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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) • Konduktimetri, Potensiometri (pH, ISE), Koulometri, Voltametri, Amperometri Potensiometri Pengukuran 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) Elektroda Pembanding Ag/AgCl: Ag(s) | AgCl (s) | Cl-(aq) || ..... Elektroda Pembanding SCE: Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||..... Elektroda Pembanding • Reaksi/Potensial setengah selnya diketahui • Tidak bereaksi/dipengaruhi oleh analit yang diukur – Reversible dan mengikuti persamaan Nernst – Potensial Konstan – Dapat kembali ke potensial awal – stabil • Elektroda Calomel – Hg in contact with Hg(I) chloride (Hg/Hg2Cl2) – Ag/AgCl Electroda 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) Hg2Cl2 + 2e 2Hg(l) + 2ClE = 0.799 + 0.05916 log [Ag+] - 0.241 V E0+= 0.799V E-= 0.241V Electroda Kerja • 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 2nd kind • Precipitate or stable complex of ion – Ag for halides – Ag wire in AgCl saturated surface • Complexes with organic ligands – EDTA rd 3 kind – Electrode responds to different cation – Competition with ligand complex Metallic Redox Indictors Inert metals – Pt, Au, Pd • Electron source or sink • Redox of metal ion evaluated – May not be reversible 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 o Low solubility - solids, semi-solids and polymers o Some electrical conductivity - often by doping o Selectivity - part of membrane binds/reacts with analyte Glass Membrane Electrode Ion selective electrodes (ISEs) A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potensial difference being created across that membrane 0 .0 1 M Ca2 + 0 .0 2 M Cl - Ca2 + 0 .1 M Ca2 + 0 .2 M Cl - + 2 ( 0 . 1 + ) M Ca + + + 0 .0 2 M Cl + Calcium selective molecular recognition ligand Ca2 + ( 0 . 1 - ) M Ca2 + 0 .2 M Cl - ISEs A1 G RT ln nFE A2 RT A1 0.0592 A1 E ln log nF A2 n A2 (@ 25C) Combination glass pH Electrode 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 isopotensial which is set to 7.00 in all meters Slope/temp control pivots line around isopotensial without changing it mV Calibrate knob raises and lowers the line without changing slope 4 7 pH Liquid Membrane Electrodes 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) Solid State Membrane Chemistry Membrane Ion Determined LaF3 F-, La3+ AgCl Ag+, ClAgBr Ag+, BrAgI Ag+, IAg2S Ag+, S2Ag2S + CuS Cu2+ Ag2S + CdS Cd2+ Ag2S + PbS Pb2+ Solid state electrodes VOLTAMETRI Pengukuran arus sebagai fungsi perubahan potensial POLAROGRAFI: • Heyrovsky (1922): melakukan percobaan voltametri yang pertama dengan elektroda merkuri tetes (DME) Cu2+ + 2e → Cu(Hg) Mengapa elektron berpindah Reduction Oxidation EF Eredox E E Eredox E F 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) Mass Transport or Mass Transfer • • • Migration – movement of a muatan listrik listrik particle in a potensial field 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 arus listriks for a given concentration = greater analytical sensitivity Nernst-Planck Equation J x D i i C i x x Diffusion x C i x D i Ci RT x F z i Migration Convection 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 = potensial gradient at distance x from electrode (x) = velocity at which species i moves (cm/s) Diffusion Fick’s 1st Law I = nFAJ Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions Simplest Experiment Chronoamperometri i time Simulation Recall-Double layer Double-Layer charging • Charging/discharging a capacitor upon application of a potensial step E t / RC Ic e R Itotal = Ic + IF Working electrode choice • Depends upon potensial window desired – Overpotensial – Stability of material – Conductivity – contamination 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. Dropping Mercury Electrode • Renewable surface • potensial window expanded for reduction (high overpotensial for proton reduction at mercury) Polarography A = 4(3mt/4d)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 the Ilkovich Equation: 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 arus listrik at the end of the drop life. The average arus listrik is obtained by integrating the arus listrik over this time period iav = 607nD1/2m2/3t1/6C Polarograms 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. 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 arus listriks Normal Pulse Polarography NPP advantage Differential pulse voltametri DPP vs DCP Ep ~ E1/2 (Ep= E1/2±E/2) where E=pulse amplitude nFAD1/2 c 1 - Ip (t m 1 = exp[(nF/RT)(E/2)] Resolution depends on E W1/2 = 3.52RT/nF when E0 Improved response because charging arus listrik is subtracted and adsorptive effects are discriminated against. l.o.d. 10-8M Resolution Stripping voltametri • Preconcentration technique. 1. Preconcentration or accumulation step. Here the analyte species is collected onto/into the working electrode 2. Measurement step : here a potensial waveform is applied to the electrode to remove (strip) the accumulated analyte. Deposition potensial ASV ASV or CSV Multi-Element Standard Addition Cyclic voltametri • Cyclic 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 Cyclic voltametri • Cyclic 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. Cyclic voltametri Cyclic voltametri For a reversible process Epc – Epa = 0.059V/n The Randles-Sevcik equation Reversible systems The Randles-Sevcik equation Reversible systems i p 0.4463nFACnFvD RT 12 i p 2.687105 n3 2v1 2 D1 2 AC • • • • • • • n = the number of electrons in the redox reaction v = the scan rate in V s-1 F = the Faraday’s constant 96,485 coulombs mole-1 A = the electrode area cm2 R = the gas constant 8.314 J mole-1 K-1 T = the temperature K D = the analyte diffusion coefficient cm2 s-1 The 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. Cyclic voltametri Cyclic voltametri Cyclic voltametri Cyclic voltametri – Stationary Electrode • Peak positions are related to formal potensial of redox process • E0 = (Epa + Epc ) /2 • Separation of peaks for a reversible couple is 0.059/n volts • A one electron fast electron transfer reaction thus gives 59mV separation • Peak potensials are then independent of scan rate • Half-peak potensial Ep/2 = E1/2 • Sign is + for a reduction 0.028/n