PRESCRIBED PRACTICAL ACTIVITIES

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PRESCRIBED PRACTICAL ACTIVITIES

• AH CHEMISTRY

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1.2Colorimetric determination of manganese in
steel

• Aim
• Use colorimetry to find the percentage of
  manganese by mass in a steel paper clip.

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Introduction

• Colorimetry can be used to determine the
  concentrations of coloured substances in
  solution.
• A colorimeter consists of a light source,
  coloured filter, light detector and recorder.
• The colour of the filter is complementary to the
  colour of the solution, for maximum absorbance.
• Light passes through the filter and then the
  solution.
• The difference between the absorbance of the
  coloured solution and water is measured and
  recorded as an absorbance value.

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• The ion-electron equations for the oxidation of
  manganese are
• Mn(s) ? Mn2(aq) 2e-
• Mn2(aq) H2O(l) ? MnO4- 8H(aq) 5e-

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Results

• Mass of steel paper clip 0.24g
• Absorbance 0.16
• Concentration 1.32 x 10-4 mol l-1
• (from calibration graph)
• If there are 1.32 x 10-4 mol in one litre, then
  100 cm3 (volume in standard flask) must contain
  1.32 x 10-5 mol. This is the number of moles of
  manganese in the sample (see ion-electron
  equations)
• Mass of Mn in sample 1.32 x 10-4 x 54.9
• 0.000725g
• Percentage yield (0.000725 / 0.24) x 100
• 0.30

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Discussion

• A 550 nm filter is used because it produces light
  with the complementary colour (green) to the
  potassium permanganate solution (purple), for
  maximum absorbance.
• The cuvettes must be optically matched (identical
  absorbing and scattering characteristics) to
  ensure that the difference in absorbance between
  the water and the potassium permanganate
  solutions is due entirely to the analyte and not
  the cuvettes.
• A calibration curve is drawn so that the
  concentration of an unknown solution can be
  determined from its absorbance.

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• It is important to wait until the pink colour in
  the solution remains constant to ensure that all
  the manganese present in the paper clip has been
  oxidised to permanganate ions.
• Precise measurements are those which are in close
  agreement with each other. Reference should be
  made to how precise the procedure was, based on
  two results.