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ELECTROANALISIS (Elektrometri) Potensiometri, Amperometri and Voltametri

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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

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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)

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Elektroda Pembanding Ag/AgCl: Ag(s) | AgCl (s) | Cl - (aq) ||.....

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Elektroda Pembanding SCE: Pt(s) | Hg(l) | Hg 2 Cl 2 (l) | KCl(aq., sat.) ||.....

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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/Hg 2 Cl 2 ) – Ag/AgCl

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Electroda Kerja Inert: Pt, Au, Carbon. Tidak ikut bereaksi. Contoh:SCE || Fe 3+, Fe 2+ (aq) | Pt(s) Elektroda Logam yang mendeteksi ion logamnya sendiri (1 st Electrode) (Hg, Cu, Zn, Cd, Ag) Contoh:SCE || Ag + (aq) | Ag(s) Ag + + e- Ag(s)E 0 + = 0.799V Hg 2 Cl 2 + 2e 2Hg(l) + 2Cl - E - = 0.241V E = 0.799 + 0.05916 log [Ag+] - 0.241 V

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Electroda Kerja E cell =E indicator -E reference Metallic – 1 st kind, 2 nd kind, 3 rd kind, redox 1 st kind – respond directly to changing activity of electrode ion – Direct equilibrium with solution

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

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

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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

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Glass Membrane Electrode

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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

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ISEs

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Combination glass pH Electrode

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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 1 st use pH 7.00 buffer to adjust calibrate knob 2 nd 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 mV pH 4 7 Calibrate knob raises and lowers the line without changing slope mV pH 4 7 Slope/temp control pivots line around isopotensial without changing it

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Liquid Membrane Electrodes

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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 MembraneIon Determined LaF 3 F -, La 3+ AgClAg +, Cl - AgBrAg +, Br - AgIAg +, I - Ag 2 SAg +, S 2- Ag 2 S + CuSCu 2+ Ag 2 S + CdSCd 2+ Ag 2 S + PbSPb 2+

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Solid state electrodes

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VOLTAMETRI Pengukuran arus sebagai fungsi perubahan potensial POLAROGRAFI: Heyrovsky (1922): melakukan percobaan voltametri yang pertama dengan elektroda merkuri tetes (DME) Cu 2+ + 2e → Cu(Hg)

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Mengapa elektron berpindah EFEF E redox EFEF Reduction Oxidation E E

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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)

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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

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Nernst-Planck Equation DiffusionMigrationConvection J i (x) = flux of species i at distance x from electrode (mole/cm 2 s) D i = diffusion coefficient (cm 2 /s) C i (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)

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

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Simplest Experiment Chronoamperometri

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Simulation

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Recall-Double layer

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Double-Layer charging Charging/discharging a capacitor upon application of a potensial step I total = I c + I F

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Working electrode choice Depends upon potensial window desired – Overpotensial – Stability of material – Conductivity – contamination

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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.

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Dropping Mercury Electrode Renewable surface potensial window expanded for reduction (high overpotensial for proton reduction at mercury)

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Polarography A = 4 (3mt/4 d) 2/3 = 0.85(mt) 2/3 Mass flow rate of drop Density of drop We can substitute this into Cottrell Equation i(t) = nFACD 1/2 / 1/2 t 1/2 Giving the Ilkovich Equation: i d = 708nD 1/2 m 2/3 t 1/6 C I has units of Amps when D is in cm 2 s -1,m is in g/s and t is in seconds. C is in mol/cm 3 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 i av = 607nD 1/2 m 2/3 t 1/6 C We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop

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Polarograms E 1/2 = E 0 + RT/nF log (D R /D o ) 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.

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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

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Normal Pulse Polarography

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NPP advantage

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Differential pulse voltametri

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DPP vs DCP Ep ~ E 1/2 (E p = E 1/2 E/2) where E=pulse amplitude = exp[(nF/RT)( E/2)] Resolution depends on E W 1/2 = 3.52RT/nF when E 0 Improved response because charging arus listrik is subtracted and adsorptive effects are discriminated against. l.o.d. 10 -8 M

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Resolution

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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.

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Deposition potensial

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ASV

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ASV or CSV

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Multi-Element

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Standard Addition

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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

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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.

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Cyclic voltametri

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For a reversible process E pc – E pa = 0.059V/n

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The Randles-Sevcik equation Reversible systems

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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 cm 2 R = the gas constant 8.314 J mole -1 K -1 T = the temperature K D = the analyte diffusion coefficient cm 2 s -1

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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.

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Cyclic voltametri

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Cyclic voltametri – Stationary Electrode Peak positions are related to formal potensial of redox process E 0 = (E pa + E pc ) /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 E p/2 = E 1/2 0.028/n Sign is + for a reduction

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