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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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  • 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

Initial pH

Buffer Region

pH after equivalence

Dominated by remaining


pH @ equivalence

Electronic spectroscopy ultraviolet and visible

Electronic SpectroscopyUltraviolet and visible

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

Beer lambert law

Beer-Lambert Law

AKA - Beer’s Law

The quantitative picture
The Quantitative Picture

  • Transmittance:

    T = P/P0


(power in)


(power out)

  • Absorbance:

    A = -log10 T = log10 P0/P

How do “we” select the


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


  • Maximum Response for a given concentration

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







Wavelength, nm

Limitations to Beer’s Law



  • 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








Wavelength, nm


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 



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.







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



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


x = determined concentration

k = number of samples

m = slope

n = number of Standards (data points)

D = ??