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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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Outline

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

Weak Base titrated with strong acid

  • Consider a 100 ml of a 0.0100 M base with 0.0500 M HCl

  • Kb = 1 x 10-5


Outline

Initial pH

Buffer Region

pH after equivalence

Dominated by remaining

[H+]

pH @ equivalence


Electronic spectroscopy ultraviolet and visible

Electronic SpectroscopyUltraviolet and visible


Where in the spectrum are these transitions

Where in the spectrum are these transitions?


Where in the spectrum are these transitions1

Where in the spectrum are these transitions?


Light is called electromagnetic radiation

Light is called electromagnetic radiation


Review of properties of em

Review of properties of EM!

  • c=ln

    • Where

      • c= speed of light = 3.00 x 108 m/s

      • l= wavelength in meters

      • n = frequency in sec-1

  • E=hn

    • or E=hc/l

      • h=Planks Constant = 6.62606 x 1034 J.s


Where in the spectrum are these transitions2

Where in the spectrum are these transitions?


Beer lambert law

Beer-Lambert Law

AKA - Beer’s Law


The quantitative picture

The Quantitative Picture

  • Transmittance:

    T = P/P0

P0

(power in)

P

(power out)

  • Absorbance:

    A = -log10 T = log10 P0/P

How do “we” select the

wavelength

to measure the absorbance?

b(path through sample)

  • The Beer-Lambert Law (a.k.a. Beer’s Law):

    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

Absorbance vs. Wavelength

Why?

  • Maximum Response for a given concentration

  • Small changes in Wavelength, result in small errors in Absorbance

A

380

400

420

460

440

Wavelength, nm


Outline

Limitations to Beer’s Law

“Fundamental”

“Experimental”

  • Not Using Peak wavelength

  • Colorimetric Reagent is limiting

  • Concentration/Molecular Interactions

  • Changes in Refractive Index


Interaction of light and matter

Interaction of Light and Matter

Start with Atoms

Finish with Molecules


Consider atoms hydrogen

Consider Atoms - hydrogen

Very simple view of Energy states

Assuming subshells have equivalent energies

n=6

n=5

Energy

n=4

A

n=3

n=2

Wavelength, nm

n=1


Molecular spectroscopy

Molecular Spectroscopy


Consider molecules

Consider molecules

  • 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

maxwith certain extinction 

UV

Visible

Electronic Spectrum

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.

1.0

Absorbance

0.0

200

400

800

Wavelength, , generally in nanometers (nm)


Uv vis and molecular structure

UV/Vis and Molecular Structure


The uv absorption process

The UV Absorption process

  •   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis.

  • n  * transitions: non-bonding electrons (lone pairs), wavelength (max) 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 max = 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

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):

h 170nm photon

 antibonding molecular orbital

 bonding molecular orbital


Examples

Examples

Napthalene

Absorbs in the UV


Experimental details

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 compoundsYou will find molar absorbtivities  in L•cm/mol, tabulated.

    • Transition metal complexes, inorganics


Final notes on uv vis

Final notes on UV/Vis

  • Qualitatively

    • Not too useful

      • Band broadening

  • Quantitatively

    • Quite Useful

      • 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

Final notes on UV/Vis (cont’d)

  • Quant (cont’d)

    • Cheap, inexpensive, can be relatively fast

    • Reasonably selective

      • Can find colorimetric method or use color of solution

    • Good accuracy ~1-5%


Chapter 5 calibration methods

Chapter 5 – Calibration Methods

  • Open Excel

  • Find data sheet

  • Input data table


Uncertainty in concentration

Uncertainty in Concentration

Where:

x = determined concentration

k = number of samples

m = slope

n = number of Standards (data points)

D = ??


What happens to the absorbed energy

What happens to the absorbed energy?


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