawkins
A Colorimetric Determination of Manganese in Steel.
