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 RangeN s3 60s2 50s6

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

Electrochemistry Potentiometry – Questions 1.The purpose of a reference electrode is to: a)provide a stable voltageb) complete the circuit c)provide a source of electrons or positive charges needed by the analyte electrode d)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 electrode b) an electrode for determining dissolved Pt c) an inert electrode for following redox reactions d) 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 measurement e.g. 5H 2 O 2 + 2MnO H + → 2Mn O 2 (g) + 8H 2 O -Can also be used with potential measurement e.g. Fe 2+ + oxidizing agent → Fe 3+ + other products potential (using inert electrode) depends on log{[Fe 3+ ]/[Fe 2+ ]}

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 (E cell > E rxn for oxidation or reduction to occur)

Spectroscopy A. Introduction 1. One of the main branches of analytical chemistry 2. 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 = frequency = # wave crests/s = wave number = # wave crests/length unit v = speed of light Note in vacuum v = c = 3.00 x 10 8 m/s Relationships: v = λ· 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

Spectroscopy Fundamental Properties of Light 1. Wave-like properties - other phenomena: diffraction, interference (covered in Ch.19) 2. Particle-like properties a) Idea of photons (individual entities of light) b) Energy of photons E = h = hv/ E = hc/  (if 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 Energy Excited State Photon in Photon out A + h → A* A* → A + h M0M0 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 Transitions These can be used for various types of emission spectroscopy Excitation MethodRelated Spectroscopy ThermalAtomic Emission Spectroscopy Charged Particle Bombardment Electron Microscopy with X- ray Emission Spectroscopy Chemical ReactionChemiluminescence 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

Spectroscopy Questions 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  -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)

Spectroscopy Interpreting 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 (nm) AoAo A* EE  EE

Absorption Based Measurements Beer’s Law Light intensity in = P o Light intensity out = P Transmittance = T = P/P o Absorbance = A = -logT Light 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: P o and P usually measured differently P o (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 02 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 Deviations 1. More than one chemical species Example: indicator (HIn) HIn ↔ H + + In - Beer’s law applies for HIn and In - species individually: A HIn = ε(HIn)b[HIn] & A In- = ε(In - )b[In - ] But if ε(HIn) ≠ ε(In - ), no “Net” Beer’s law applies A meas ≠ ε(HIn) total b[HIn] total Standard prepared from dilution of HIn will have [In - ]/[HIn] depend on [HIn] total In example, ε(In - ) = 300 M -1 cm -1 ε(HIn) = 20 M -1 cm -1 ; pK a = 4.0