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Analytical Chemistry Final Notes
Analytical Chemistry Final Notes

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Western University
Chemistry 2272F
Ken Yeung

Analytical Chemistry Final Electrochemistry Electrochemistry – Study of chemical energy and electrical energy based on reduction & oxidation reactions. Electro-analytical Chemistry – Use of electrical measurements on a chemical system for analytical purposes… Redox Reaction (Titration) – Transfer of electrons from one species to the other Oxidation – The increase in oxidation number/loss of electrons Oxidizing Agent – Substance causing the oxidation reaction Reduction – The decrease in oxidation number/gain of electrons Reducing Agent – Substance causing the reduction Voltaic Cell – Generates a spontaneous voltage by a chemical redox reaction e.g. battery Cathode – Electrode where the reduction takes place Anode – Electrode where the oxidation takes place Line Notation  Anode is on the left, Cathode is on the right  “|” Indicates phase boundary  “||” Indicates the salt bridge  Ignore Spectator ions **If half-cell has no solid species to serve as electrode, use an inert electrode such at Pt or graphite** **Concentrations should be entered if applicable.** Standard Potentials – Standard reduction potentials (1 M and 1 atm) are outlined in table Cell Potential – Comparison between the anode and cathode reduction potentials **Standard Reduction Potentials used for both, negative sign is valid** Nernst Equation – To determine cell potential at non-standard conditions or… Redox Titration Curve Initially – Only one species is present (ie. Fe ). Therefore no potential may be calculated. Before Equivalence – Analyte in excess, however some product is formed. Analyte therefore controls potential Equivalence Point – Tricky to calculate (use equation) After Equivalence – Titrant determines potential. Equivalence Point Equation: Factors affecting the sharpness of the curve: 1. Concentration 2. Completeness/Spontaneity 3. Strength of Reducing Agent/Oxidizing Agent ** Endpoint detection can occur are either potential measurements or visual indication ** Introduction to Spectrometric Methods ** Absorbance is evident by the intensity of color, which is proportional to the concentration ** Energy States of Matter - Energy is quantized in discrete energy levels; which is a property of the molecule/atom Excited State – Any energy state that is higher than the lowest energy state Ground State – The lowest level energy state of an atom/molecule Emission – Light given off during relaxation – Electromagnetic radiation emitted as an excited particle relaxes Absorption – Electromagnetic energy is transferred from a photon to a molecule/atom – Causes transition from ground state to excited state – Only occurs if the energy level exactly matches the delta E Quantitative Aspects of Spectrochemical Measurements Emission – Signal is proportional to the concentration of analyte – At some point this falls off due to interactions [ ] Absorption – Beer-Lambert‟s Law outlines the relationship – Absorption of a wavelength is linearly related to concentration Chemical Analysis: 1. Choose wavelength by conducting a scan (λ max should be chosen) a. This is because a iλ most constant for slit width 2. Run calibration Standards (construction of linear model) 3. Run unknown sample 4. Determine unknown conc. from calibration curve Instruments for Optical Spectroscopy Absorbance Spectrometer – Measures absorbance of monochromatic light across a sample. A narrow band of light is isolated Fluorescence Spectrometer – Measures light emitted by the analyte that has been excited by absorption of light. Emission Spectrometer – Measures light emitted by a sample excited thermally or chemically. Sources of Radiation Continuum Sources – Broad spectrum of light – Requires a wavelength selector to be useful – Example: Blackbody source of light (temperature dependent) Line Source – Few discrete (monochromatic) wavelengths – Based on emission of atoms (nothing to do with temperature) Laser Conditions  Based on a population inverse  More electrons are found in the higher energy state… lower chances of „absorption‟ Components of Optical Instruments Wavelength Selector Interference Filter – Transmits a particular wavelength and reflects others – Transmitted light constructively interferes within filter – Light that is reflected interferes destructively Monochromators – Isolates a single wavelength of light – Variable selection of wavelength – Light is first dispersed via Prism/Grating and then selected Grating – Reflective surface etched with close
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