electroanalytical chemistry potentiometry, voltammetry and polarography

Electroanalytical chemistry

Potentiometry, Voltammetry and Polarography

Electroanalysis

• measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration)

• Conductimetry, potentiometry (pH, ISE), coulometry, voltammetry

Potentiometry

the measure of the cell potential to yield chemical information (conc., activity, charge)

Measure difference in potential between two electrodes:

reference electrode (E constant)indicator electrode (signal α analyte)

Reference electrodes

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

- +

Ag/AgClSalt bridge

KCl

Pt

Fe2+, Fe3+

- +

Ag

Soln. aq. satdin KCl + AgCl

Pt

Fe2+, Fe3+AgCl + KCl

AgCl

Porous glass

AgCl(s) + e - <=> Ag(s) + Cl -

E0=0.222V

Fe3+ + e - <=> Fe2+

E0=0.771VE(KCl sat.)=0.197V

Reference Electrodes

SCE:

Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....

Hg(l)

Soln. sat. in KCl

Pt

KCl

Hg, Hg2Cl2 et KCl

Porous glass

E0=0.268V

E(KCl sat.)=0.241VGlass wool

Hg2Cl2 + 2e - <=> 2Hg(l) + 2Cl -

Indicator Electrodes• Inert:

Pt, Au, Carbon. Don’t participate in the reaction.

example: SCE || Fe3+, Fe2+(aq) | Pt(s)

• Certain metallic electrodes: detect their ions(Hg, Cu, Zn, Cd, Ag)example SCE || Ag+(aq) | Ag(s)Ag+ + e- Ag(s)

E0+= 0.799VHg2Cl2 + 2e 2Hg(l) + 2Cl-

E-= 0.241V

E = 0.799 + 0.05916 log [Ag+] - 0.241 V

Ion selective electrodes (ISEs)

A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being

created across that membrane

C a 2 + C a 2 + 0 . 0 1 M C a 2 +

0 . 0 2 M C l -

0 . 1 M C a 2 +

0 . 2 M C l -

( 0 . 1 + ) M C a 2 + ( 0 . 1 - ) M C a 2 +

0 . 0 2 M C l - 0 . 2 M C l -

+

+

+

+

-

-

-

-

Calcium selective molecular recognition ligand

ISEs

25C) (@

log0592.0

ln

ln

2

1

2

1

2

1

A

A

nA

A

nF

RTE

nFEA

ARTG

Combination glass pH Electrode

Ag

Soln. aq. satdin KCl + AgCl

AgCl(s) + KCl(s)

AgCl porousglass

+ -

0.1M HCl inAgCl sat.

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 isopotential which is set to

7.00 in all meters

mV

pH 4 7

Calibrate knob raisesand lowers the linewithout changing slope

mV

pH 4 7

Slope/temp control pivots line around isopotentialwithout changing it

Liquid Membrane Electrodes

Solid State Membrane Electrodes

Ag wire

Filling solutionwith fixed[Cl-] andcation thatelectroderesponds to

Ag/AgCl

Solid state membrane(must be ionic conductor)

Solid State Membrane Chemistry

Membrane Ion Determined

LaF3 F-, La3+

AgCl Ag+, Cl-

AgBr Ag+, Br-

AgI Ag+, I-

Ag2S Ag+, S2-

Ag2S + CuS Cu2+

Ag2S + CdS Cd2+

Ag2S + PbS Pb2+

Solid state electrodes

Voltammetry

The measurement of variations in current produced by variations of the potential applied to a working electrode

polarography:• Heyrovsky (1922): first voltammetry experiments

using a dropping mercury working electrode

In voltammetry, once the applied potential is sufficiently negative, electron transfer occurs between the electrode and the electroactive species: Cu2+ + 2e → Cu(Hg)

Why Electrons Transfer

EF

Eredox E

F

Eredox

•Net flow of electrons from M to solute•Ef more negative than Eredox

•more cathodic •more reducing

Reduction Oxidation

•Net flow of electrons from solute to M•Ef more positive than Eredox

•more anodic •more oxidizing

E E

Steps in an electron transfer eventO 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 charged particle in a potential 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 electrodeDifficult to get perfect reproducibility with stirring, better to move the electrodeConvection is considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity

Nernst-Planck Equation

xx

x

RT

F

x

xx CCDzCDJ iii

iiii

Diffusion 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 = potential gradient at distance x from electrode(x) = velocity at which species i moves (cm/s)

DiffusionFick’s 1st Law

Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions

I = nFAJ

Simplest ExperimentChronoamperometry

time

i

Simulation

Recall-Double layer

Double-Layer charging

• Charging/discharging a capacitor upon application of a potential step

RCtc e

R

EI /

Itotal = Ic + IF

Working electrode choice

• Depends upon potential window desired– Overpotential– Stability of material– Conductivity– contamination

The polarogrampoints a to b

I = E/Rpoints 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 Epoints c to d

when E is sufficiently negative, every molecule that reaches the electrode

surface is reduced.

Dropping Mercury Electrode

• Renewable surface

• Potential window expanded for reduction (high overpotential for proton reduction at mercury)

PolarographyA = 4(3mt/4d)2/3 = 0.85(mt)2/3

Mass flow rate of dropDensity of drop

We can substitute this into Cottrell Equation

i(t) = nFACD1/2/ 1/2t1/2

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 current at the end of the drop life. The average current is

obtained by integrating the current over this time period

iav = 607nD1/2m2/3t1/6C

We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop

Polarograms

E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple)

Usually D’s are similar so half wave potential is similar to formal potential. Also potential 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 currents

Normal Pulse Polarography

•current measured at a single instant in the lifetime of each drop.

•higher signal because there is more electroactive species around each drop of mercury.

•somewhat more sensitive than DC polarography.

•data obtained have the same shape as a regular DCP.

NPP advantage

• IL = nFAD1/2c/(tm)1/2

• (tm = current sampling t)

• IL,N.P./IL,D.C. = (3t/7tm)1/2

• Predicts that N.P.P.

5-10 X sensitive than D.C.P.

Differential pulse voltammetry

DPP

• current measured twice during the lifetime of each drop difference in current is plotted.

• Results in a peak-shaped feature, where the top of the peak corresponds to E1/2, and the height gives concentration

• This shape is the derivative of the regular DC data. • DPP has the advantage of sensitive detection limits and

discrimination against background currents. Traditionally, metals in the ppm range can be determined with DPP.

• Derivative improves contrast (resolution) between overlapping waves

DPP vs DCP

Ep ~ E1/2 (Ep= E1/2E/2)

1

-1

(

cnFAD1/2

mp tI

where E=pulse amplitude

= exp[(nF/RT)(E/2)]

Resolution depends on EW1/2 = 3.52RT/nF whenE0

Improved response because charging current is subtracted and adsorptive effects are discriminated against.l.o.d. 10-8M

Resolution

Square wave voltammetry

SWV

SWV Response

SWV

• advantage of square wave voltammetry is that the entire scan can be performed on a single mercury drop in about 10 seconds, as opposed to about 5 minutes for the techniques described previously. SWV saves time, reduces the amount of mercury used per scan by a factor of 100. If used with a pre-reduction step, detection limits of 1-10 ppb can be achieved, which rivals graphite furnace AA in sensitivity.

•data for SWV similar to DPP

•height and width of the wave depends on the exact combination of experimental parameters (i.e. scan rate and pulse height

Stripping Voltammetry• Preconcentration technique.

1. Preconcentration or accumulation step. Here the analyte species is collected onto/into the working electrode

2. Measurement step : here a potential waveform is applied to the electrode to remove (strip) the accumulated analyte.

Deposition potential

ASV

ASV or CSV

Adsorptive Stripping Voltammetry

• Use a chelating ligand that adsorbs to the WE.

• Can detect by redox process of metal or ligand.

Multi-Element

Standard Addition