Outline. Final Comments on Titrations/Equilibria Titration of Base with a strong acid End-point detection Choice of indicators Titration Curve method Start Chapter 18 Spectroscopy and Quantitative Analysis. Weak Base titrated with strong acid.
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pH after equivalence
Dominated by remaining
pH @ equivalence
Electronic SpectroscopyUltraviolet and visible
AKA - Beer’s Law
T = P/P0
A = -log10 T = log10 P0/P
How do “we” select the
to measure the absorbance?
b(path through sample)
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)
Limitations to Beer’s Law
Interaction of Light and Matter
Start with Atoms
Finish with Molecules
Very simple view of Energy states
Assuming subshells have equivalent energies
Interactions between other molecules and with the solvent result in an increase in the width of the spectra.
maxwith certain extinction
Make solution of concentration low enough that A≤ 1
(Helps to Ensure Linear Beer’s law behavior)
UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed.
Wavelength, , generally in nanometers (nm)
UV/Vis and Molecular Structure
Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.
Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W):
h 170nm photon
antibonding molecular orbital
bonding molecular orbital
Absorbs in the UV
x = determined concentration
k = number of samples
m = slope
n = number of Standards (data points)
D = ??
What happens to the absorbed energy?