Colorimetric Determination of Manganese in Steel

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Introduction

Spectrophotometric Determination of Manganese in Steel Introduction The aim of this experiment was to determine the average percentage concentration of manganese in steel. This is useful when it is necessary to determine the strength of the steel. Spectrophotometric analysis was used in this experiment. This is a technique that can be successfully used to determine the concentrations of coloured substances in solution. It works on the basis that a coloured substance absorbs light of a colour complementary to its own and also that the amount of light absorbed is proportional to the substance's concentration.Spectrophotometric analysis was relevant in this experiment because the permanganate ions that are formed from the manganese were coloured, allowing spectrophotometry to be used to work out the absorbance by absorbing all the other colours other than the colour of the permanganate ions. The conversion of the manganese into permanganate ions took place in two stages. Firstly, the manganese was oxidised to manganese ions using the oxidising agent, nitric acid: 3 Mn (s) + 2 NO3- (aq)

Middle

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.

Conclusion

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. Lee McFadzean

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