Kearns_Experiment 3

Kearns_Experiment 3 - Experiment 3 Spectroscopic...

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Experiment 3, Spectroscopic Determination of Formulas and Stability Constants of Complex Ions Author: Jeremy Kearns Group 3 Section 2, T 11:00 AM – 2:00 PM Partners: Kevin Smith and Bryce Duncan 9/21/2010 Abstract: This experiment uses spectroscopy to determine the formula and stability constant of a metal-ligand complex consisting of copper and iminodiacetic acid (IDA). Through the use of analyzing actual absorbance data compared against hypothetical absorbance data generated, it was found that copper complexes with IDA is a 1:1 ratio. The hypothetical absorbance was generated by analyzing the absorbance near the extreme mole fraction values and creating trend- lines that should ideally model hypothetical behavior. In addition to finding the formula of the copper-IDA complex, the absolute stability constant of the complex was also found to be 3.35*10 10 ± 9.19*10 9 M -1 . This fell within the 25% uncertainty of the literature value of 4*10 10 M -1 .
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I. Introduction Spectroscopy is an excellent way to determine the composition of ions formed in a solution, as well as the stability constant of the ion of interest. In order to find the composition of a metal-ligand complex formed in solution, one can continuously vary the concentrations of each of the components making up metal-ligand complex. By varying the concentrations of each of the metal-ligand components (while keeping a constant total solute concentration) and measuring the absorbance at each concentration combination, one can begin to determine the chemical formula of the metal-ligand complex of interest. In order to obtain an accurate measure of the amount of metal-ligand complex present in each of the prepared solutions, one must set the spectrometer to a wavelength where the metal- ligand complex absorbs the light well, and each of the separate components do not absorb the light very well. If one of the compounds used to make the complex does absorb light well at the wavelength of interest, one can formulate a calibration in order to subtract a certain value from the measured absorbance and obtain the actual absorbance of the metal-ligand complex. If the absorbance of the metal-ligand complex is graphed against the mole fraction of the ligand, a convex-down curve is formed. Towards the extreme values of mole fraction of the ligand, linear trend lines can be formed. By extrapolating trend lines for both of the extreme values of mole fraction of ligand, one can find the theoretical absorbance of the metal-ligand complex is the reaction between the metal and ligand went to completion. An intersection of these extrapolated lines results in the theoretical mole fraction of ligand that corresponds to the maximum yield of the metal-ligand complex. From here, one can determine the composition of the metal-ligand complex using the following formula: = - n fmax1 fmax ( ) 1 where n is the number of ligand atoms bonding to each metal atom in the metal ligand complex and f max represents the mole fraction of ligand that would result in the theoretical maximum yield
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Kearns_Experiment 3 - Experiment 3 Spectroscopic...

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