Presentation on theme: "ELECTROANALISIS (Elektrometri)"— Presentation transcript:
1ELECTROANALISIS (Elektrometri) Potensiometri, Amperometri and Voltametri
2ElectroanalysisMengukur berbagai parameter listrik (potensial, arus listrik, muatan listrik, konduktivitas) dalam kaitannya dengan parameter kimia (reaksi ataupun konsentrasi dari bahan kimia)Konduktimetri, Potensiometri (pH, ISE), Koulometri, Voltametri, Amperometri
3PotensiometriPengukuran potensial listrik dari suatu Sel Elektrokimia untuk mendapatkan informasi mengenai bahan kimia yang ada pada sel tsb (conc., aktivitas, muatan listrik)Mengukur perbedaan potensial listrik antara 2 electroda:Elektroda Pembanding (E constant)Elektroda Kerja/Indikator(sinyal analit)
4Elektroda PembandingAg/AgCl: Ag(s) | AgCl (s) | Cl-(aq) || .....
6Elektroda Pembanding Reaksi/Potensial setengah selnya diketahui Tidak bereaksi/dipengaruhi oleh analit yang diukurReversible dan mengikuti persamaan NernstPotensial KonstanDapat kembali ke potensial awalstabilElektroda CalomelHg in contact with Hg(I) chloride (Hg/Hg2Cl2)Ag/AgCl
7Electroda Kerja Inert: Pt, Au, Carbon. Tidak ikut bereaksi. Contoh: SCE || Fe3+, Fe2+(aq) | Pt(s)Elektroda Logam yang mendeteksi ion logamnya sendiri (1st Electrode)(Hg, Cu, Zn, Cd, Ag)Contoh: SCE || Ag+(aq) | Ag(s)Ag+ + e- Ag(s) E0+= 0.799VHg2Cl2 + 2e 2Hg(l) + 2Cl- E-= 0.241VE = log [Ag+] V
8Electroda Kerja 1st kind Ecell=Eindicator-Ereference Metallic 1st kind, 2nd kind, 3rd kind, redox1st kindrespond directly to changing activity of electrode ionDirect equilibrium with solution
92nd kind 3rd kind Precipitate or stable complex of ion Ag for halides Ag wire in AgCl saturated surfaceComplexes with organic ligandsEDTA3rd kindElectrode responds to different cationCompetition with ligand complex
10Metallic Redox Indictors Inert metalsPt, Au, PdElectron source or sinkRedox of metal ion evaluatedMay not be reversible
11Membrane 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 FPolycrystallineor mixed crystal - AgS for S2- and Ag+PropertiesLow solubility - solids, semi-solids and polymersSome electrical conductivity - often by dopingSelectivity - part of membrane binds/reacts with analyte
16Proper pH Calibration E = constant – constant.0.0591 pH Meter measures E vs pH – must calibrate both slope & intercept on meter with buffersMeter has two controls – calibrate & slope1st use pH 7.00 buffer to adjust calibrate knob2nd step is to use any other pH bufferAdjust slope/temp control to correct pH valueThis will pivot the calibration line around the isopotensial which is set to 7.00 in all metersSlope/temp control pivotsline around isopotensialwithout changing itmVmVCalibrate knob raisesand lowers the linewithout changing slopepHpH
17Liquid Membrane Electrodes Persamaan nikolsky untuk pengukuran sampel campuran dengan kurva kalibrasi dan standard adisi
18Solid State Membrane Electrodes Ag wireSolid State Membrane ChemistryMembraneIon DeterminedLaF3F-, La3+AgClAg+, Cl-AgBrAg+, Br-AgIAg+, I-Ag2SAg+, S2-Ag2S + CuSCu2+Ag2S + CdSCd2+Ag2S + PbSPb2+Fillingsolutionwith fixed[Cl-] andcation thatelectroderesponds toAg/AgClSolid state membrane(must be ionic conductor)
22Mengapa elektron berpindah ReductionOxidationEFEredoxEredoxEEEF
23Steps 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)
24Mass Transport or Mass Transfer Migration – movement of a muatan listrik listrik particle in a potensial fieldDiffusion – 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 electrodeDifficult to get perfect reproducibility with stirring, better to move the electrodeConvection is considerably more efficient than diffusion or migration = higher arus listriks for a given concentration = greater analytical sensitivity
25Nernst-Planck Equation DiffusionMigrationConvectionJi(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 = potensial gradient at distance x from electrode(x) = velocity at which species i moves (cm/s)
26Diffusion I = nFAJ Fick’s 1st Law Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions
30Double-Layer charging Charging/discharging a capacitor upon application of a potensial stepItotal = Ic + IF
31Working electrode choice Depends upon potensial window desiredOverpotensialStability of materialConductivitycontamination
32electron transfer to the electroactive species. The polarogrampoints a to bI = E/Rpoints b to celectron transfer to the electroactive species.I(reduction) depends on the no. of molecules reduced/s: this rises as a function of Epoints c to dwhen E is sufficiently negative, every molecule that reaches the electrode surface is reduced.
33Dropping Mercury Electrode Renewable surfacepotensial window expanded for reduction (high overpotensial for proton reduction at mercury)
34Polarography A = 4(3mt/4d)2/3 = 0.85(mt)2/3 Density of dropMass flow rate of dropWe can substitute this into Cottrell Equationi(t) = nFACD1/2/ 1/2t1/2We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding dropGiving the Ilkovich Equation:id = 708nD1/2m2/3t1/6CI has units of Amps when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3This expression gives the arus listrik at the end of the drop life. The average arus listrik is obtained by integrating the arus listrik over this time periodiav = 607nD1/2m2/3t1/6C
35Polarograms E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple) Usually D’s are similar so half wave potensial is similar to formal potensial. Also potensial is independent of concentration and can therefore be used as a diagnostic of identity of analytes.
42DPP vs DCP Ep ~ E1/2 (Ep= E1/2±DE/2) s = exp[(nF/RT)(DE/2)] where DE=pulse amplitudes = exp[(nF/RT)(DE/2)]Resolution depends on DEW1/2 = 3.52RT/nF when DE0Improved responsebecause charging arus listrikis subtracted and adsorptiveeffects are discriminated against.l.o.d M
44Stripping voltametri Preconcentration technique. 1. Preconcentration or accumulation step. Here the analyte species is collected onto/into the working electrode2. Measurement step : here a potensial waveform is applied to the electrode to remove (strip) the accumulated analyte.
52Cyclic voltametriCyclic voltametri is carried out at a stationary electrode.This normally involves the use of an inert disc electrode made from platinum, gold or glassy carbon. Nickel has also been used.The potensial is continuously changed as a linear function of time. The rate of change of potensial with time is referred to as the scan rate (v). Compared to a RDE the scan rates in cyclic voltametri are usually much higher, typically 50 mV s-1
53Cyclic voltametriCyclic voltametri, in which the direction of the potensial is reversed at the end of the first scan. Thus, the waveform is usually of the form of an isosceles triangle.The advantage using a stationary electrode is that the product of the electron transfer reaction that occurred in the forward scan can be probed again in the reverse scan.CV is a powerful tool for the determination of formal redox potensials, detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics.
57The Randles-Sevcik equation Reversible systems n = the number of electrons in the redox reactionv = the scan rate in V s-1F = the Faraday’s constant 96,485 coulombs mole-1A = the electrode area cm2R = the gas constant J mole-1 K-1T = the temperature KD = the analyte diffusion coefficient cm2 s-1
58The Randles-Sevcik equation Reversible systems As expected a plot of peak height vs the square root of the scan rate produces a linear plot, in which the diffusion coefficient can be obtained from the slope of the plot.
62Cyclic voltametri – Stationary Electrode Peak positions are related to formal potensial of redox processE0 = (Epa + Epc ) /2Separation of peaks for a reversible couple is 0.059/n voltsA one electron fast electron transfer reaction thus gives 59mV separationPeak potensials are then independent of scan rateHalf-peak potensial Ep/2 = E1/2 0.028/nSign is + for a reduction