Experiment 3:
Spectroscopic Determination of Formulas and
Stability Constants of Complex Ions
Author:
Ben Rainey
Group 2
Section 3, T 2:00 PM – 5:00 PM
Partner: Ben Huffer
November 9, 2010
Abstract
:
In this experiment, the two main scientific goals were to determine the
formula of and to determine the stability constant of a metalligand complex.
The
complex consisted of copper and iminodiacetic acid (IDA).
To accomplish these goals a
Shimadzu UV/Vis and the method of continuous variations was used.
The hypothetical
absorbencies were generated by analyzing the absorbance near the extreme mole fraction
values.
Linear regression was used to determine absorbencies that should ideally model
hypothetical behavior.
These hypothetical absorbencies were then compared with
measured absorbencies.
It was found that copper complexes with IDA in a 1:1 ratio.
The
measured absolute stability constant was determined to be 3.66*10
10
±6.14*10
9
M
1
.
The
measured absolute stability constant had an uncertainty of 16.8%.
The value obtained for
the absolute stability constant was in agreement with the literature value of 4*10
10
M
1
with a 25% uncertainty.
I. Introduction
Spectroscopy is one method used to determine the composition of ions formed in
a solution and to determine the stability constant of the ion of interest.
In order to find
the composition of a metalligand complex formed in solution, the method of continuous
variations can be utilized.
One can continuously vary the concentrations of each of the
components making up the metalligand complex.
The metalligand complex under
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View Full DocumentExperiment 3: Spectroscopic Determination of Formulas and Stability Constants of Complex Ions
study in this experiment is the copperIDA complex. By varying the concentrations of
each of the metalligand components, while keeping the total solute concentration
constant, and measuring the absorbance at each concentration combination, the chemical
formula of the metalligand complex of interest can be determined.
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 complex absorbs light strongly and the metal ion and the ligand do not. If one
of the compounds used to make the complex absorbs light well at the wavelength of
interest, a calibration can be obtained in order to determine the actual absorbance of the
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, the theoretical absorbance of
the complex for a reaction that went to completion can be determined.
The intersection
of these extrapolated lines yields the theoretical mole fraction of ligand that corresponds
to the maximum yield of the metalligand complex. The composition of the metalligand
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 Spring '11
 James
 Chemistry, IDA

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