Chemistry Reference
In-Depth Information
6
Instrumental Measurements
with Carbon Paste Electrodes,
Sensors, and Detectors
6.1 ELECTROCHEMICAL TECHNIQUES: FUNDAMENTALS
AND BASIC PRINCIPLES
In.general,.heterogeneous.carbon.electrodes.and.sensors.are.very.versatile.and.can.be.employed.in.
connection. with. all. conventional. electrochemical. techniques—of. course,. except. polarography—
such.as.voltammetry,.amperometry,.potentiometry,.and.all.their.variants,.as.well.as.many.others..
Compared.with.the.use.of.homogeneous.electrodes,.the.combinations.with.carbon.paste.electrode.
(CPEs),.chemically.modiied.carbon.paste.electrodes.(CMCPES),.and.carbon.paste.(CP)-biosensors.
show.various.advantages.and.disadvantages,.which.are.sketched.briely.in.the.following.sections..
Otherwise,.the.signiicant.characteristics.of.each.technique.are.summarized.in.a.condensed.form.
only,.as.they.can.be.referred.to.in.the.corresponding.handbooks.[3,162-165,1915-1925].
6.1.1 d
ouble
l
ayer
C
onCepT
and
C
apaCiTiVe
C
urrenT
CPEs,.as.most.of.the.electrodes.for.electrochemical.analysis,.are.mainly.exposed.to.solutions.con-
taining.a.supporting.electrolyte..All.methods.relying.on.the.measurement.of.currents.(voltammetry,.
amperometry,. but. indirectly. also. many. other. methods. such. as. coulometry. or. measurements. of.
resistivity).apply.a.working.potential.(static.or.changing.with.time).
The.application.of.a.potential.to.an.electrode.surface.exposed.to.a.solution.causes.the.formation.of.
a.double.layer..Basic.concepts.have.already.been.established.by.Helmholtz;.in.the.course.of.improve-
ments,.the.model.was.reined.to.describe.properly.the.behavior.of.charged.particles.from.the.surface.
into.the.solution..The.double.layer.is.caused.by.the.distribution.of.charge.on.the.electrode.surface.
by.enhancing.or.depleting.electrons.from.the.current.source.(potentiostat)..This.charge.is.compen-
sated.by.mobile.charged.particles.in.the.solution,.which.results.in.the.formation.of.a.double.layer.
corresponding.to.a.charged.capacitor..The.structure.of.the.solution-electrode.boundary.in.the.liquid.
phase.close.to.the.electrode.surface.may.be.divided.into.a.rigid.compact.layer.directly.in.contact.with.
the. surface. (caused. by. speciically. adsorbed. ions. [inner. Helmholtz-plane]. and. by. nonspeciically.
adsorbed.species,.for.example,.hydrated.ions.[outer.Helmholtz.plane]),.and.an.adjacent.diffuse.layer.
caused.by.thermal.agitation.of.the.solution.and.extending.from.the.compact.layer.into.the.solution.
bulk.(Stern.layer).according.to.the.Gouy-Chapman-Stern.model.([1915,1918,1919];.see.Figure.6.1).
The.current.necessary.to.charge.the.interface.is.called.capacitive.current..When.applying.a.con-
stant. potential. (potential. step),. it. decreases. rapidly. to. small. values. according. to. an. exponential.
decay.with.practically.vanishing.magnitude.after.some.time,.assuming.a.simpliied.circuit.of.the.
system.with.a.serial.arrangement.of.a.resistor.(solution).and.a.capacitor.(solution-electrode.inter-
face;.Equation.6.1):
E
R
e
−
t R C
s
/(
⋅
)
.
i
=
(6.1)
d
C
.
s
123
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