Chem. 133 – 3/10 Lecture

Announcements I

• Exam 1– Average (66.5) + Distribution– A little worse than average

• Today’s Lecture– Electrochemistry (just

questions)– Spectroscopy

• Introduction• Properties of Light• Relating light to molecular scale

changes

Range N

80-88 3

70s 3

60s 2

50s 6

Announcements II

• Today’s Lecture – cont.– Spectroscopy – cont.

• Alternative transitions between ground and excited states

• Interpreting spectra• Beer’s Law

• Lab– End of Period 2 today– Tomorrow is make up day– Set 2, period 1 lab report due 3/19

ElectrochemistryPotentiometry – Questions

1. The purpose of a reference electrode is to:a) provide a stable voltage b) complete the circuitc) provide a source of electrons or positive charges needed by

the analyte electroded) all of the above

2. For modern pH measurement, one probe will go into solution. How many reference electrodes exist in in this probe?

3. An F- ion selective electrode is to be used to check that water is properly fluoridated. It is found to work well in most cases, but gives errors in water samples at higher pH. Give a possible explanation for the error, and a possible solution to decrease the error.

4. A platinum electrode is used as:a) reference electrodeb) an electrode for determining dissolved Ptc) an inert electrode for following redox reactionsd) ion selective electrode

Electrochemistry What we are not covering

A. Chapter 15 – Redox Titration- Not heavily used- High precision method of measuring

analyte concentrations- Can be used without potential

measuremente.g. 5H2O2 + 2MnO4

- + 6H+ → 2Mn2+ + 5O2(g) + 8H2O

- Can also be used with potential measuremente.g. Fe2+ + oxidizing agent → Fe3+ + other productspotential (using inert electrode) depends on log{[Fe3+]/[Fe2+]}

Electrochemistry What we are not covering

B. Chapter 16 – Current-based Electrochemical Measurements- These tend to be more modern

electrochemical measurements- Used frequently in electrochemical detectors

in chromatography- Cells used are electrolytic cells (electrical

energy used to drive chemical reactions) - Analyte concentration derived from charge

(from current) measured- Potential allows for selectivity (Ecell > Erxn for

oxidation or reduction to occur)

Spectroscopy

A. Introduction1. One of the main branches of analytical chemistry2. The interaction of light and matter (for purposes of quantitative and qualitative analysis)3. Topics covered:

- Theory (Ch. 17)- General Instruments and Components (Ch. 19)- Atomic Spectroscopy (Ch. 20)- NMR (Rubinson and Rubinson)

Spectroscopy

B. Fundamental Properties of Light

1. Wave-like properties:λ

λ = wavelength = distance between wave crests

n = frequency = # wave crests/s

= wave number = # wave crests/length unit

v = speed of light

Note in vacuum v = c = 3.00 x 108 m/s

Relationships: v = λ·n and = 1/λ

In other media, v = c/n where n = index of refraction

Note: when n > 1, v < c

Even if light travels through other media, wavelength often is defined by value in vacuum

SpectroscopyFundamental Properties of Light

1. Wave-like properties- other phenomena: diffraction, interference (covered in Ch.19)

2. Particle-like propertiesa) Idea of photons (individual entities of light)b) Energy of photons

E = hn = hv/lE = hc/ l (if l is defined for a vacuum)

Spectroscopy Absorption vs. Emission

1. Absorption- Associated with a

transition of matter from lower energy to higher energy

2. Emission- Associated with a

transition from high energy to low energy

Ground State

EnergyExcited State

Photon in

Photon out

A + hn → A*

A* → A + hn

M0

M*

Spectroscopy Regions of the Electromagnetic Spectrum

1. Many regions are defined as much by the mechanism of the transitions (e.g. outer shell electron) as by the frequency or energy of the transitions

Long wavelengths

Short wavelengths

High Energies

Low Energies

Gamma rays

X-rays

Nuclear transitions

Inner shell electrons

UV + visible

Outer shell electrons

Infrared

Bond vibration

Molecular rotations

Microwaves Radio waves

Electron spin

Nuclear spin

Spectroscopy Regions of the Electromagnetic Spectrum

Note: Higher energy transitions are more complex because of the possibility of multiple ground and excited energy levels

Ground electronic state

Excited electronic state

Vibrational levels

Rotational levels

Spectroscopy Alternative Ground – Excited State

TransitionsThese can be used for various types of emission spectroscopy

Excitation Method Related Spectroscopy

Thermal Atomic Emission Spectroscopy

Charged Particle Bombardment

Electron Microscopy with X-ray Emission Spectroscopy

Chemical Reaction Chemiluminescence Spectroscopy (analysis of NO)

Transition from even higher levels

Fluorescence, Phosphorescence

1. Collisional Deactivation (A* + M → A + M + kinetic energy)

2. Photolysis (A* → B∙ + C∙)3. Photoionization (A* → A+ + e-)4. Transition to lower excited state (as in

fluorescence or phosphorescence)5. Some of the above deactivation methods

are used in spectroscopy (e.g. photoaccustic spectroscopy and photoionization detector)

Spectroscopy Alternative Excited State – Ground State

Transitions

SpectroscopyQuestions

1. Light observed in an experiment is found to have a wave number of 18,321 cm-1. What is the wavelength (in nm), frequency (in Hz), and energy (in J) of this light? What region of the EM spectrum does it belong to? What type of transition could have caused it?

2. If the above wave number was in a vacuum, how will the wave number, the wavelength, the frequency and the speed change if that light enters water (which has a higher refractive index)?

3. Is a lamp needed for chemiluminescence spectroscopy? Explain.

4. Light associated with wavelengths in the 0.1 to 1.0 Å region may be either X-rays or g-rays. What determines this?

5. What type of transducers could be used with photoionization to make a detector?

Spectroscopy Transitions in Fluorescence and

Phosphorescence• Absorption of light leads to

transition to excited electronic state

• Decay to lowest vibrational state (collisional deactivation)

• Transition to ground electronic state (fluorescence) or

• Intersystem crossing (phosphorescence) and then transition to ground state

• Phosphorescence is usually at lower energy (due to lower paired spin energy levels) and less probable

Ground Electronic State

Excited Electronic State

higher vibrational states

Triplet State (paired spin)

SpectroscopyInterpreting Spectra

• Major Components– wavelength (of

maximum absorption) – related to energy of transition

– width of peak – related to energy range of states

– complexity of spectrum – related to number of possible transition states

– absorptivity – related to probability of transition (beyond scope of class)

A

l (nm)

Ao

A*

DE

dl

dE

Absorption Based MeasurementsBeer’s Law

Light intensity in = Po

Light intensity out = P

Transmittance = T = P/Po

Absorbance = A = -logTLight source

Absorbance used because it is proportional to concentration

A = εbC

Where ε = molar absorptivity and b = path length (usually in cm) and C = concentration (M)

b

ε = constant for given compound at specific λ value

sample in cuvette

Note: Po and P usually measured differently

Po (for blank)

P (for sample)

Beer’s Law – Specific Example

A compound has a molar absorptivity of 320 M-1 cm-1 and a cell with path length of 0.5 cm is used. If the maximum observable transmittance is 0.995, what is the minimum detectable concentration for the compound?

Beer’s Law– Best Region for Absorption Measurements

• Determine the Best Region for Most Precise Quantitative Absorption Measurements if Uncertainty in Transmittance is constant

A

% uncertainty

0 2

High A values - Poor precision due to little light reaching detector

Low A values – poor precision due to small change in light

Beer’s Law– Deviations to Beer’s Law

A. Real Deviations- Occur at higher C - Solute – solute interactions become important- Also absorption = f(refractive index)

Beer’s Law– Deviations to Beer’s Law

B. Apparent Deviations1. More than one chemical species

Example: indicator (HIn)HIn ↔ H+ + In-

Beer’s law applies for HIn and In- species individually: AHIn = ε(HIn)b[HIn] & AIn- = ε(In-)b[In-]

But if ε(HIn) ≠ ε(In-), no “Net” Beer’s law applies Ameas ≠ ε(HIn)totalb[HIn]total

Standard prepared from dilution of HIn will have [In-]/[HIn] depend on [HIn]total

0

0.05

0.10.15

0.2

0.25

0.3

0.350.4

0.45

0.5

0 0.005 0.01 0.015

Total HIn Conc.A

bso

rban

ce

In example, ε(In-) = 300 M-1 cm-1

ε(HIn) = 20 M-1 cm-1; pKa = 4.0