Instrumental Measurements with Carbon Paste Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 125

mass transport .or. mass transfer ..A.basic.description.of.the.mass.transfer.in.an.electric.ield.is.given.
by.the.Nernst-Planck.equation.whose.one-dimensional.form.is.shown.in.Equation.6.2:

∂ C x

x

( ) z F

RT DC

∂ ( ) x

x

f

i i

J x ( ) = − D − + C v x ( ) . (6.2)

i i i i i

∂ ∂

.

where
J i ( x ).is.the.low.of.species.“ i ”.(mol.s −1 .m −2 )
x .is.the.distance.from.surface.(m)
D i .is.the.diffusion.coeficient.(m 2 .s −1 )
C i .is.the.concentration.of.species. i .(mol.m −3 )
z i .is.the.charge
ϕ .is.the.potential
v .is.the.velocity.of.moving.volume.element.(m.s −1 )

Reasons.for.the.mass.transfer.of.the.ionic.species.may.be.as.follows:

. (i). Diffusion . along. a. concentration. gradient. (chemical. potential). is. described. by. the. irst.
term.in.Equation.6.2.and.corresponds.to.the.irst.Fick.law..Concentration.gradients.occur.
in. electrochemical. measurements. if. a. transfer. of. electrons. occurs. at. the. electrode. sur-
face.due.to.reduction.or.oxidation.(faradic.process),.which.consumes.the.species.of.inter-
est..The.depletion.zone.around.the.electrode.surface.is.called.diffusion.layer..In.most.of.
the.voltammetric.and.amperometric.measurements,.the.aim.is.to.determine.the.diffusion.
current.
. (ii). Migration .is.represented.by.the.second.term.in.the.Nernst-Planck.equation.and.is.caused.
by.the.gradient.of.the.electric.ield;.it.can.be.suppressed.by.the.addition.of.a.supporting.
electrolyte,.which.also.decreases.the.resistivity.of.the.solution.
.(iii). Convection .(expressed.in.the.third.term.of.Equation.6.2).is.usually.the.forced.movement.of.
the.solution.relative.to.the.electrode,.caused.by.streaming.liquids.or.stirring.of.the.solution.
but.also.by.displacement.of.the.electrode.(e.g.,.rotation)..The.situation.where.convection.
has. a. signiicant. inluence. on. the. measurement. is. usually. referred. to. as. hydrodynamic.
condition.

6.1.3 F aradiC p roCesses
Faradic.processes.involve.an.electron.transfer.of.the.target.species.at.the.electrode.surface;.the.cor-
responding.current.is.called.faradic..The.reaction.can,.in.general,.be.represented.by.Equation.6.3:

−

O ne + ↔ R . (6.3)

.

where
O .means.the.oxidized.species
R .means.the.reduced.form
n .means.the.number.of.electrons.transferred

A. reduction. proceeds. from. the. left. to. the. right,. an. oxidation. in. the. opposite. direction.. Electron.
transfers.of.this.type.provoke.a.depletion.of.the.target.species,.which.as.a.consequence.launches.
a.mass.transfer.of.the.bulk.species.toward.the.electrode..Thus,.the.mass.transfer.and.the.electron.
transfer.determine.the.kinetics.of.a.faradic.process.(see.Figure.6.2).