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
metalligand 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
copperIDA 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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View Full DocumentI. 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 metalligand complex formed in solution, one can continuously vary the concentrations of each
of the components making up metalligand complex. By varying the concentrations of each of
the metalligand 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 metalligand complex of interest.
In order to obtain an accurate measure of the amount of metalligand 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 metalligand complex. If the
absorbance of the metalligand complex is graphed against the mole fraction of the ligand, a
convexdown 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 metalligand 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 metalligand complex. From here, one can determine the composition of the metalligand
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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 Spring '11
 James
 Chemistry, IDA, metalligand complex

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