Topics: Chemistry, Concentration, Analytical chemistry Pages: 8 (1772 words) Published: January 15, 2014
Department of Chemistry

University of Kentucky

Molecular Absorption Spectroscopy: Determination of Iron
With 1,10-Phenanthroline

Submit a clean, labeled 100-mL volumetric flask to the instructor so that your unknown iron solution can be issued. Your name, section number, and locker number should be written legibly on this flask. The flask does not need to be dry on the inside, but needs to have been rinsed with distilled water after it has been washed. The flask must be turned in at least 1 lab period before you plan to do the experiment so that the Teaching Assistants will have time to prepare the unknown. Each student will have his or her own unknown to analyze even if you are working in pairs.

As a backup precaution, bring an MS-DOS formatted floppy disk, or other data-storage medium, so that you can copy the files obtained during the experiment. BACKGROUND
1,10-phenanthroline (C12H8N2, ortho-phenanthroline or o-Phen) is a tricyclic nitrogen heterocyclic compound that reacts with metals such as iron, nickel, ruthenium, and silver to form strongly colored complexes. This property provides an excellent and sensitive method for determining these metal ions in aqueous solution. For example, o-Phen reacts with ferrous ion to produce a deeply colored red complex:


Fe2+ + 3 o-Phen

The molar absorptivity (ε) of the ferrous complex, [(C12H8N2)3Fe]2+, is 11,100 L/mol-cm at the wavelength of maximum absorbance intensity, λmax = 508 nm. This large value indicates the complex absorbs very strongly. The intensity of the color is independent of pH in the range 2 to 9. The complex is very stable and the color intensity does not change appreciably long periods of time. Beer’s law is obeyed, over about 1.5-2 orders of magnitude of iron concentration. Beer’s Law is a very simple relationship: A = εbc

where A is the absorbance of a substance at a specified wavelength λ, in units of nm for light in the ultraviolet and visible regions of the electromagnetic spectrum; b is the length of the light path through the sample, usually in cm; ε is the molar absorptivity of the absorbing species at λ, when the concentration is in M; and c is the concentration in molar units, M.

CHE 226 – Analytical Chemistry Laboratory


Iron Absorption

Department of Chemistry

University of Kentucky

The critical point of Beer’s Law for use in quantitative analysis is that, theoretically, the absorbance of a solution is linear with the concentration of the absorbing substance, if the wavelength, the pathlength, and other experimental conditions are kept constant. There are a number of factors, however, that limit the concentration range over which Beer’s Law is valid for a particular analysis. Perhaps the most critical is concentration. There are invariably deviations from Beer’s law at high enough concentrations.

To determine the total iron in the sample, it must be completely in the ferrous state, and Fe2+ can readily be air-oxidized to the ferric state, Fe3+. o-Phen will form a colored complex with Fe3+, but its spectrum is different from that of the ferrous complex and the color is not as intense. Thus, one could not determine the total iron present by making measurements at only one wavelength. Hence, a mild reducing agent is added before the color is developed in order to provide a measure of the total Fe present in solution. Hydroxylamine, as its hydrochloride salt, can be used. The reaction is

2 Fe3+ + 2 NH2OH HCl + 2 OH-

2 Fe2+ + N2 + 4 H2O + H+ + Cl-

This is an equilibrium reaction, but even a mild reducing agent will drive it >99.99% to the right. The pH is adjusted to a value between 6 and 9 by addition of an ammonia or sodium acetate buffer.

Varian Cary 50 UV-Vis Spectrophotometer.
This is a computer-controlled double-beam grating spectrophotometer with a constant 20nm bandpass. A high-intensity Xe flash lamp is used as the source for both UV and visible...
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