Encyclopedia of Electrochemistry, Organic Chemistry

641 Pages·2002·5.59 MB·English

by  Bard A.J.

1
1
Methods to Investigate
Mechanisms of Electroorganic
Reactions
BerndSpeiser
.. ..
InstitutfurOrganischeChemie,AufderMorgenstelle18,Tubingen,Germany
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Scope:MethodsofMolecularElectrochemistry . . . . . . . . . . . . . . . 3
1.1.2 HistoricalDevelopment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 WhyandHowtoInvestigateMechanismsofElectroorganicReactions 4
1.2.1 StepsofElectrodeReactionMechanisms . . . . . . . . . . . . . . . . . . . 4
1.2.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1.3 ElectronTransfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1.4 ChemicalKineticSteps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1.5 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 OrganicElectrodeReactionMechanisms . . . . . . . . . . . . . . . . . . . 6
1.2.2.1 ElectronTransferInitiatesChemistry . . . . . . . . . . . . . . . . . . . . . 6
1.2.2.2 NomenclatureofElectrodeReactionMechanisms . . . . . . . . . . . . . 6
1.2.3 FormalDescriptionofEventsatanElectrode . . . . . . . . . . . . . . . . 7
1.2.3.1 Current-Potential-TimeRelationships . . . . . . . . . . . . . . . . . . . . . 7
1.2.3.2 ConcentrationProfiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.4 MethodsofMechanisticElectroorganicChemistry . . . . . . . . . . . . 7
1.2.4.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.4.2 Controlled-PotentialTechniques . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.4.3 Controlled-CurrentTechniques . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.2.4.4 HydrodynamicVoltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.4.5 ExhaustiveElectrolysisTechniques . . . . . . . . . . . . . . . . . . . . . . . 13
1.3 HowtoGainAccesstoKinetics,Thermodynamics,andMechanisms
ofElectroorganicReactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.1 Qualitative and Quantitative Investigation of Electrode Reaction
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.2 GeneralRecommendationsforMechanisticAnalysis . . . . . . . . . . . 14
2 1 MethodstoInvestigateMechanismsofElectroorganicReactions
1.3.3 SomeMechanisticExamples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3.3.1 PureETReactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3.3.2 Follow-upReactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.3.3.3 PreequilibriatoETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3.3.4 CatalyticReactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.4 Howto GainAdditionalInformationabout ElectroorganicReaction
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.4.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.4.2 Ultramicroelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.4.3 Electrogravimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.4.4 Spectroelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3
1.1 molecular electrochemistry[1] or dynamic
Introduction electrochemistry[2]havebeenusedforthat
part of electrochemistry that studies the
1.1.1
mechanisticeventsatornearanelectrode
Scope:MethodsofMolecular
onamolecularlevel.
Electrochemistry
There are a large number of methods
(oftenalsocalledelectroanalyticalmethods)
Reactionmechanismsdividethetransfor-
for such studies of which only the most
mations between organic molecules into
important ones can be covered in this
classes that can be understood by well-
chapter. Moreover, technical details of
defined concepts. Thus, for example, the
the methods cannot be described, and
S 1orS 2nucleophilicsubstitutionsare
N N emphasis will be placed on their use in
examplesoforganicreactionmechanisms.
mechanisticelectroorganicchemistry.
Eachmechanismischaracterizedbytran-
sition states and intermediates that are
1.1.2
passed over while the reaction proceeds.
HistoricalDevelopment
Itdefinesthekinetic,stereochemical,and
product features of the reaction. Reaction
Although organic electrochemistry had
mechanismsarethusextremelyimportant
alreadybeenestablishedinthenineteenth
to optimize the respective conversion for
century, only the 1960s saw the advent
conditions,selectivity,oryieldsofdesired of detailed electroorganic mechanistic
products. studies.
Reaction mechanisms are also defined Mostofthetechniquesemployedcanbe
for electroorganic reactions, induced by tracedbacktopolarography,whichwasal-
or including an electron transfer at an ready in use in 1925, to determine the
electrode. Knowledge of such electrode concentrations of organic molecules[3].
reaction mechanisms includes, prefer- Technical developments in instrumenta-
ably but not exclusively, the potential at tion(potentiostats)[4],theuseofnonaque-
which the reaction proceeds, the proof ouselectrolytes[5],andthedigitalcontrol
of intermediates, the electron stoichiom- of experiments[6] led to the spread of
etry, the kinetics of the various reaction electroanalyticaltechniques.Forexample,
steps, and the transport properties of cyclic voltammograms are frequently and
the species involved. Recently, the terms routinely used today to define the redox
4 1 MethodstoInvestigateMechanismsofElectroorganicReactions
properties of newly synthesized organic (substrate)fromthebulkoftheelectrolyte
compounds similar to the use of NMR to the electrode plays an important, often
spectraforstructuralcharacterization. rate-determiningrole. The electrontrans-
Numerical simulation of the experi- ferstepoccursattheinterface.Theproduct
ments[7] became increasingly available of the redox reaction is transported back
during the 1980s, and ultramicroelec- tothebulk.Purelychemicalreactionsmay
trodes[8] opened the way not only to precede or follow these steps. Specific in-
ever-faster timescales but also to finer teractions of any species present in the
lateralresolutionwhencharacterizingelec- electrolytewiththeelectrodesurfaceleads
trode processes. Finally, combinations toadsorption,whichmayconsiderablyin-
withspectroscopicandmass-sensitivede- fluencetheoverallprocess.
vicesopenednewways to augment infor-
mation available from molecular electro-
1.2.1.2 Transport
chemicalexperiments.
Three types of mass transport are impor-
This development contributes to a still-
tantatanelectrode:
increasing body of knowledge about the
fate of organic molecules upon oxidation 1. Diffusion (along a concentration gradi-
andreduction. ent)isobservedifthesolutionnearthe
electrodeisdepletedfromasubstrateor
a product is accumulated. Diffusion is
1.2
characterizedby a diffusion coefficient
WhyandHowtoInvestigateMechanisms D (typical value: 10−5cm2/s) and ex-
ofElectroorganicReactions
tendsoveradiffusionlayer(thickness:
δ)thatdevelopsfromtheelectrodeinto
1.2.1
StepsofElectrodeReactionMechanisms the electrolyte. At the outward bound-
ary the concentrations approach their
1.2.1.1 General bulkvalues.
As heterogeneous reactions at the inter- 2. Migration (in the electrical field be-
faceelectrode–electrolyte,electrochemical tween the anode and the cathode)
reactions are intrinsically more complex contributestothemovementofcharged
than typical (thermal) chemical transfor- species.Inmostpracticalexperiments,
mations(Figure1).Wemostlyneglectthe however,theconcentrationofsupport-
exact structure of the interface in the fol- ing electrolyte ions is much higher
lowingdescription.Transportoftheeduct (100–1000:1) than that of other ions.
Adsorption Diffusion layer Bulk
E E E’
Transport
e
Electrod E Etrlaencstfreorn Crehaecmtioicnasl Fig.1 Stepsconstitutinga
P typicalorganicelectrode
Transport reaction;E,E(cid:1):educt,P,P(cid:1):
P P P’
product;circlesindicate
d adsorbedmolecules.
1.2 WhyandHowtoInvestigateMechanismsofElectroorganicReactions 5
(cid:1) (cid:2)
Hence, migration of the latter is sup- c nF
ox =exp (E−E0) (1)
pressed.Ontheotherhand,migration c RT
red
becomes important at modified elec-
trodes or in electrolytes of low ion (n=number of electrons transferred,
concentration[9]. F =Faraday constant, R =gas con-
3. Convection (of the electrolyte liquid stant,T =temperature).Thecurrentis
phase as a whole) can be natural (due proportionaltotheamountofmaterial
tothermaleffectsordensitygradients) transported to the electrode in a time
or forced (principal mass transport unit.
mode in hydrodynamic techniques). 2. ET much slower than transport (ET
Still, however, close to the electrode control). The current follows the But-
surfaceadiffusionlayerdevelops. ler–Volmerequation(2)
(cid:3) (cid:1) (cid:2)
−αnF
Ifweneglectmigration,experimentscan i =i exp (E−E0)
0
beperformedunderconditionsofminimal RT
(cid:1) (cid:2)(cid:4)
convection, which are thus dominated (1−α)nF
by diffusion. Since δ increases with −exp (E−E0) (2)
RT
time t in such a case, nonstationary
conditions exist. On the other hand, if where i0 defines the exchange current
convection dominates in the electrolyte at E =E0 (irreversible ET). A physical
bulk,δ (cid:2)=f(t),andweapproachstationary interpretation of α is related to the ET
conditions,asfarasdiffusionisconcerned. transitionstate(seethecomprehensive
discussioninref.[10]).Itisfurthermore
1.2.1.3 ElectronTransfer expected that α is potential dependent
Theelectrontransfer(ET)attheinterface andimportantmechanisticconclusions
betweenelectrodeandelectrolyteiscentral follow[11,12].
to an electrode reaction. Electrons pass 3. ET and transport have comparable
throughtheinterface.Macroscopicallywe rates. This mixed-control situation is
observeacurrenti. characterizedasquasi-reversible.
The transfer of an electron to (reduc-
tion)orfrom(oxidation)thesubstrateisan A given electrode reaction may corre-
activated process, characterized by a rate spondtoanyofthesesituationsdepending
constant k , defined as the standard (or ontheexperimentalconditions,inparticu-
s
formal)potentialE0,andthetransfercoef- larontheexternalcontrolofmasstransfer.
ficient α. The three situations mentioned
belowcanbedistinguished: 1.2.1.4 ChemicalKineticSteps
Mostelectrodereactionsofinteresttothe
1. ET much faster than transport (trans- organic electrochemist involve chemical
port control). Electrochemical equilib- reactionsteps.Theseareoftenassumedto
riumisattainedattheelectrodesurface occurinahomogeneoussolution,thatis,
atalltimesanddefinedbytheelectrode notattheelectrodesurfaceitself.Theyare
potentialE.Theconcentrationsc and described by the usual chemical kinetic
ox
c of oxidized and reduced forms of equations, for example, first- or second-
red
the redox couple, respectively, follow order reactions and may be reversible
theNernstequation(1)(reversibleET) (chemicalreversibility)orirreversible.
6 1 MethodstoInvestigateMechanismsofElectroorganicReactions
Chemical steps may precede or follow strong acids or bases are not necessary)
the transport and ET processes. In the and/ortheadditionalselectivityintroduced
former case, the electroactive species is incontrolled-potentialexperiments.
formed in a preequilibrium. In the latter The reaction mechanisms of organic
case, we produce by ET some reactive electrodereactionsarethuscomposedofat
species, which undergoes a (possibly leastoneETstepattheelectrodeaswellas
complex) chemical transformation to a preceding and follow-up bond-breaking,
morestableproduct. bond-forming, or structural rearrange-
ment steps. These chemical steps may
be concerted with the electron trans-
1.2.1.5 Adsorption
fer[15, 16]. The instrumental techniques
The involvement of specific attractive in-
described in this chapter allow the in-
teractionsofmoleculeswiththeelectrode
vestigation of the course of the reaction
surface (adsorption) makes the electrode
accompanyingtheoverallelectrolysis.
process even more complex. The inten-
sityofsuchinteractionsrangesfromweak
(physisorption)tostrong(chemicalbonds 1.2.2.2 NomenclatureofElectrode
ReactionMechanisms
formedbetweenadsorbateandelectrode).
In order to classify the various mech-
Forsomecommonorganicelectrochem-
anisms of organic electrode reactions,
ical reactions, for example, the Kolbe
a specific nomenclature has been de-
electrolysisofcarboxylates[13],theadsorp-
veloped[17]. It is often extended in an
tionofintermediateshasbeendiscussed.
informal way to accommodate particular
1.2.2 reaction features, and one may find addi-
OrganicElectrodeReactionMechanisms tionalordeviantsymbols.
Usually, however, electron transfers
1.2.2.1 ElectronTransferInitiates at the electrode are denoted by ‘‘E’’,
Chemistry while chemical steps not involving the
Themajorityoforganicelectrodereactions electrode are denoted by ‘‘C’’. The ET
is characterized by the generation of a may further be characterized as ‘‘E ’’,
r
reactiveintermediateattheelectrodebyET ‘‘E ’’, or ‘‘E’’ in the reversible, quasi-
qr i
and subsequent reactions typical for that reversible,orirreversiblecase.Itisusually
species. Thus, the oxidation or reduction notindicatedhowtransportoccurs.Ifthe
step initiates the follow-up chemistry to C-stepisadimerization,thesymbol‘‘D’’is
the reaction products (‘‘doing chemistry common,whileanETbetweentwospecies
withelectrodes’’[14]). in a (homogeneous) solution is denoted
Species with electron deficiency (e.g. ‘‘SET’’ (for solution electron transfer)[18]
carbocations), unpaired electrons (e.g. or‘‘DISP’’(see,e.g.[19]).
radicals, radical ions), electron excess For more complex mechanisms, pic-
(e.g. carbanions), or those with unusual turesque names such as square, ladder,
oxidationstates(e.g.metalcomplexeswith fence[18]orcubicschemes[20]havebeen
low- or high-valent central atoms) are selected. In redox polymer films, addi-
producedattheelectrode.Electrochemical tional transport of counterions, solvation,
generation of such intermediates may be and polymer reconfiguration are impor-
advantageousbecauseofthemildreaction tantandfour-dimensionalhyper-cubesare
conditions employed (room temperature, neededtodescribethereactions[21].
1.2 WhyandHowtoInvestigateMechanismsofElectroorganicReactions 7
1.2.3 profile. In general, the electrode islocated
FormalDescriptionofEventsatan at x =0, and the electrolyte extends into
Electrode the positive x half-space. The bulk of
the solution is assumed at the right-hand
1.2.3.1 Current-Potential-Time
side of the profile. Often, concentration
Relationships
values are normalized with respect to
The equations given in Section1.2.1 in-
the bulk concentration of one species,
clude the most important quantities for
and space coordinate values are normal-
understanding a reaction mechanism at
ized with respect to the extension of
an electrode: current i, potential E, and
the diffusion layer δ. Such concentra-
time t. Consequently, most techniques to
tion profiles will be used in the following
investigateelectroorganicreactionmecha-
discussion.
nismsinvolvethedeterminationofi orE
asafunctionoftime(whiletheotherone
1.2.4
ofthesequantitiesiskeptconstant)orasa MethodsofMechanisticElectroorganic
functionofeachother(whileoneisvaried Chemistry
witht inadefinedmanner).
Similari–E–t relationshipsarederived 1.2.4.1 Classification
theoretically from basic equations (simu- Oneofseveralpossibilitiestoclassifyelec-
lation, see Section1.4.1), on the basis of troanalyticalmethodsisbasedonthequan-
a hypothesisforthe reactionmechanism, tity that is controlled in the experiment,
and the experimental and the theoretical that is, current or potential. Alternatively,
resultsarecompared.Inthisway,thehy- since diffusion is an important mode of
pothesis is either disproved, or proven to mass transport in most experiments, we
be consistent with the events at the elec- distinguish techniques with stationary or
trode. nonstationary diffusion. Finally, transient
methodsaredifferentfromthosethatwork
1.2.3.2 ConcentrationProfiles inanexhaustiveway.
The current through the electrode is pro- Onlyasmallselectionofthevariantsin
portional to the flux of redox-active mate- theelectrochemicalliteraturecanbemen-
tionedhere.Thus, impedancetechniques
rialtothesurface,which,inturnisrelated
to the concentrations c of various species (small amplitude sinusoidal perturbation
at the electrode with observation of the
neartheinterface.Thus,anequivalentde-
system’s response[22]) as well as polaro-
scription is based on the dependence of
c on space x and t. Often a single space- graphic methods (at mercury electrodes)
will not be described. Since the notion of
coordinatesuffices.Morecomplexsystems
areactionmechanismrequiresconsump-
(e.g.ultramicroelectrodes)mayrequireup
tionofsubstance,equilibriumtechniques
tothreespace-coordinates.
(such as potentiometry) will also not be
Althoughitisdifficulttodeterminethe
discussedhere.
spatial distribution of species experimen-
tally, it provides an illustrative view of
the electrode reaction. Simulations usu- 1.2.4.2 Controlled-PotentialTechniques
allyprovidevaluesofc=f(x,t)foreach Control of the potential E of that elec-
species as the primary result. The space trode where the electrode reaction occurs
dependenceofc istermedaconcentration (working electrode) is accomplished by a
8 1 MethodstoInvestigateMechanismsofElectroorganicReactions
Potentio/galvanostat
Computer,
Recorder
E
Function
generator
R
C W
Electrolyte
Fig.2 Schematicrepresentationofexperimentalset-upfor
controlled-potentialexperiments;W:working,C:counter,R:
referenceselectrodes.
potentiostat in a three-electrode arrange- transport (diffusion) limited region. After
ment (Figure2). The current is passed some (pulse) time τ, E may be switched
through the working (W) and counter back to E or another appropriate value
R
(C) electrodes, while E is measured with (double-stepchronoamperometry).
respecttoacurrentlessreference(R)elec- Starting at E guarantees that at t <
R
trode. Often, a recording device and a 0, the concentration of the redox-active
functiongeneratorcomplementtheexper- compound A, c , equals c0 at all x.
A A
imentalsetup. The product concentration c is usually
B
Wewillassumeasimplereversibleone- assumed to be zero. After E is stepped,
electron redox process A−(cid:4)−±−−e−(cid:3)−−B in all the concentrations of A and B at x =0
adjust to conform to equation(1). These
casestointroducethetechniques.
concentrations deviate from the bulk
An important property of the solution
concentrationsthat remainat their initial
tobeinvestigatedistherestoropen-circuit
values throughout the experiment, and
potential E .Thisisthe potentialthatthe
R
a concentration gradient develops. As a
workingelectrodedevelopsinthesolution
result,Adiffusestotheelectrode,whileBis
at equilibrium, that is, when no current
producedatx =0anddiffusestothebulk.
flows through the electrode. The value of
E depends on the components of the The resulting diffusion layer grows into
soRlutionandtheelectrodeitself. thesolutionwitht (typically10−2cmafter
Chronoamperometry is a technique in 10s in common organic solvents). The
which a potential step is applied to the steepness of the concentration gradient
workingelectrodeinaquietsolutionatt = is high shortly after t =0, and decreases
0(Figure3).Initially(t <0),theelectrode thereafter.This isreflectedinthe current
attains E . For t >0, a potential is responsegivenbytheCottrellequation(3)
R
selected,whichdrivesthedesiredelectrode √
reaction. Often, but not necessarily (see, nFAc0 D
i = √A (3)
e.g.References[23–25])thelatterisinthe πt
1.2 WhyandHowtoInvestigateMechanismsofElectroorganicReactions 9
0.6 30
0.5 20
6
E 0.4 10 10
Potential, [V] 000...123 i urrent, • [A]−−21000
0.0 C −30
−0.1 −40
0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0
Time, t Time, t
(a) [s] (b) [s]
1.2 1.2
c 1.0 c 1.0
n, 0.8 n, 0.8
o o
ati 0.6 ati 0.6
ntr 0.4 ntr 0.4
e e
nc 0.2 nc 0.2
o o
C 0.0 C 0.0
−0.2 −0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
(c) Distance, x (d) Distance, x
Fig.3 Chronoamperometry:(a)typical profilesrespectivelyatvarioustimes,increasing
excitationsignal;(b)currentresponse;and timeshownbyarrows]foradouble-step
concentrationprofiles[(c)firststep;(d)second chronoamperometricexperiment(pulsetime
step;educt:solidlines,product:dottedlines;five τ =1s).
in the most simple case (with A= thechargeQ,isrecorded(Figure4).This
e√lectroactive area of the electrode). Thus, quantitycontinuouslyincreasesduringthe
i t isaconstant,andaplotofi vs.t−1/2 first part of the experiment (0<t <τ).
isastraightline. Integrationofequation(3)yields
Switching back E to E causes the √
concentrations at x =0 to rReturn to their Q= 2nFA√cA0 D√t (0<t <τ) (4)
original values with the concentration π
profiles changing accordingly. Now, B,
As soon as the potential is stepped back
which has accumulated in the diffusion
suchthatacurrentinthereversedirection
layer, diffuses toward the electrode and
flows, the accumulated charge decreases
is transformed back to A. We observe a
asshown:
currentintheoppositedirection. √
AnyreactionthatremovesBfromtheso- 2nFAc0 D √ √
Q= √A ( t − t −τ)
lutionwillinfluencethecurrentresponse, π
allowing qualitative mechanistic conclu-
(τ <t <2τ) (5)
sions. Furthermore, quantitative analysis √
of chronoamperometric curves includes Thus, a√plot o√f Q vs. t for the first, and
determinationofn,A,orD,providedtwo Q vs. t − t −τ for the second part
ofthesequantitiesareknown. of the curve results in two straight lines
Chronocoulometry is similar to chrono- (‘‘Anson plot’’[26]). Adsorption of redox
amperometry, but the time integral of i, active species can simply be diagnosed
10 1 MethodstoInvestigateMechanismsofElectroorganicReactions
3.7 4
60 3.2 60 3
1 2.2 1 2
Q ge, • [C] 11..27 Q ge, • [C] 01
har 0.7 har −1
C 0.2 C −2
−0.−30.2 0.3 0.8 1.3 1.8 2.3 −30.0 0.2 0.4 0.6 0.8 1.0 1.2
Time, t Square root of time, t1/2
(a) [s] (b) [s1/2]
Fig.4 Chronocoulometry:(a)typicalchargeresponse;(b)Ansonplotforadouble-step
chronocoulometricexperiment.
iftheextrapolatedAnsonplotlinesdonot thanchronoamperometry.Again,n,A,or
crossclosetotheorigin[27].Aninteresting D are accessible from chronocoulometric
characteristic of the chronocoulometric data.
curve is that Q(2τ)/Q(τ)=0.414, if no Linearsweepandcyclicvoltammetry(LSV
follow-up reaction destroys B. If B reacts, and CV) are probably the most widely
however,thischargeratioincreases. used techniques to investigate electrode
Because of its integral nature, chrono- reaction mechanisms. They are easy to
coulometry is less susceptible to noise apply experimentally, readily available in
0.7 400
0.6 300
6
E 0.5 10 200
Potential, [V] 0000....1234 i urrent, • [A]−1100000
0.0 C −200
−0.1 −300
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 −0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Time, t Potential, E
(a) [s] (b) [V]
1.2 1.2
c 1.0 c 1.0
n, 0.8 n, 0.8
o o
ati 0.6 ati 0.6
ntr 0.4 ntr 0.4
e e
nc 0.2 nc 0.2
o o
C 0.0 C 0.0
−0.2 −0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
(c) Distance, x (d) Distance, x
Fig.5 Linearsweepandcyclicvoltammetry:(a) dottedlines;fiveprofilesrespectivelyatvarious
typicalexcitationsignal;(b)currentresponse; times,increasingtimeshownbyarrows]fora
andconcentrationprofiles[(c)forwardscan; cyclicvoltammetricexperiment.
(d)reversescan;educt:solidlines,product: