Transcript Slide 1

Outline

Final Comments on Titrations/Equilibria
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Titration of Base with a strong acid
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End-point detection
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Choice of indicators
Titration Curve method
Start Chapter 18
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Spectroscopy and Quantitative Analysis
Weak Base titrated with strong
acid
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Consider a 100 ml of a 0.0100 M base
with 0.0500 M HCl
Kb = 1 x 10-5
[OH ]  Kb  Cb
Initial pH
Buffer Region
[base]
pH  pKa  log
[acid]
pH @ equivalence
[ H  ]  K a  Ca
pH after
equivalence
Dominated by
remaining
[H+]
Electronic Spectroscopy
Ultraviolet and visible
Where in the spectrum are
these transitions?
Where in the spectrum are
these transitions?
Light is called electromagnetic
radiation
Review of properties of EM!
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c=ln
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Where
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c= speed of light = 3.00 x 108 m/s
l= wavelength in meters
n = frequency in sec-1
E=hn
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or E=hc/l
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h=Planks Constant = 6.62606 x 1034 J.s
Where in the spectrum are
these transitions?
Beer-Lambert Law
AKA - Beer’s Law
The Quantitative Picture
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Transmittance:
T = P/P0
Absorbance:
A = -log10 T = log10 P0/P
P0
(power in)
P
(power out)
How do “we” select the
wavelength
The
(a.k.a.
Beer’s Law):
toBeer-Lambert
measureLaw
the
absorbance?
b(path through sample)
A = ebc
Where the absorbance A has no units, since A = log10 P0 / P
e is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample in cm
c is the concentration of the compound in solution, expressed in mol L-1 (or M,
molarity)
Absorbance vs. Wavelength
Why?
1. Maximum Response for a given
concentration
2. Small changes in Wavelength,
result in small errors in
Absorbance
A
380
400
420
Wavelength, nm
440
460
Limitations to Beer’s Law
“Fundamental”
1.
1. Concentration/Molecular Interactions
2. Changes in Refractive Index
2.
“Experimental”
Not Using
Peak
wavelength
Colorimetric
Reagent is
limiting
Interaction of Light and
Matter
Start with Atoms
Finish with Molecules
Consider Atoms - hydrogen
Very simple view of Energy states
Assuming subshells have equivalent energies
Energy
n=6
n=5
n=4
n=3
A
n=2
n=1
Wavelength, nm
Molecular Spectroscopy
Consider molecules
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With molecules, many energy levels.
Interactions between other molecules and with the solvent
result in an increase in the width of the spectra.
Electronic Spectrum
Make solution of
concentration low enough
that A≤ 1
(Helps to Ensure Linear
Beer’s law behavior)
1.0
lmaxwith certain
extinction e
UV
Visible
Absorbance
UV bands are much
broader than the photonic
transition event. This is
because vibration levels
are superimposed.
0.0
200
400
Wavelength, l, generally in nanometers (nm)
800
UV/Vis and Molecular
Structure
The UV Absorption process
•  * transitions: high-energy, accessible in vacuum
UV (lmax <150 nm). Not usually observed in molecular
UV-Vis.
•n  * transitions: non-bonding electrons (lone pairs),
wavelength (lmax) in the 150-250 nm region.
•n  * and   * transitions: most common
transitions observed in organic molecular UV-Vis,
observed in compounds with lone pairs and multiple
bonds with lmax = 200-600 nm.
Any of these require that incoming photons match in
energy the gap corresponding to a transition from ground
to excited state.
What are the nature of
these absorptions?
Example:   * transitions responsible for ethylene UV
absorption at ~170 nm calculated with semi-empirical
excited-states methods (Gaussian 03W):
hn 170nm photon
antibonding
bonding molecular
molecular
orbital
orbital
Examples
Napthalene
Absorbs in the UV
Experimental details
•What compounds show UV spectra?
•Generally think of any unsaturated compounds as
good candidates. Conjugated double bonds are
strong absorbers.
•The NIST databases have UV spectra for many
compounds You will find molar absorbtivities e in
L•cm/mol, tabulated.
•Transition metal complexes, inorganics
Final notes on UV/Vis
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Qualitatively
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Not too useful
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Band broadening
Quantitatively
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Quite Useful
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Beer’s Law is obeyed through long range of
concentrations
Thousands of methods
Most commonly used
Detection Limits ~ 10-4 – 10-6 M
Final notes on UV/Vis (cont’d)
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Quant (cont’d)
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Cheap, inexpensive, can be relatively fast
Reasonably selective
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Can find colorimetric method or use color of
solution
Good accuracy ~1-5%
Chapter 5 – Calibration
Methods
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Open Excel
Find data sheet
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Input data table
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Uncertainty in Concentration
sconcentration 
(x

D
x
2
i
i
D
sy
m
)
2 x xi
1 x n  xi



k D
D
D
2
2
x
i
n
Where:
x = determined concentration
k = number of samples
m = slope
n = number of Standards (data points)
D = ??
(x )  n  x   x 
2
i
i
i
What happens to the
absorbed energy?