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CM2011

Andrew Susilo Chemistry None English 41 cards

Cards in this deck (41)

  1. 1Analytical approach to solving problems: identify context of problem + determine type of analysis -> design experiment + establish criteria -> run experiment -> transform + analyse + verify data -> design experiment again // propose solution (external evaluation)Types of analysis: qualitative (what is there) // quantitative (how much) // characterisation (chemical / physical properties) // fundamental (improve measurement's capabilities)
    Precision relates to reproducibility
  2. 2Error = difference between answer and true answer Types of error: systematic (eg instrumental / method / human, same size + magnitude + direction) // random (can only be treated statistically) Q for bad data: gap ./. rangeRelative uncertainty = absolute uncertainty ./. magnitude; summed uncertainty (absolute) = sqrt [sum of squared absolute uncertainties]; multiplied uncertainty (percent) = sqrt [sum of squared percent uncertainties]
    Rules for reporting: percentage to whole number, 1dp if <1%
  3. 3Confidence interval = t * sqrt [var./.N]; t = error ./. sqrt [var./.N]; t between two sets of data = difference ./. sqrt [pooled var * (N1 + N2) ./. N1N2]; t between two methods @ many samples = average difference ./. sqrt [SD of difference / #] Rules for reporting: mean / SD / t to +1 dp if data set >10, otherwise follow.
  4. 4AQI - developed by USEPA, to provide timely and easily understandable information about daily levels of air pollution; replaces PSI in most countries (except Singapore NEA, < AQI due to treatment of data).
  5. 5Particle = cluster of molecules; aerosol = stable suspension of solid / liquid particles @ gas; PM = airborne particles, mixture of organic / inorganic / biological materials, may have toxic metals / PAH bound PM_10 = aerodynamic diameter <10 um; PM_2.5 <2.5 umBreathing mode: ~8 dm^3 min^-1 (resting, --> ~12 m^3 per day) // 40-60 dm^3 min^-1 @ exercise
    Cigarette - ~12 mg PM_2.5
    Problems with AQI measurements: uncertainty of exact health effects / does not account for particle composition / time scale? / no error analysis
  6. 6PM sampling process: large vacuum -> particles separated based on size -> small particles collected @ filter paper -> put @ concentrated HCl + HNO3 -> pressurised and heated from microwave acid digestion -> filter and dissolve -> ICP-MS (atomic measurement for composition of samples)Elemental analysis - based on volume @ solution; common elements measured in ppm-ppb, trace elements @ air in ppt
    EF used to determine effect of non-crustal elements on observed concentrations, Al commonly used as reference (very common @ crust)
  7. 7Possible sources of PM: combustion of vegetation / re-mobilisation of particles previously deposited @ vegetation (inclusions released when plants burned) Similarity of PGE / REE trend may indicate same source (eg refinery / catalytic converter degradation)PM composition: ~80% ions, ~8% soluble organic, ~6% insoluble elements, <1% soluble inorganic and soluble elements, ~5.4% unidentified (mainly carbon-based)
    Non-routine hazardous compounds increased during burning: PAH / VOC / dioxin
  8. 8Electrochemistry analysis - equilibrium (battery @ discharge mode, potentiometry) // non-equilibrium (battery charged with external voltage, voltammetry) Electrolyte = fully dissociate @ solution; ionic strength pushes equilibrium towards more charged speciesElectrolyte effect: electrostatic repulsive / attractive forces between electrolyte ions x reactant ions -> reactant ions surrounded by sheath of solutions with slight excess of electrolyte ions @ ionic atmosphere -> solvation cage -> effective concentration (activity) affected
  9. 9Debye-Huckel equation constants only apply to aqueous solutions @ 298 K; denominator simplifies to unit if I <0.01 M, y = unity if extremely dilute, cannot determine y experimentally if too high concentration Effective behaviour indicated by y @ all equilibria.
  10. 10Electrochemical cell (= galvanic) - oxidant // reductant physically separated, \ salt bridge (high concentration) maintains electrical contact; potential drops to 0 as reaction progresses Representation of cells: anode metal | anode solution || cathode solution | cathode metal*No voltage-measuring device can measure absolute half cells
    SHE: hydrogen gas @ 1atm, unit activity for H+ (not in reality), n_e- = 2
    Calomel reference electrode = +0.242 V, Cl- saturated, n_e = 1 \ no longer used because toxic; silver-AgCl electrode = +0.205 V, replaces Hg-Hg2Cl2
  11. 11Electrode potential = half cell reaction vs reference electrode; standard potential = electrode potential @ 298 K and unity activities; formal potential = measured when [Ox]^a ./. [Red]^b is unity, specified substances @ specified concentrations.
  12. 12Types of transducers: electrochemical (potentiometric / voltammetric / amperometric etc) / optical (spectroscopic measurement @ optode, associated with chemical reaction) / mass sensitive (piezoelectric effect @ oscillating crystal, use gas sensor) / heat sensitive (calorimetric, use thermistor).
  13. 13Potentiometry = measure potential of electrochemical cells without drawing appreciable current, uses reference electrode + indicator electrode + potential-measuring device Ag+ hydrated @ solid Ag surface, leaving free e- @ solid side*Ion-selective electrode surface must contain a component reacting selectively and reversibly with analyte
    Types of membranes in potentiometric sensors: glass (optimised NaSiO3 and Al2O3) / inorganic salt (sparingly soluble) / ionophore (polymer-immobilised)
  14. 14Glass membrane conditioned by soaking in 0.1M HCl 1h, involving formation of SiO-H+ groups @ glass surface; interference happens with alkali M+ Potential @ water//glass interface measured with impendance mV meter, require reference electrodeGlass sensor components: Ag | AgCl | satd KCl aq || sample | glass | c | AgCl | Ag
    E = E_G + E_G' + E_R' minus (E_R + E_J)
    LJP results from difference in rates of H+ transfer vs Cl- transfer; steady state will be attained with less concentrated side being more +ve
  15. 15NH3 gas sensing - NH3 diffuses via gas permeable membrane to NH4Cl solution inside barrel -> pH shift detected via pH probe; interferences are small molar amines Other gases sensed via pH glass membrane: CO2 (inner solution NaHCO3) / SO2 (inner solution NaHSO3)F- sensing - moves through lattice of crystalline LaF3 (sparingly soluble, doped with europium difluoride) via Frenkel defect (hop from one vacancy to next); interferences mainly OH-
    Ag2S membrane can be pressed into stable thin film, sense either Ag+ (direct via defect) or S^2- (indirect)
  16. 16Polymer membrane sensors are highly viscous liquids PVC sensor components: 30% PVC + ~68% plasticiser + 1% ionophore (complexing agent, selectively binds to ion, move about @ liquid channels of plasticiser) + ~1% salt (for conductivity / counter-ion; not always need)Problem of selectivity of K+ vs Na+: ~4mM @ blood serum vs ~140mM
    K+ sensing: 10% NH4NO3 bridge inserted between satd KCl /x/ sample solution to prevent contamination, use valinomycin as ionophore (high cavity size for fast and reversible exchange), K+ fully shielded from membrane
  17. 17Nernstian sensors need slope within 2mV from theoretical value Linear range good for exp-5 to exp-1, sensor becomes less sensitive below exp-5 resulting in curvature Calibration to reach steady state by not fluctuating over 0.1mV for >60sISA = electrolyte added to every solution to adjust ionic strength, helps to get electrode to respond to concentration (vs activity); F- ISA buffer: NaCl + NaOAc (adjust pH to ~5) + CDTA (complex out Al^3+ and Fe^3+)
    Selectivity coefficient determined by two-point mixed solution method
  18. 18Rotational transition - molecules rotate about various axes (can be hindered if not separated enough) -> absorb hu @ far IR / microwave; vibrational - atoms / groups of atoms within molecule vibrate relative to each other (can be altered by collisions) -> @ mid-far IR; electronic - e- absorb @ UV-VisBand spectra - arise from numerous quantised vibrational levels -> series of closely-spaced lines not fully resolved; radiant power of line / band spectra directly proportional to concentration
    Continuum spectra - arise from oscillations @ heated solids, peak shifts left with heat
  19. 19Intensity (radiant power) = power of light beam per unit area (in m^2) Scatchard plot - determine equilibrium constant via measurement of non-absorbing species via plot of dA ./. [X] = {K * dE * [total]} minus {K * dA}; dE for 1cmLimits to Beer Lambert law: real deviations (particles affecting charge distributions of close neighbours, or electrostatic interactions with electrolytes in dilute solutions) // chemical deviations // instrumental deviations (eg molar absorptivity differences if non-monochromatic)
  20. 20Components of atomic spectroscopy instrument: excitation source (continuous) -> monochromator (concave mirror collimates light -> diffracted @ different angles by reflection grating -> rotate to pass different wavelengths to exit) -> sample -> detector + amplifier (PMT)Continuous sources: tungsten (3500K, emits 320-2500nm) / D2 (emits 200-400nm via splitting of D2* into D atoms)
    Effective bandwidth of monochromator = width @ half peak; should be ~2nm (narrower for gaseous phase), depends on grating size and quality / slit width / focal length
  21. 21Fluorescence vs absorbance spectroscopy: fixed vs continuous excitation wavelength, more sensitive (because zero background), extra monochromator (perpendicular) Fluorescence vs phosphorescence: shorter lifespan, more likely, same spin vs differentTypes of excess energy relaxation: radiative (fluorescence // phosphorescence) // non-radiative (vibrational relaxation to lowest vibrational // internal conversion from lowest vibrational to upper of same spin electronic state // intersystem crossing to different spin multiplicity)
  22. 22Stokes shift: fluorescence emission longer wavelength than absorbed due to reoptimisation of geometry in excited electronic state (/ environmental interactions); mirror image if spacings equal for both electronic states + similar transition probabilitiesFluorescence quantum yield = ratio of number of fluorescent molecules ./. total number of excited molecules = photons emitted ./. photons absorbed; greater if highly fluorescent (eg substituted ar / rigid) / lower temperature
  23. 23AAS components: hollow cathode (needs lines <1-10pm, Ar+ sputters M @ cathode -> collide with higher energy e- -> excited -> emit hu) + atomiser (nebulised sample into aerosol -> mix with O2 + fuel -> desolvated @ 1o combustion -> volatilised @ interzonal); O2 and N2O used for less readily excitedAES components: ICP plasma inlet (RF power source give Ar+ + e- via spark from Tesla coil, maintained @ 6k-10k K, interact with fluctuating magnetic field + sample aerosol inlet (completely atomised) + coolant (prevents tube from melting); spectral observation made @ 15-20mm above induction coil
  24. 24AES vs AAS: higher temperature for more complete atomisation (substantial population of excited states), chemically inert atmosphere, uniform temperature cross-section, thinner optical pathlength, can measure many elements simultaneouslySources of interferences in AES: spectral (overcome by multi-element analysis with multiple wavelengths for correction) // physical (eg on viscosity / temperature) // chemical (eg on volatilisation) // self-absorption (esp @ high concentrations)
  25. 25Temperature affects excited state population \ no effect on ground state -- AES more sensitive to temperature than AAS Boltzmann distribution equation: relative populations = relative dengeneracy * exp(-dE ./. kT).
  26. 26Atomic linewidth broadening factors: natural (Heinsberg uncertainty principle: relative uncertainty in energy or wavelength inversely relates to lifetime) // collisional (shortens lifetime, worse with increasing T or p) // Doppler (higher frequency detected when atoms move towards detector vs away).
  27. 27Solvent extraction = separate compounds / metal complexes based on relative solubilities in 2 immiscible solvents Metal ions require chelation into neutral complexes for organic solubility Partition coefficient (K) = [A]_2 ./. [A]_1 @ equilibrium mixturei = number of extractions; f = [ V_aq ./. [V_aq + K*V_org]]^i (D instead of K if ionised); [A]_final = f * [A]_original
    *More efficient to do multiple small extractions vs one big extraction
  28. 28Distribution coefficient (D) = [A]_org ./. [total [A]]_aq; total includes ionised forms D_base = K * K_a ./. [K_a + [H^+]]; D_acid = K * K_b ./. [K_b + [OH^-]] = K * [H^+] ./. [K_a + [H^+]]Chelating ligands usually weak acids
    Chelation constant (b) = [ML_n] ./. [[M^n+] * [L^-]^n]; K_M = [ML_n]_org ./. [ML_n]_aq; K_L = [HL]_org ./. [HL]_aq; D = [K_M * b * Ka^n * [HL]^n_aq] ./. [H^+]^n_aq ~= [K_M * b * Ka^n * [HL]^n_org] ./. [K_L^n * [H^+]^n_aq]
  29. 29Chromatography = separation based on different partitioning between mobile phase (gas / liquid) vs stationary phase; solutes washed through stationary phase by mobile phase (elution) Eluent = fluid in; eluate = fluid outTypes of chromatographic bed shape: column (packed / open) / planar
    Separation mechanisms: adsorption / partition (stationary high-boiling liquid bonded to solid surface) / ion-exchange / size-exclusion (smaller solutes retained) / affinity (*most selective)
  30. 30Retention time for each component (t_r) = void time (= t_m; minimum time for unretained species to pass) + adjusted retention time (= t_s; effective time solute spends @ stationary phase) Retention factor (k_p) = t_s(p) ./. t_m = K * V_s ./. V_m; selectivity factor (a) = k_q ./. k_p = t_s(q) ./. t_s(p) = K_q ./. K_pLinear flow rate (u) = cm min^-1; volume flow rate (u_v) = V_r ./. t_r = e * A * u = e * A * L ./. t_m; e = 1 for open tubular
    Narrow column - good for analytical (good separation); fatter column - preparative (more load)
    Scaling rules: V <-x-> m, m <-x-> A (if u kept constant)
  31. 31Resolution (R_s) = d(t) ./. [average of W_p and W_q] = 0.5835 d(t) ./. [average of W_half(p) and W_half(q)] = 0.25 * sqrt(N) * [[t_q ./. t_p] - 1] Theoretical plate: evaluate column efficiency as if column made up of series of small platesNumber of theoretical plates (N) = L ./. H = 16 * [t_r ./. W]^2
    W = 4*sig; W_half = 2.35*sig; diffusion coefficient (D) = sig^2 ./. 2*t; distance travelled (x) = sig^2 ./. H
  32. 32Reasons for band spread: multiple flow paths (non-homogeneous packing, improve by smaller range of particle size) / longitudinal diffusion (greater for gaseous phase, improve by faster linear flow) / mass transfer resistance (finite equilibration time, improve by thinner films / smaller packing / slow velocity)Band of solute emerge @ detector with SD (sig); var_injection = V_injection^2 ./. 12
    Contribution of laminar flow via capillary tubing to bandwidth: var_tubing = pi * r^4 * L * u_v ./. [24*D]
    H = 'A (for multiple paths) + ['B ./. u] (for diffusion) + [C * u] (for mass transfer)
  33. 33Real vs ideal columns: K depends on [solute] vs independent on [solute] --> skewed vs Gaussian bandshape Increase K - fronting peaks; decrease K - tailing peaks Isotherm: K = C_s ./. C_m; upper and lower isotherms arise from overloading of soluteTailing peaks: 'A > 'B; eluted too early because adsorption sites significantly saturated by strong solute binding, eg silica hydroxyls vs polar solutes, improve by silanisation to block hydroxyl
    Fronting peaks: 'A < 'B; excess solute makes solute more soluble @ stationary phase
  34. 34GC - for gas / volatile liquid (stable between 50-300degC), use non-volatile chemically inert stationary phase, gaseous mobile phase (must be inert, low viscosity); most commonly use OT column (WCOT / SCOT / PLOT) Derivatisation used for making analyte more suitable for GCPLOT used for extremely volatile, PLOT / SCOT offer greater surface area
    OT vs packed: no multiple paths, higher resolution, higher sensitivity, lower pressure needed, shorter analysis time (faster u) \ lower capacity
    Packed column adaptations: silanisation of silica, small uniform particles
  35. 35Polar stationary phase better for more polar solutes (poly-methyl-phenyl-siloxane > dimethyl) Cross-linking protects silanol from bleeding @ high temperature (due to exposure of O2) + prevent tailing Thicker films better for shielding / capacity / resolution \ long retentionOptimal flow rate for carrier gas: N2 < He < H2 (fastest diffusion of analyte --> least mass transfer --> best resolution @ high flow rate \ breaks down vacuum pump for MS, explosive)
    Pressure programming - increase solute vapour pressure -> increase flow rate (no need high temperature)
  36. 36Split injection - best for high resolution work (require low volume, preferred for >0.1% analyte), split ratio typically 50-600:1; splitless - for trace analysis, large volume @ low-boiling solvent, column temperature 40degC below bp for solvent trapping; on-column preferred for quantitative / if unstable to heatSolvent trapping: solvent condenses and saturates @ first few metres of column -> solutes re-dissolved @ solvent plug -> purge turn on to let solvent evaporate as temperature increase -> solutes concentrated @ narrow band -> greater resolution when eluted as temperature increases
  37. 37Thermal conductivity detector - universal for all analytes (lowers conductivity of gas stream -> filament heats up) \ not highly sensitive (unable to detect minute quantities from <0.53mm diameter OT); slower flow rate for greater sensitivityFlame ionisation detector: eluate burned @ H2 + air (sample destroyed) -> produce CHO+ + e- -> volatage amplified -> filtered to remove high-frequency noise; sensitive for small capillaries (especially hydrocarbon samples)
    MS detector measures m:z of ions (magnetic field deflects lightest most)
  38. 38HPLC needs packed column for high resolution (more extreme method needed to decrease mass transfer because diffusion is slow) Particle diameter inversely proportional to N (more uniform filling + decrease mass transfer) \ directly to L; \ creates resistance to solvent flow.
  39. 39Silica-based materials dissolved @ pH > 8 (polystyrene support used instead) Function of guard columns: remove contaminants from solvents -> increase life of analytical column; same packing material and stationary phase as analytical \ shorter and less expensiveDissolved gases in liquids causes band spreading; need to remove by bubbling inert gas
    Mobile phase solvents pulled from reservoirs via action of pump (need to generate pressures ~6k psi + flow rate 0.1-1 mL/min + flow reproducibility >0.5% + resistant to corrosion)
  40. 40Normal vs reverse phase HPLC: stationary phase polar (eg amino / diol) vs non-polar (eg octadecyl / octyl), increase elution strength with more non-polar mobile phase vs more polar Stationary phase held by physical absorption // chemical bonding (reaction of silanol +x organo-chloro-silane)Isocratic vs gradient elution: single solvent (or constant solvent mixture) vs continuous change of solvent composition to increase eluent strength (required to elute more strongly retained solutes)
    Eluent strengths independent of solute in adsorption chromatographya
  41. 41UV spectroscopic detector: use Hg / Xe / tungsten lamps as light source + coupled to monochromator -> commonly photodiode detector; most common in HPLC, linear range >5 orders of magnitude of soluteRefractive index detector: sample cell refracts light from reference cell (flow solvent only; universal \ 1000x lower detection limit than UV, need higher concentration of analyte for significant change in refractive index
    Electrochemical detector only for redoxable compounds (very sensitive)

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None Chemistry Flashcards on CM2011, created by Andrew Susilo on 07/09/2022.
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