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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Potensiometri, Amperometri and Voltametri
PengukuranpotensiallistrikdarisuatuSelElektrokimiauntukmendapatkaninformasimengenaibahankimia yang adapadaseltsb(conc., aktivitas, muatanlistrik)
Mengukurperbedaanpotensiallistrikantara 2 electroda:
Ag(s) | AgCl (s) | Cl-(aq) || .....
Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....
Pt, Au, Carbon. Tidakikutbereaksi.
Contoh: SCE || Fe3+, Fe2+(aq) | Pt(s)
(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
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
Slope/temp control pivots
line around isopotensial
without changing it
Calibrate knob raises
and lowers the line
without changing slope
Solid state membrane
(must be ionic conductor)
Cu2+ + 2e → Cu(Hg)
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
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)
Fick’s 1st Law
Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions
I = nFAJ
Itotal = Ic + IF
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.
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
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
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.
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 DE0
because charging aruslistrik
is subtracted and adsorptive
effects are discriminated against.
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.
For a reversible process
Epc – Epa = 0.059V/n
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-1The 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.