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The iron content of runoff from a bananaranch is a necessary analyticalparameter to analyze. A 25.0mL

sample of the runoff was acidified withHNO3 and treated with excess KSCN toform a red complex. (KSCN itself iscolorless.) The solution then wasdiluted to 100.0mL and put in a

variable pathlength cell. Forcomparison, a 10.0mL referencesample of 6.80x10-4 M Fe3+ wastreated with HNO3 and KSCN anddiluted to 50.0mL. The reference was

placed in a cell with a 1.00cm lightpath. The runoff sample exhibited thesame absorbance as the referencewhen the pathlength of the runoff cellwas 2.48cm. What was the

concentration of iron in the banana

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UV Spectroscopy and Qualitative Analysis

1) UV-vis spectroscopy is usually notvery useful for qualitative analysisbecause there are few absorption

maxima and minima2) Solvents:

a. Must be transparent in region ofinterest

b. Should not interfere with absorbingspecies (but usually it does). Polarsolvents tend to obliterate finestructural detail in molecular spectra

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UV Spectroscopy and Qualitative Analysis

3) UV-vis spectroscopy does provide someinformation on functional groups

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UV-vis spectroscopy and

Quantitative Analysis

1)Scope is huge

a.)95% of all quantitative analyses inhealth care field are done by UV-vis

spectroscopyb.) wide applicability to organic andinorganic analyses

c.) even non-absorbing species canbe used by doing colorimetricreactions (reactions must go near tocompletion)

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UV-vis spectroscopy and

Quantitative Analysis

2. L.O.D. is low,typically 10-4

10-5M but can be

as low as 10-6 10-7M

3. Moderate to highselectivity

4. Accuracy to within1-3% with minimaltraining

5. Easy and accuratedata acquisition

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Construction of Calibration Curves

is often done on the absorption

max. Why?

Sample and Reference cells should be matched

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ExampleThe ultraviolet absorbances of

a series of 9 standardshaving different nitrateconcentrations were

determined at 220nmusing a 1.0cm cell; 8samples of river waterwere taken downstreamfrom a chemical plant, avg.absorbance 0.642. What is

the nitrate content of theriver in mg/mL?

NO3(mg/mL

0 .004 .015 .025 .035 .04 .05 .06 .07

Abs. .003 .10 .211 .350 .453 .556 .623 .671 .691

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The n p* (T1)transition occurs at397nm, the n p*

(S1) transitionoccurs at 355nm.What is thedifference inenergy betweenthe n p* (T1)

state?

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Relaxation Processes

1. Radiationless loss of energy in smallsteps; excitation energy converted intokinetic energy by small collisions withother molecules, small increase in

temperature2. Fluorescence: radiative form of

relaxationResonance fluorescence: no change in

wavelength from excitation to emissionLowest e- state, vibrational, rotationalstate of each excited state producesresonance mostly in atoms

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Stokes Shift

Molecules see morenon-resonancefluorescence

Not all excited energyis transmitted as

radiation (some isnon-radiative)

Makes emissionspectrum look like

mirror image ofexcitation spectrum

This shift in thespectrum towardlonger wavelengths is

called Stokes Shift

Results fromthermal energylosses

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More about fluorescence

Why do somemolecules fluoresceand others dont?

Want as fast of way aspossible to get down to theground state, generallynon-radiative internalconversion is fastest butsometimes due tomolecules configuration,fluorescence may be faster

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More about excess energy loss

1. Emission of radiation

Excited particles (ions, atoms, ormolecules) relax to lower energy level

by giving up excess energy as photons Excitation brought about by

bombardment with e-, exposure to high

potential current, or heat treatment byarc or flame

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More about excess energy loss

2.Thermal radiation a.k.a blackbodyradition

1. radiation emission lmax prop. To

1/T2. Energy emitted varies as the 4th

power of temperature

3. Emissive power varies 1/l5Heated solids produce IR, vis, and

longer l UV

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3. Phosphorescence (another way

to lose excess energy)

Radiative form of relaxation, involvesinter-system crossing where an e-flips spin. Long lifetime!

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Draw the spectra for Li and

acetone Relevent energy transitions for Li:

pp* 1.33 x 10-18 J

N s* 1.06 x 10-18 J

N p* 7.12 x 10-19 J

Relevent energy transitions for acetone

3.63 x 10-19 J

3.26 x 10-19 J

2.96 x 10-19 J

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You are in graduate school and justsynthesized a new organic complex thatabsorbs UV light. Your advisor is really

excited and tells you that you can write apaper about it for submission to the

Journal of Organic Chemistrybut firstwants more information about the new

compound for inclusion in the paper. Onepiece of useful information would be the e.Design an experiment to do this.

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Riboflavin Demo