Colorimetric Determination of Manganese in Steel

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                             Aakanksha Sadekar

          5th January 2006

Colorimetric Determination of Manganese in Steel

Aim:-  The aim  of this experiment was to find out the percentage of manganese

present in a steel paper clip.

Introduction:-  

Colorimetry is an analytical technique that is used to determine the concentrations of coloured substances in solution. It relies on the fact that a coloured substance absorbs light of a complimentary colour to its own and that the amount of light absorbs is proportional to its concentration. Colorimetry is particularly suited to the determination of manganese in steel because the manganese can be converted to the permanganate (VII) ion which is strongly coloured.

The oxidation is achieved in two stages:

1st stage oxidation

   Mn(s)        Mn2+(aq)

This oxidation is brought about by the nitric acid which itself is reduced to nitrogen monoxide, which forms nitrogen dioxide (brown fumes) on contact with the air.

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 (2, 4, 6, 8, 10, 12, 14cm3) stock standard solution (of 9.78 x 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.

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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 ...

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