SPECTROPHOTOMETRIC DETERMINATION OF IRON IN AQUEOUS IN AQUEOUS SOLUTIONS AS SOLUTIONS AS A A COMPLEX COMPLEX OF 1,10-PHENANTHROLINE RESULTS AND DISCUSSION The goal of this experiment is to become familiar with basic methods in UV-Visible molecular absorption spectrophotometry for quantitative chemical analysis. Specifically, it aims to determine the amount of iron in aqueous solution as a complex of 1,10-phenanthroline by means of spectrophotometric method of analysis. Spectrophotometry is the science that deals with the quantitative study of the electromagnetic spectrum, particularly the intensity of light [1]. By using the ability of atoms to absorb light energy of specific wavelength, it can be used in the calculation of the concentration of an unknown solution. In order for a species to be analyzed using the spectrophotometric spectrophotometric method, certain requirements must be met. First, its absorbance must be within the range of the wavelength of the photometer. Second, the solution must be colored in order for it to be absorbed in the visible region. And if a transmission type photometer is used, the sample to be analyzed must be transparent. A spectrophotometer spectrophotometer is the primary device used in spectroscopy. It is capable of measuring the absorbance of a solution by quantifying the amount of light passing through a solution placed in a specialized tube called cuvette [2]. One important principle involved in spectroscopy is the Beer ’s Law. It directly relates the concentration of a colored substance in a solution to the amount of light it absorbs [3]. A = a b c
(1)
Where A is the absorbance, a is the absorptivity constant (M-1 cm -1 ), b is the path length (cm), and C is the analyte molar concentration. Absorbance is proportional to the concentration of the solution whereas transmittance is proportional to the intensity of the light that has entered the sample. Conceptually, transmittance is an easier quantity to understand as compared to
absorbance. But since absorbance is directly proportional to the other parameters, that is, it displays a simple dependence on the concentration and cell path length , Beer’s law is expressed i n terms of molar absorptivity instead of transmittance [2]. Also, transmittance is stated in percentage, while absorbance is unitless. Because of this, absorbance must be used in order to balance the equation. However, the Beer’s Law has a lot of limitations. First, the solutions solutions to be analyzed analyzed must be highly diluted. Second, reagent of high purity must be used. And third, the temperature must be constant [3]. If these conditions are not followed, departures from the law would be observed.
In this experiment, the system that is under observation is the reaction between Fe 2+ and orthophenanthroline phenanthroline to produce a deep orange complex. Fe2+ + 3 o-Phen
Fe(o-Phen) 32+
→
(2)
To determine the total iron in the sample, it must completely be in the ferrous state. However, Fe 2+ can readily be air-oxidized to the ferric state, Fe 3+. oPhen will form a colored complex with Fe 3+,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, in the preparation of standard solutions, a mild reducing agent is added before the color is developed in order to prevent Fe 2+ from being oxidized to Fe 3+ and to provide a measure of the total Fe present in solution [4]. Hydroxylamine, as its hydrochloride salt can be used. The reaction is shown by 2 Fe3+ + 2 NH2OH-HCl + 2 OH- → 2 Fe2+ + N2 + 4 H2O + H+ + Cl- (3) After adding 10% hydroxylamine hydrochloride solution to the working standard Fe(II) solution, 1,10-phenanthroline was then added to the mixture. Note that phenanthroline was added in excess in order to prevent metals such as silver, bismuth, chromium and copper from causing interferences to the solution. Then, acetate buffer of pH 4 was added to adjust the pH value of the solution between 6 and 9. This sequence of addition of reagents was also
followed in the preparation of unknown Fe(II) solutions. It is important that the reagents were added in this sequence because changing this order would defeat their purpose. Once the standard and unknown solutions were already prepared, their absorbance values were then measured using a photometer. Spectral scanning was first done to determine the real peak wavelength of the spectrum. Since the solutions are reddish-orange, a blue-green wavelength of light (509 nm) was selected from the spectrophotometer. The complex absorbs very strongly at this wavelength and is also very stable, that is, the color intensity of the solution does not change drastically over long periods of time. Note that not all substan ces obey the Beer’s Law over all concentration ranges. This is why a calibration procedure is essential. Using the recorded data, a calibration curve can now be plotted which will provide the relationship between concentration and absorbance under the conditions used for the analysis.
Figure 1. Calibration Curve
Using the linear equation, the concentration of the unknown Fe(II) solution was computed in the three trials. After which, the concentration of Fe(II) in the stock sample was then calculated and expressed in ppm and molarity.
REFERENCES [1] Institute of Chemistry. Analytical Chemistry Laboratory Manual . University of the Philippines Diliman. 2007. p. 81. [2] Rice University. Retrieved last 10
Principles of Spectrophotometry . September 2013 from
http://www.ruf.rice.edu/~bioslabs/methods/protein/ spectrophotometer.html. [3] Skoog, et al. Fundamentals of Analytical Chemistry, Eighth Edition . Brooks/Cole – Thomson Thomson Learning. 2004. [4] CHEMetrics, Inc. Iron(total, soluble, ferrous) Phenanthroline Method. Retrieved last 17 September 2013 fromhttp://www.ospreyscientific.com/uploads/Tech %20Papers/CHEM%20iron_phenanthroline.pdf. [5] D.C Harris. Quantitive Chemical Analysis, 8 th ed. New York. W.H. Freeman and Company. Company. 2007.
