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ELECTROANALISIS ( Elektrometri )

ELECTROANALISIS ( Elektrometri ). Potensiometri , Amperometri and Voltametri. Electroanalysis. Mengukur berbagai parameter listrik ( potensial , arus listrik , muatan listrik , konduktivitas ) dalam kaitannya dengan parameter kimia ( reaksi ataupun konsentrasi dari bahan kimia )

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ELECTROANALISIS ( Elektrometri )

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  1. ELECTROANALISIS(Elektrometri) Potensiometri, Amperometri and Voltametri

  2. Electroanalysis • Mengukurberbagai parameter listrik (potensial, aruslistrik, muatanlistrik, konduktivitas) dalamkaitannyadengan parameter kimia (reaksiataupunkonsentrasidaribahankimia) • Konduktimetri, Potensiometri(pH, ISE), Koulometri, Voltametri, Amperometri

  3. Potensiometri PengukuranpotensiallistrikdarisuatuSelElektrokimiauntukmendapatkaninformasimengenaibahankimia yang adapadaseltsb(conc., aktivitas, muatanlistrik) Mengukurperbedaanpotensiallistrikantara 2 electroda: ElektrodaPembanding(E constant) ElektrodaKerja/Indikator(sinyalanalit)

  4. ElektrodaPembanding Ag/AgCl: Ag(s) | AgCl (s) | Cl-(aq) || .....

  5. ElektrodaPembanding SCE: Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....

  6. ElektrodaPembanding • Reaksi/Potensialsetengahselnyadiketahui • Tidakbereaksi/dipengaruhiolehanalit yang diukur • Reversible danmengikutipersamaan Nernst • PotensialKonstan • Dapatkembalikepotensialawal • stabil • Elektroda Calomel • Hg in contact with Hg(I) chloride (Hg/Hg2Cl2) • Ag/AgCl

  7. ElectrodaKerja • Inert: Pt, Au, Carbon. Tidakikutbereaksi. Contoh: SCE || Fe3+, Fe2+(aq) | Pt(s) • ElektrodaLogam yang mendeteksi ion logamnyasendiri (1st Electrode) (Hg, Cu, Zn, Cd, Ag) Contoh: SCE || Ag+(aq) | Ag(s) Ag+ + e-  Ag(s) E0+= 0.799V Hg2Cl2 + 2e  2Hg(l) + 2Cl- E-= 0.241V E = 0.799 + 0.05916 log [Ag+] - 0.241 V

  8. ElectrodaKerja • Ecell=Eindicator-Ereference • Metallic • 1st kind, 2nd kind, 3rd kind, redox 1st kind • respond directly to changing activity of electrode ion • Direct equilibrium with solution

  9. 2ndkind • Precipitate or stable complex of ion • Ag for halides • Ag wire in AgCl saturated surface • Complexes with organic ligands • EDTA 3rd kind • Electrode responds to different cation • Competition with ligand complex

  10. Metallic Redox Indictors • Inert metals • Pt, Au, Pd • Electron source or sink • Redox of metal ion evaluated • May not be reversible

  11. Membrane Indicator electrodes • Non-crystalline membranes: • Glass - silicate glasses for H+, Na+ • Liquid - liquid ion exchanger for Ca2+ • Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3- • Crystalline membranes: • Single crystal - LaF3 for FPolycrystalline • or mixed crystal - AgS for S2- and Ag+ • Properties • Low solubility - solids, semi-solids and polymers • Some electrical conductivity - often by doping • Selectivity - part of membrane binds/reacts with analyte

  12. Glass Membrane Electrode

  13. Ion selective electrodes (ISEs) A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potensialdifference being created across that membrane

  14. ISEs

  15. Combination glass pH Electrode

  16. Proper pH Calibration • E = constant – constant.0.0591 pH • Meter measures E vs pH – must calibrate both slope & intercept on meter with buffers • Meter has two controls – calibrate & slope • 1st use pH 7.00 buffer to adjust calibrate knob • 2nd step is to use any other pH buffer • Adjust slope/temp control to correct pH value • This will pivot the calibration line around the isopotensialwhich is set to 7.00 in all meters Slope/temp control pivots line around isopotensial without changing it mV mV Calibrate knob raises and lowers the line without changing slope 4 7 4 7 pH pH

  17. Liquid Membrane Electrodes

  18. Solid State Membrane Electrodes Ag wire Filling solution with fixed [Cl-] and cation that electrode responds to Ag/AgCl Solid state membrane (must be ionic conductor)

  19. Solid state electrodes

  20. VOLTAMETRI Pengukuranarussebagaifungsiperubahanpotensial POLAROGRAFI: • Heyrovsky (1922): melakukanpercobaanvoltametri yang pertamadenganelektrodamerkuritetes (DME) Cu2+ + 2e → Cu(Hg)

  21. EF Eredox Mengapaelektronberpindah Reduction Oxidation Eredox E E EF

  22. Steps in an electron transfer event • O must be successfully transported from bulk solution (mass transport) • O must adsorb transiently onto electrode surface (non-faradaic) • CT must occur between electrode and O (faradaic) • R must desorb from electrode surface (non-faradaic) • R must be transported away from electrode surface back into bulk solution (mass transport)

  23. Mass Transport or Mass Transfer • Migration – movement of a muatanlistriklistrikparticle in a potensialfield • Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution) • Convection – mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode Difficult to get perfect reproducibility with stirring, better to move the electrode Convection is considerably more efficient than diffusion or migration = higher aruslistriksfor a given concentration = greater analytical sensitivity

  24. Diffusion Migration Convection Nernst-Planck Equation Ji(x) = flux of species i at distance x from electrode (mole/cm2 s) Di = diffusion coefficient (cm2/s) Ci(x)/x = concentration gradient at distance x from electrode (x)/x = potensialgradient at distance x from electrode (x) = velocity at which species i moves (cm/s)

  25. Diffusion Fick’s 1st Law Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions I = nFAJ

  26. Simplest ExperimentChronoamperometri

  27. Simulation

  28. Recall-Double layer

  29. Double-Layer charging • Charging/discharging a capacitor upon application of a potensialstep Itotal = Ic + IF

  30. Working electrode choice • Depends upon potensialwindow desired • Overpotensial • Stability of material • Conductivity • contamination

  31. The polarogram points a to b I = E/R points b to c electron transfer to the electroactive species. I(reduction) depends on the no. of molecules reduced/s: this rises as a function of E points c to d when E is sufficiently negative, every molecule that reaches the electrode surface is reduced.

  32. Dropping Mercury Electrode • Renewable surface • potensialwindow expanded for reduction (high overpotensialfor proton reduction at mercury)

  33. Polarography A = 4(3mt/4d)2/3 = 0.85(mt)2/3 Density of drop Mass flow rate of drop We can substitute this into Cottrell Equation i(t) = nFACD1/2/ 1/2t1/2 We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop Giving theIlkovichEquation: id = 708nD1/2m2/3t1/6C I has units of Amps when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3 This expression gives the aruslistrikat the end of the drop life. The average aruslistrikis obtained by integrating the aruslistrikover this time period iav = 607nD1/2m2/3t1/6C

  34. Polarograms E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple) Usually D’s are similar so half wave potensialis similar to formal potensial. Also potensialis independent of concentration and can therefore be used as a diagnostic of identity of analytes.

  35. Other types of Polarography • Examples refer to polarography but are applicable to other votammetric methods as well • all attempt to improve signal to noise • usually by removing capacitive aruslistriks

  36. Normal Pulse Polarography

  37. NPP advantage

  38. Differential pulse voltametri

  39. DPP vs DCP Ep ~ E1/2 (Ep= E1/2±DE/2) where DE=pulse amplitude s = exp[(nF/RT)(DE/2)] Resolution depends on DE W1/2 = 3.52RT/nF when DE0 Improved response because charging aruslistrik is subtracted and adsorptive effects are discriminated against. l.o.d. 10-8M

  40. Resolution

  41. Stripping voltametri • Preconcentrationtechnique. 1. Preconcentrationor accumulation step. Here the analyte species is collected onto/into the working electrode 2. Measurement step : here a potensialwaveform is applied to the electrode to remove (strip) the accumulated analyte.

  42. Deposition potensial

  43. ASV

  44. ASV or CSV

  45. Multi-Element

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