Spectrophotometric Determination of Iron
Purpose To become familiar with the principles of calorimetric analysis and to determine the iron content of an unknown sample. Summary Iron +II is reacted with o-phenanthroline to form a coloured complex ion. The intensity of the coloured species is measured using a Spectronic 301 spectrophotometer. A calibration curve (absorbance versus concentration) is constructed for iron +II and the concentration of the unknown iron sample is determined. Theory Colorimetric analysis is based on the change in the intensity of the colour of a solution with variations in concentration. Colorimetric methods represent the simplest form of absorption analysis. The human eye is used to compare the colour of the sample solution with a set of standards until a match is found. An increase in sensitivity and accuracy results when a spectrophotometer is used to measure the colour intensity. Basically, it measures the fraction of an incident beam of light which is transmitted by a sample at a particular wavelength. You will use a Spectronic 21 in this experiment. There are two ways to measure the difference in intensity of the light beam. One is the percent transmittance, %T, which is defined as:
%T =
Io 1 = log = − log T I T
For any given compound, the amount of light absorbed depends upon (a) the concentration, (b) the path length, (c) the wavelength and (d) the solvent. Absorbance is related to the concentration according to the Beer-Lambert law:
A = εbc
where ε is the extinction coefficient (M-1 cm-1 ), b is the solution path length (cm) and c is the concentration (moles litre-1 ). Not all substances obey the linear Beer-Lambert law over all concentration ranges. Therefore you will construct a calibration curve that will provide the relationship between concentration and absorbance under the conditions used for the analysis.
In this experiment, you will analyze for iron by reacting iron +II with o-phenanthroline to form an...