PRESCRIBED PRACTICAL ACTIVITIESAH CHEMISTRY

1.2Colorimetric determination of manganese in steel

AimUse colorimetry to find the percentage of manganese by mass in a steel paper clip.

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.

The ion-electron equations for the oxidation of manganese are:

Mn(s) Mn2+(aq) + 2e-

Mn2+(aq) + H2O(l) MnO4- + 8H+(aq) + 5e-

ResultsMass 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%

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.

• 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.