Analytical Electrochemistry

الکترو شيمی تجزيه ای

واحد2 –کارشناسی ارشد شيمی تجزيه

مرکز شيراز -حسين توللی: تھيه کننده

Potentiometry

Reference Electrodes

Ideally, reversible (=Nernstian) stable in potential nonpolarizable show little hysteresis with temperature

Calomel Electrode

||Hg2Cl2(sat), KCl (x M)|Hg(l)Hg2Cl2(s) + 2 e– ¾

2 Hg(l) + 2 Cl– (aq)E° = +0.268 V

Hydrogen Electrode

H2 (g), H+(aq)|Pt (s)2 H+ (aq) ¾ H2 (g)

E° = +0.000 V

Inconvenient Easily poisoned Sensitive to oxidants and

reductants

Silver-Silver Chloride Electrode

AgCl (s) + e– ¾ Ag (s) + Cl– (aq) E° = +0.222 V ||AgCl (sat), KCl (x M)|Ag (s) Stable to +275°C

Mercury-Mercurous Sulfate electrode

||Hg2SO4 (sat), K2SO4(x M)|Hg (l) Hg2SO4(s) + 2 e– ¾ 2 Hg (s) + SO4

2– (aq) E° = +0.615 V Useful for low chloride ion applications

Indicator Electrodes

Metallic Inert Membrane

Metallic Electrodes

||Cu2+(aq)|Cu(s) Cu2+(aq) + 2 e– Cu (s)

||AgCl (s)|Ag(s) AgCl(s) + e– Ag (s) + Cl– (aq)

]Cu[1log

2303.2

2F

RTEE

]Cllog[303.2 F

RTEE

Note: 2.303RT/F 0.0592 at 25°C

Inert Electrodes

Pt, Au, or Pd are ideal

Fe3+(aq) + e– ¾ Fe2+(aq)

]Fe[]Fe[log303.2

3

2

F

RTEE

Membrane Electrodes

pH electrode Solid state electrodes Liquid membrane electrodes Gas-sensitive electrodes

Definitions of pH

pH = –log[H+] (Sørensen) pH = –log aH+ (Linderstrøm-Lang) Operational NIST scale

10ln

HHRT

FEEpp stdstd

Glass Electrode Ag(s)|AgCl(s)|HCl (0.1 M)|glass membrane|ext. soln|| Hg(s)|Hg2Cl2(s)|HCl (0.1 M)|glass membrane|ext. soln||

Ext. soln. Int. soln.

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+ H+

500 Å 500 Å

Na+

Na+Na+Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+ Na+

Na+

Na+Na+

Na+Na+

Glass = 22% Na2O, 6% CaO, 72% SiO2 (Corning 015)

Glass Electrode R > 100 M Must be calibrated frequently Other glasses selectively exchange

Na+, Li+, or NH4+

Divalent cation ion-selective glass electrodes not possible due to tight binding of cation

H0592.0]Hlog[0592.0

pEEE

const

constcell

Basic Nernst Equation of a pH Electrode(only the pH sensing cell, not the reference electrode)….

EpH cell constant x 0.05916 x log AH outside

AH inside

the constant is cancelled out by calibration in similar ionic strength solutions

is an efficiency factor that cancels out by calibration in similar ionic strength solutions.

So, what really matters are the activities of hydrogen ions!Don' t forget, electrodes respond to ACTIVITIES, even if we pretend

that they are concentrations to simplify our calculations.

ISFETA “new”pH electrode

What does 0.05916 mean? It is a constant, if there is a

one-electron reaction It can be considered as the

equivalent of a constant of 59.16 mV

A pH meter is a high-impedance potentiometer (measures voltage)

A pH change of “1”imparts a change in 59.16 mV to the potential recorded by the pH meter!

1 pH unit change= 59.16 mV

7.00395.80

6.00336.64

5.00277.48

4.00218.32

3.00159.16

2.00100.00

pHmV (relative readings)

Errors in pH Measurement….

1. Uncertainty in your buffer pH due to normal weighing, diluting errors.2. Junction potential due to the salt bridge and differences in junction

potentials over time due to contamination of the junction1. Overcome by regular recalibration

3. Sodium Error will result in high concentration of sodium solutions. The sodium can also impart a charge across the glass membrane.

4. Acid error (strong acids) can saturate or contaminate the membrane with hydrogen ion!

5. Equilibration Error is overcome by letting the electrode equilibrate with the solution

6. Dried out glass membrane (ruins electrode)7. Temperature. Since temperature affects activities, it is best to have all

solutions at the same, constant temperature!8. Strong bases. Strongly basic solutions (>pH 12) will dissolve the glass

membrane!

Single Crystal Pressed Pellet Electrodes

Membrane composed of a single crystal or pure pellet of an ionic salt

Nearly perfectly selective for the appropriate cation, anion

Fluoride Electrode Ag(s)|AgCl(s)|NaCl, NaF (aq)|LaF3 crystal|ext. soln||

Fluoride is mobile species:LaF3 (s) ¾ LaF2

+(s) + F–(aq) A nearly perfect ion-selective electrode

F0592.0]Flog[0592.0

pEEE

const

constcell

Silver Sulfide Electrode

Sensing membrane is Ag2S Ag+ is mobile speciesAg2S (s) ¾ 2 Ag+ (aq) + S2–(aq)

Ag0592.0]Aglog[0592.0

pEEE

const

constcell

Mixed Pellet Electrodes

Mixed pellets of Ag2S and AgX respond to X–, where X = Cl, Br, I

AgX must be relatively insoluble:AgX (s) ¾ Ag+(aq) + X–(aq)

X0592.0]Xlog[0592.0]X[

log0592.0]Aglog[0592.0

pEE

KEEE

constconst

spconstconstcell

Mixed Pellet Electrodes

Ion-selective electrodes for Cu2+, Cd2+, and Pb2+

can be made byMixing CuS, CdS, or PbS with Ag2S

Note: All mixed pellet Ag2S electrodes also respond to Ag+, so this will be an interferent

Liquid Membrane Electrodes Selectivity depends on the

organic ion exchanger Must be refilled periodically Ion exchanger and solvent in

membrane must be water-immiscible

X0592.0

]Xlog[0592.0

pn

E

nEE

const

nconstcell

Liquid Membrane Electrodes

Membrane typically cellulose acetate Typical ion exchangers:

(RO)2PO2–, R = C8 to C16 in polar solvent Ca2+

RSCH2CO2– Cu2+ or Pb2+

M(o-phen)32+ NO3

–, BF4–, ClO4

– by ion assoc.valinomycin K+ (4000:1 over Na+)

Gas-Sensing Electrode

Gas diffusion into electrode causes shift in pH or solubility equilibrium in the filling solution

Examples:SO2 + H2O ¾ HSO3

– + H+ (pH sensor)

H2S ¾ 2 H++ S2– (S2– sensor)22 SO0592.0]SOlog[0592.0 pEEE constconstcell

SH2

0592.0]SHlog[2

0592.022 pEEE constconstcell

The CO2 Electrode

Used clinically to determine blood CO2concentration

Based on the reactionCO2 + H2O ¾ HCO3

– + H+

pH sensor

2

2

CO0592.0]COlog[0592.0

pEEE

const

constcell

Potentiometric Titrations Determination of acid with NaOH

(pH) Determination of Cl– with AgNO3

(AgCl, Ag+ ISE) Determination of Al3+ with NaF

(AlF63–, F– ISE)

Determination of Fe2+ with K2Cr2O7(Fe3+, Pt)

What would E vs. Volume curves look like?

Electrogravimetry and Coulometry

Distinguishing Characteristics Electrochemical reaction carried out to completion If analyte is deposited, weighed

electrogravimetry If analyte is consumed by current coulometry

Precision

Very precise (0.1% or better) but time-consuming mass can be measured to 0.01mg (and atomic weights

are known to 3 ppm or better charge can be measured accurately, and the Faraday is

known to be 96484.560.27 C/mol

Electrogravimetry

Typically, a metal is electroplated onto a previously weighed electrode

The initial applied potential is usually slightly higher than that predicted for the reduction to account for anode and cathode overvoltage, and resistive losses

Voltage adjusted to maintain a current flow of several amperes

ApparatusforElectro-gravimetry

Controlled-Potential Electrolysis

Electrodeposition of CopperCu2+ + 2 e– Cu (cathode) E° = +0.34 VH2O ½ O2 + 2 H+ + 2 e– (anode) E° = +1.23 V2 H+ + 2 e– H2 (unwanted) E° = +0.00 V Easy reaction carried out at electrode to suppress hydrogen evolution:NO3

– + 10 H+ + 8 e– NH4+ + 3 H2O (depolarizer)

Thus, Cu often plated in a nitrate medium to prevent spongy deposits In chloride media, anodic depolarizers are often used to prevent Cl2

generation:N2H4 N2 + 4 H+ + 4 e–

Electrodeposition Technique

Cathode is removed with current on (why?) Cathode washed and dried quickly to suppress

oxidation Good (=non-flaky) deposition encouraged by

stirring, low current density, depolarizers

Controlled-Potential Coulometry

Measure charge consumed by electrolysis of analyte

Faraday’s Law:w = weight of product in gramsq = charge in coulombsM = atomic weight in g/moln = # of electrons transferredF = Faraday’s constant in C/mol

nFqMw

t

dtiq0

Measurement of Charge (q)

i = current in amperes (C/s)Coulometers:

Precision:Mass-based (0.1%)Electronic (0.01%)

Amperometric Titrations

Voltammetricdetection of

endpoint

Pb2+ + SO42–

PbSO4 (s)

Direct Coulometric Titration at Constant Current Analyte is consumed at either anode or

cathode Auxiliary reactions maintain electrolytic

efficiency (=prevent side reactions) Endpoint is determined by another method,

e.g. potentiometry, photometry, amperometry Analyte is quantified by amount of charge

consumed

Determination of Fe(III) in HBr

Fe(III) consumed at cathode:Fe3+ + e– Fe2+ (desired)2 H+ + 2 e– H2 (undesired)

Toward end of reaction H+ will be easier to reduce than Fe3+

Auxiliary reaction prevents this:TiO2+ + 2 H+ + e– Ti3+ + H2O (auxiliary)Ti3+ + H2O + Fe3+ TiO2+ + 2 H+ + Fe2+ (net)

Indirect Coulometric Titration at Constant Current

Titrant is generated at generator electrode

Endpoint is detected by another method, e.g.,potentiometry or voltammetry

Analyte quantified by charge consumed

Determination of As(III) with I2

Analyte added to solution of buffered NaI Generator electrode (Pt): 2 I– I2 + 2 e–

Titration Reaction: I2 + As(III) 2 I– + As(V) Endpoint can be detected colorimetrically with starch

(I2-starch complex is blue)

Coulometric Titrants

Voltammetric Methods

Votammetry and Polarography Measuring current as a function

of applied potential Conditions chosen to favor

polarization of the working electrode

Polarography dropping mercury electrode

Today, not used so much for analysis as for basic research

Reactions which affect working range for polarizable electrodes Solvent

2 H+ + 2 e– ¾ H2 E° = 0.00 VO2 + 4 H+ + 4 e– ¾ 2 H2O E° = +1.23 VO2 + 2 H+ + 2 e– ¾ H2O2 E° = +0.70 V

ElectrodeHg2+ + 2 e– ¾ Hg E° = +0.80 VHg2Cl2 + 2 e– ¾ 2 Hg + 2 Cl – E° = +0.27 VPt2+ + 2 e– ¾ Pt E° = +1.20 V

Working Range for PolarizableElectrodes

SCE = +0.244 V

Dropping Mercury Electrode Heryovsky, 1922 5-30 drops/min At typical potential scan

rates, each drop period is at constant potential

Insignificant electrolysis of sample

Self-renewing Useful from –1.0 V to

+0.4 V vs. SCE Supporting electrolyte 0.1 -

1.0 M

Typical Polarogram

Residual (charging, condenser) current,ir

Diffusion (limiting) current, id or iL

Half-wave potential, E½

Oxygen Removal is Essential

Air saturated solution is 4 mM 5 A diffusion currentO2 + 4 H+ + 4 e– ¾ 2 H2O E° = +1.23 VO2 + 2 H+ + 2 e– ¾ H2O2 E° = +0.70 V

Sparge solutions with high purity N2 or Ar for 5-20 min Commercial N2 must be passed through V2+ or Cr2+

solution, or over hot Cu to remove trace O2; presaturatewith electrolyte solution

Ilkovic Equation

iD is diffusion current n is number of electrons transferred in redox reaction F is Faraday’s constant A is area of electrode D is diffusion constant of electroactive species The concentration gradient at electrode c/ x c under conditions of

diffusion-limited current Note: Note: iiDD c at diffusion limit, no significant migration, and no c at diffusion limit, no significant migration, and no

significant convectionsignificant convection

0

x

D xcnFADi

Other factors that can limit current Kinetics, e.g. A + B Ox + e– ¾ Red Catalysis, e.g. where a chemical species present oxidizes

Red back to Ox Adsorption: if either Red or Ox is adsorbed to the

electrode surface, imax may be determined by the available surface of the electrode, e.g., adsorption prewave where Red sticks to and inactivates electrode surface

Polarographic Maxima

Complex artifactualphenomenon

Less likely at low drop rates, in concentrated electrolyte, or low concentration of electroactive species

Lessened by inclusion of surfactants in medium

Importance of Charging CurrentLate-drop-life measurements more sensitive

Instrumentation

Small volume (1-10mL) Temperature control desirable Oxygen removal required Three electrode potentiostat

arrangement preferred Polarography uses DME

Modern DME

Drops form very quickly (residual current dies out quickly)

Drops dislodged by “drop-knocker” at reproducible time intervalsmore consistent id values

Potentiostat

Counter electrode carries most of the current Reference electrode must be physically close to working electrode Virtually no current flows between working and reference

electrodeaccurate potential measurement

Quantitative Analysis

iD c, E½ may help identify analyte Must correct iD for residual current Detection limit of classical polarography is about 10

M Precision and accuracy 2-5% typically

Electroactive Species Transition metal ions, e.g., Cu, Tl, Pb, Cd, Zn, Fe, Ni, etc. Strongly hydrolyzed metal ions only in nonaqueous media, e.g. Al,

Th, Zr etc. Not alkali metals—these best handled by flame photometry Oxyanions, e.g. BrO3

–, IO4–, SO3

2–, etc. Inorganic molecules, e.g. O2, H2O2, SOx, NOx etc. Organic species with C–X, N–N, N–O, S–O bonds, enes, ynes,

etc.

Interferences Components whose E½ differ by <100 mV may be

indistinguishable in conventional DC polarography

Typical Applications

Determination of metals in ores, food Determination of morphine (requires nitration to

make electroactive) Determination of DDT (a chlorinated organic

compound) Determination of ascorbic acid in food (anodic

wave)

Dissolved Oxygen (Clark) Electrode Ag|AgCl anode Pt cathode (where O2 is reduced) Maintained at a constant potential of –

0.8 V Molecular oxygen diffuses through

teflon membrane, iL [O2] Working range 0.2-50 ppm Other gases which reduce at –0.8 V will

interfere, e.g. SO2, H2S, X2

Current Sampled Polaragraphy

Potential is applied continuously

Diffusion current is measured only during latter part of drop life, when charging current has largely died out

Less noisy, slightly more sensitive than DC polaragraphy

Pulse Polarography

A short (40 ms) voltage pulse is applied during the last ¼of drop life

Charging current allowed to decay for 20 ms Diffusion current measured during last 20 ms Succeeding pulses increase in E with time More sensitive than conventional DC or current sampled

polarography

Differential Pulse Polarography Pulses of 5-100 mV are superimposed on a linear

increasing voltage gradient Current is sampled just before and near the end of the

pulse, and id is measured The result is a derivative plot ( id / E) of a conventional

DC current sampled polarogram Much more sensitive than conventional DC polarography Excellent resolution of different electroactive species

Comparison of Conventional and Differential Pulse Polarography

Summary of Polarography Techniques

Voltammetry

Slow E scan at constant fixed electrode surface, or

Fast E scan at DME during the lifetime of a single drop

Negligible bulk electrolysis of solution

Voltammetric Parameters

The peak potential,is offset about 28 mV at 25°C

The peak current,is proportional to the square root of the scan ratenF

RTEE p 1.121

212123 cvADknip

Voltammetry Apparatus

Various working microelectrodes used: hanging mercury drop (depicted), glassy carbon, etc.

Typically small volume (1-10 mL)

Stripping Voltammetry

For cations of amalgam-forming metals and anions of sparingly soluble Hg salts

First step is electrochemical deposition (preconcentration) at the Hg electrode

Second step is anodic or cathodic potential sweep, in which peak currents are measured

Extremely sensitive (nM or ppb range) depending on preconcentration time

Anodic Stripping Voltammagram

Typical Applications

Trace metal analysis in water Electrodeposition for 60 s at –1.2 V Differential pulse stripping for analysis

Lead in blood 50 L of blood is wet-ashed Metals electrodeposited for 5 min at –0.7 V Differential pulse stripping for analysis

Cyclic Votammetry

Continuous, cyclic sweeping of potential

Especially useful for reversible reactions

Can provide information about transient electroactive species

Summary of Voltammetry Techniques

Cyclic Voltammetry(CV)

Important parameters: Epa and Epc

ipc and iac

E’ E = |Epa - Epc|

Time, s

E app

, VExcitationExcitation

E1

E2

Eapp, V

I, A

Epa

Epc

E1 E2

Response Response

forw

ard reverse

R - ne- = O

For Nernstian CV

Ep = |Epa - Epc| = 59/n mV at 250C independent of

Eo = (Epa + Epc)/2

Ipc/Ipa = 1

For Nernstian Process Potential excitation controls [R]/[O] as in Nernst

equation:Eapp = E0- 0.059/n log [R]/[O]

if Eapp > E0, [O] ___ [R] and ox occurs if Eapp < E0, [O] ___ [R] and red occurs i.e., potential excitation CONTROLS [R]/[O]

Criteria for Nernstian Process

Ep independent of scan rate

ip 1/2 (diffusion controlled)

Ipc/Ipa = 1 (chemically reversible)

Quasi-reversible or Irreversible

Quasi-reversible: Ep > 59 mV and Ep increases with increasing iR can mascarade as QR system

Irreversible: chemically - no return wave slow ET - 2 waves do not overlap

EXAMPLE: ElectrocatalyticOxidation of Guanine in DNA

Top: non-faradaiccontribution

Bottom: shape and magnitude of redox waves

P.M.Armistead; H.H.Thorp Anal. Chem. 2000, 72, 3764-70.

EXAMPLE: UME’sin Sol-Gels Q: Identify the waves in

the CV’s shown at left Top: UME - slow scan

rate (sigmoidal shape) Bottom: UME - fast scan

rate

Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.

UME’s:

0.1 m

Fe3+

Fast scan rates30 V/s

planar diffusion

Slow scan rates5 mV/s

radial diffusion

0.1 m

UME’s Radial vs. Planar Diffusion

Radial Diffusion Redox wave:

sigmoidal shape Iss = 4nFrDoCo

*

Iss scan rate independent DoCo

*

Planar Diffusion Redox wave:

normal shape Ip 1/2 Do

1/2 C

EXAMPLE: UME’sin Sol-Gels

Learn Do from CA

Obtain Co*from slow scan

rate CV (Iss)

Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.

EXAMPLE 2: Look Ma, No Electrolyte! [S2Mo18O62]4- + e- =

[S2Mo18O62]5- + e- = [S2Mo18O62]6-

BAS 100-A 3-electrode cell:

GC macrodisk/Pt wire/ Pt wire ACN with no electrolyte

Alan M. Bond,* Darren C. Coomber, Stephen W. Feldberg, Keith B. Oldham, and Truc Vu ; Analytical Chemistry; 2001; 73(2); 352-359.

20 mV/s

20 mV/s

100 mV/s

Applications of CV Many organic functional groups are reducible:

C=OC=CC=NN=NS-S

see Handbook of Organic Compounds

Applications of CV Many functional group are not reducible so we

can derivatize these groups convert them into electroactive groups by chemical

modification

EXAMPLES: alcohols + chromic acid = aldehyde group phenyl + nitration = nitro group

Adsorption Phenomena

Non-specifically adsorbed No close-range interaction with electrode Chemical identity of species not important

Specifically adsorbed Specific short-range interactions important Chemical identity of species important

CV and Adsorption If electroactive adsorbed

species: Ep = Eo - (RT/nF) ln (bo/bR) ip = (n2F2/4RT) A o

*

If ideal Nernstian,Epa = Epc and Ep/2 = 90.6 mV/n at 250C

Eapp

I90 mV

EXAMPLE 2: Oxidation of Cysteineat BDD

Nicolae Spãtaru, Bulusu V. Sarada, Elena Popa, Donald A. Tryk, and Akira Fujishima* ; VoltammetricDetermination of L-Cysteine at Conductive Diamond Electrodes, Analytical Chemistry; 2001; 73(3); 514-519.

Stripping Analysis or Stripping Voltammetry 2 Flavors:

Anodic (ASV) Good for metal cations

Cathodic (CSV) Good for anions and oxyanions

Stripping Voltammetry - Steps1. Deposition

2. Concentration

3. Equilibration

4. Stripping

Example of ASV: Determination of Pb at HDME

Deposition (cathodic) reduce Pb2+

Stir (maximize convection) Concentrate analyte

Stop stirring = equilibration/rest period

Scan E in anodic sense and record voltammogram oxidize analyte (so redissolution

occurs)

EappI

Pb Pb2+ + 2e-

Ip

Stripping Voltammetry - Quantitation

Ip Co*

Concentrations obtained using either Standard addition Calibration curve

HDME ASV

Usually study M with Eo more negative than Hg EX: Cd2+, Cu2+, Zn2+, Pb2+

Study M with Eo more positive than Hg at GC EX: Ag+, Au+, Hg

Can analyze mixture with Eo 100 mV

CSV Anodic deposition

Form insoluble, oxidized Hg salt of analyte anion Stir (maximize convection)

Equilibrate (stop stirring) Scan potential in opposite sense (cathodic)

Reducing salt/film and forming soluble anion Record voltammogram

HDME CSV

Can study halides, sulfides, selenides, cyanides, molybdates, vanadates

EX: FDA 1982-1986 used to confirm CN- (-0.1 V) in Tylenol Crisis

Comparison of Potential Methods

Pulse methods Differential pulse

Good selectivity Reason: peak shape

Square wave Good for chromatography Reason: Rapid response

3 min diff. pulse expt = 30 s sq. wave expt

Comparison of Potential Methods LSV

Poorest dl (10-5 M) of any method Reason: inability to distinguish against charging

current CV

Good for mechanistic study

Comparison of Potential Methods

Stripping Voltammetry Good for trace analysis Reasons: lowest dl, most sensitive, good relative

precision EX: 30 min conc. of Ag+ At Hg (ASV)

detection limit = 2 pM relative precision 2-3%

Controlled Current Methods -Chronopotentiometry

Excitation: I vs. time Constant current (step) Linearly increasing

current (ramp) Response E vs. time

Excitation

Response

time

time

I

E

to

to Instrument: galvanostat

Chronopotentiometry

Experimental 3-electrode cell

Luggin capillary Counter isolated with frit Working insulated against

convection Pt, Au, C, Hg pool

quiescent solution

CFrit WR

Sand Equation Response: Boundary condition: I = i/A = nFD (C/x)x=0 = constant

Cx=0 = Co* - (2 it1/2/nFA (Do)1/2)

So, concentration decreases linearly with t1/2

When CX=o = 0 (all O reduced):0 = Co

* - (2 it1/2/nFA (Do)1/2)

So, nFA(Do)1/2Co*/ 2i = 1/2

Note:1. The larger i the smaller 2. < 30 s to minimize convection (natural)

Sand Equation (cont’d)

The Sand Equation (cont’d) At 250C, a more useful form of the Sand equation

is:i 1/2/Co

* = 85.5 n Do1/2 A (mA s1/2/mM)

For 2nd component of 2-component mixture: (n1FAD1

1/2 1/2 C1*/2) + (n2FAD2

1/2 1/2 C2*/2) = I

(1+ 2)1/2

NB: 2 is affected by first reduction

Shape of the Chronopotentiogram

where

when Do = DR,E/4 = Eo

time

E

dl dl

O + e- = R

new rxn

E/4

ttE nF

RTE 2/1

2/12/1

4ln

DDEE

o

RnFRTo

2/1

4ln

Analysis in Chronopotentiometry

Test for reversibility Plot E vs. ln (…) Plot i1/2 vs. I

useful diagnostic for adsorption, coupled reactions

E/4

tt

2/1

2/12/1

ln

Slope: (RT/nF) = 0.059 V/n

E

i

i1/2 adsorption

precedingreactions

Adsorption

ElectroactiveOsoln + e- = R (long t)Oads + e- = R (short t)

Electroinactive

i1/2 i1/2

i i

Applications

Adsorption Coupled Chemical Electrochemical Reactions Quantitation of mixtures of metals

Pb2+, Cd2+, Zn2+ (10-2 - 10-4 M)

Advantages of Chronopotentiometry

Simpler instrumentation No feedback from reference electrode required

Theory simpler and amenable to closed from analytical solution

Can measure higher concentrations - 0.01 M

Disadvantages of Chronopotentiometry

Response waveform less well definedElectroactive impurities that are reduced before

analyte will artificially lengthen transition time and distort wave

Difficult to quantitate at low concentrations

Double layer charging currentsOften largerDifficult to correct for since E is varying

Comparison: Which deals with double-layer capacitance and

uncompensated resistance better? LSV Potential step voltammetry Chronopotentiometry

Jan C. Myland and Keith B. Oldham* ; Which of Three Voltammetric Methods, When Applied to a Reversible Electrode Reaction, Can Best Cope with Double-Layer Capacitance and Severe

Uncompensated Resistance?, Analytical Chemistry; 2000; 72(14); 3210-3217.

Comparison: Which deals with double-layer capacitance and

uncompensated resistance better?LSV Potential step voltammetry Chronopotentiometry

Jan C. Myland and Keith B. Oldham* ; Which of Three Voltammetric Methods, When Applied to a Reversible Electrode Reaction, Can Best Cope with Double-Layer Capacitance and Severe

Uncompensated Resistance?, Analytical Chemistry; 2000; 72(14); 3210-3217.

Exercise

Q: What Experiment is This? Name of experiment

type of excitation

Response i ____ slope

Deficiency

Excitation

Response

time

time

E

I

to

to

What Experiment Is This? Name of experiment

Type of excitation

Response Q ____ intercept slope

Excitation

Response

time

time

E

Q

to

to

Qdl

Q: What Is This Experiment? Name of experiment

Excitation

Response i ____ Ep ____ of E’ = _____________

Time, s

E app

, VExcitationExcitation

E1

E2

Eapp, V

I, A

Ep

E1 E2

Response Response

Eo

Eo X

X

Exercise E1

Identify the species that have undergone oxidation and reduction in the reaction CuS(s) + O2(g) Cu(s) + SO2(g)

Solution: The Cu(II) on the left is reduced from the +2 to the 0 oxidation state. The S2- on the left has been oxidized from the -2 to the +4 oxidation state. The O on the left has been reduced from the 0 to the -2 oxidation state.

Exercise E2

Express the formation of H2O from H2 and O2 in acid solution as the difference of two reduction half-reactions.

Solution: The overall reaction is 2 H2(g) + O2(g) 2 H2O(l)

Oxygen is reduced according to O2 2 H2O and in acid solution this is balanced by adding H+ and e to getO2(g) + 4 H+(aq) + 4e 2 H2O(l)

The reaction to be subtracted from this is 4 H+(aq) + 4 e 2 H2(g)

Exercise E3 Express the oxidation of NADH (nicotinamide adenine

dinucleotide, which participates in the chain of oxidations that constitutes respiration) to NAD+ by oxygen, when the latter is reduced to H2O2 in aqueous solution, as the difference of two reduction half-reactions.

Solution: O2 is the oxidizing agent; therefore O2is reduced: O2 + 2 H+ + 2 e H2O2

The reverse oxidation reaction to be subtracted from this is then NAD+ + H+

+ 2 e NADH

Exercise E4

Write the half-reaction and the reaction quotient for a chlorine-gas electrode.

Solution: Written as a reduction, the half-reaction at the chlorine electrode is ½ Cl2(g) + e Cl(aq)

And the reaction quotient then becomes Q = a(Cl) / a(Cl2 )½

Exercise E5 Write the half-reaction and the reaction quotient for the

calomel electrode, Hg(l)|Hg2Cl2(s)|Cl(aq), in which mercury(I) chloride (calomel) is reduced to mercury metal in the presence of chloride ions. This electrode is a component of instruments used to measure pH, as explained later.

Solution: The reduction of Hg2Cl2 to Hg is given by Hg2Cl2(s) + 2 e 2 Hg(l) + 2 Cl(aq)

Notice that everything except the chloride ion is a pure solid or liquid. Therefore only the chloride appears in the reaction quotient. Q = a(Cl2

Cells

Varieties of Cell

The two basic types are concentration cells and chemical cells.

Concentration cells are either electrolyte concentration cells, where the electrode compartments are identical except for the concentrations of the electrolytes, or electrode concentration cells, in which the electrodes themselves have different concentrations, such as amalgams or gas electrodes at different pressures.

Most cells are chemical cells.

Two Versions of the Daniell Cell

Constructing a Daniell Cell

Two Practical Cells At left is a

pri-mary cell (used once only).

At right is a secon-dary cell (may be re-charged)

Cell Notation

In the version of the Daniell cell with the porous pot there is a liquid junction. This is denoted as Zn(s)|ZnSO4(aq):CuSO4(aq)|Cu(s)

When the liquid junction potential has been essentially eliminated by use of a salt bridge the Daniell cell is denoted Zn(s)|ZnSO4(aq)||CuSO4(aq)|Cu(s)

Other punctuation in cell notations includes a comma to separate two species present in the same phase.

Cells with a Common Electrolyte

A cell in which the anode is a hydrogen electrode and the cathode is a silver-silver chloride electrode is denoted Pt|H2(g)|H+(aq), Cl(aq) | . AgCl(s)|Ag(s)

or, without the comma, Pt|H2(g)|HCl(aq)|AgCl (s)|Ag(s)

Exercise E6

Give the notation for a cell in which the oxidation of NADH by oxygen could by studied.

Solution: Refer to Exercise E3, where the half-reactions are O2 + 2 H+ + 2 e H2O2 and NAD+ + H+ + 2 e NADH

Putting the oxidation half-reaction into conven-tional cell notation, as written on the left, it becomes Pt|NADH(aq), NAD+(aq), H+(aq)

The reduction half-reaction is written Pt|O2(g)|H+(aq),H2O2(aq)

Putting these together, Pt|NADH(aq), NAD+(aq), H+(aq)|| H2O2(aq), H+(aq)|O2(g) | Pt

Exercise E7

Write the chemical equation for the cell in Exercise E6. Solution: We can again refer to Exercise E3, where the

half-reactions are O2 + 2 H+ + 2 e H2O2 and NAD+ + H+ + 2 e NADH

Subtracting the oxidation half-reaction from the reduction half-reaction gives NADH(aq)+ O2(g) + H+(aq) NAD+ (aq) + . H2O2 (aq)

The Cell Potential

Since w = G = work output, and since electrical work output = (charge) x (vol-tage) = FE ,

G = FE A spontaneous rxn

has a neg. G and a pos. E E is intensive

Electrochemical sensors Expected to exhibit changes in resistance (conductivity) or

changes in capacitance (permittivity) due to substances or reactions.

These may carry different names. Potentiometric sensors do not involve current –

measurement of capacitance and voltage. Amperimetric sensors rely on measuring current Conductimetric sensors rely on measurement of

conductivity (resistance).

Electrochemical sensors These are different names for the same properties since

voltage, current and resistance are related by Ohm’s law. Electrochemical sensors include a large number of

sensing methods, all based on the broad area of electrochemistry. Many common sensors including fuel cells, surface conductivity sensors, enzyme electrodes, oxidation sensors and humidity sensors belong to this category.

Potentiometric sensors A large subset of electrochemical sensors Principle: electric potential develops at the surface of a

solid material immersed in solution containing ions that exchange at the surface.

The potential is proportional to the number or density of ions in the solution.

A potential difference between the surface of the solid and the solution occurs because of charge separation at the surface.

Potentiometric sensors The contact potential, analogous to that used to set up a voltaic cell

cannot be measured directly. If a second electrode is provided, an electrochemical cell is setup and

the potential across the two electrodes is directly measurable. To ensure that the potential is measured accurately, and therefore

that the ion concentration is properly represented by the potential, it is critical that the current drawn by the measuring instrument is as small as possible (any current is a load on the cell and therefore reduces the measured potential).

Potentiometric sensors For a sensor of this type to be useful, the potential

generated must be ion specific – that is, the electrodes must be able to distinguish between solutions.

These are called ion-specific electrodes or membranes. The four types of membranes are: Glass membranes, selective for H+, Na+ and NH4

+ and similar ions.

Potentiometric sensors Polymer-immobilized membranes: In this type of

membrane, an ion-selective agent is immobilized (trapped) in a polymer matrix. A typical polymer is PVC

Gel-immobilized enzyme membranes: the surface reaction is between an ion specific enzyme which in turn is either bonded onto a solid surface or immobilized into a matrix - mostly for biomedical applications

Soluble inorganic salt membranes: either crystalline or powdered salts pressed into a solid are used. Typical salts are LaF3 or mixtures of salt such as Ag2S and AgCl. These electrodes are selective to F, S and Cl and similar ions.

Glass membrane sensors By far the oldest of the ion-selective electrodes, Used for pH sensing from the mid-1930’s and is as

common as ever. The electrode is a glass made with the addition of sodium

(Na2O) and aluminum oxide (Al2O3), Made into a very thin tube-like membrane. This results in a high resistance membrane which

nevertheless allows transfer of ions across it. The basic method of pH sensing is shown in figure

pH sensor

pH sensor

Consists of the glass membrane electrode on the left and a reference electrode on the right.

The reference electrode is typically an Ag/AgCl electrode in a KClaqueous solution or a saturated Calomel electrode (Hg/Hg2Cl2 in a KClsolution).

The reference electrode is normally incorporated into the test electrode so that the user only has to deal with a single probe as shown in Figure

The sensor is used by first immersing the electrode into a conditioning solution of Hcl (0.1.mol/liter) and then immersing it into the solution to be tested. The electric output is calibrated in pH.

A sensor of this type responds to pH from 1 to 14.

pH probe with reference electrode

Glass membrane sensors

Modifications of the basic configuration, both in terms of the reference electrode (filling) as well as the constituents of the glass membrane lead to sensitivity to other types of ions as well as to sensors capable of sensing dissolved gas in solutions, particularly ammonia but also CO2, SO2, HF, H2S and HCN

Soluble inorganic salt membrane sensors Based on soluble inorganic salts which undergo ion-

exchange interaction in water and generate the required potential at the interface.

Typical salts are the lanthanum fluoride (LaF3) and silver sulfide (Ag2S).

The membrane may be either a singe crystal membrane, a sintered disk made of powdered salt a polymer matrix embedding the powdered salt each has its own application and properties

Soluble inorganic salt membrane sensors The structure of a commercial sensor used to sense

fluoride concentration in water is shown next The sensing membrane, made in the form of a thin disk

grown as a single crystal. The reference electrode is created in the internal solution

(in the case: NaF/NaCl at 0.1 mol/liter). The sensor shown can detect concentrations of fluoride in

water between 0.1 and 2000 mg/l. This sensor is commonly used to monitor fluoride in

drinking water (about 1mg/l).

Soluble inorganic salt membrane sensors for fluoride

Soluble inorganic salt membrane sensors Membranes may be made of other materials such as

silver sulfide. The latter is easily made into thin sintered disks from

powdered material and may be used in lieu of the single crystal.

Other compounds may be added to affect the properties of the membrane and hence sensitivities to other ions.

This leads to selective sensors sensitive to ions of chlorine, cadmium, lead and copper and are often used to sense for dissolved heavy metals in water.

Polymeric salt membranes

Polymeric membranes are made by use of a polymeric binder for the powdered salt

About 50% salt and 50% binding material. The common binding materials are PVC,

polyethylene and silicon rubber. In terms of performance these membranes are

quite similar to sintered disks.

Polymer-immobilized ionophoremembranes A development of the inorganic salt membrane Ion-selective, organic reagents are used in the production

of the polymer by including them in the plasticizers, particularly for PVC.

A reagent, called ionophore (or ion-exchanger) is dissolved in the plasticizer (about 1% of the plasticizer).

This produces a polymer film which can then be used as the membrane replacing the crystal or disk in sensors.

Polymer-immobilized ionophoremembranes

The construction of the sensor is simple

Shown in Figure 8.9 and includes an Ag/AgCl reference electrode.

The resulting sensor is a fairly high resistance sensor.

Polymer-immobilized ionophoremembranes A different approach to building

polymer-immobilized ionophoremembranes is shown in Figure It is made of an inner platinum wire on which the polymer membrane is coated

The wire is protected with a coating of paraffin.

This is called a coated wire electrode. To be useful a reference membrane

must be added.

Gel-immobilized enzyme membranes Similar in principle to polymer immobilized ionophore

membranes A gel is used and the ionophore is replaced by an enzyme

which is selective to a particular ion. The enzyme, (a biomaterial) is immobilized in a gel

(polyacrylamide) and held in place on a glass membrane electrode as shown in Figure The choice of the enzyme and the choice of the glass electrode define the selectivity of the sensor.

Gel-immobilized enzyme membrane sensor

Gel-immobilized enzyme membrane sensors Gel sensors exist for the sensing of a variety of important

analytes including urea glucose, L-amino acids, penicillin and others.

The operation is simple; the sensor is placed in the solution to be sensed which diffuses into the gel and reacts with the enzyme.

The ions released are then sensed by the glass electrode. These sensors are slow in response because of the need

for diffusion but they are very useful in analysis in medicine including blood and urine.

The Ion-sensitive field effect transistor ISFET Also called the ChemFet Essentially a MOSFET in which the gate has been

replaced by an ion-selective membrane. Any of the membranes above may be used - most often

the glass and polymeric membranes In its simplest form, a separate reference electrode is

used but the reference electrode may be easily incorporated within the gate structure as shown in Figure.

Ion-sensitive field effect transistor ISFET

Ion-sensitive field effect transistor ISFET The gate is then allowed to come in contact with

the sample to be tested The drain current is measured to indicate the ion

concentration. The most important use of this device is

measurement of pH Available commercially.