Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 77

are between.5%.and.10%.(w/w).of.the.paste..Too.small.concentrations.do.not.provoke.the.desired.
effect,. whereas. too. high. concentrations. may. change. the. physical. and. physicochemical. properties.
of. the. electrode. material. in. an. unfavorable. way. by. increasing. its. ohmic. resistivity. and. the. back-
ground.current..(However,.there.are.also.the.paste.mixtures.containing.50%.(w/w).modiier.or.even.
more.(see,.e.g.,.[7].and.references.therein),.where.the.resultant.CPEs.are.reported.to.be.operating.
satisfactorily.)
4.2.2 a CTiVaTion and r esToraTion oF The M odiFier
Sometimes,. it. is. necessary. to. create. the. actually. reactive. groups. or. species. of. the. modiier. by.
conditioning,.a.procedure.that.can.be. (physico)chemical .or. electrochemical ..Typical.examples.are.
protonation.of.weak.amines.or.deprotonation.of.weak.acids.for.creating.ion.exchangers.[55,69].or.
electrolytic.generation.of.the.active.form.[804].
Due. to. electrochemical. reactions. during. the. measurement,. the. modiier. may. be. consumed. or.
products. may. adsorb. strongly. at. the. electrode. surface.. In. case. of. repetitive. use. of. the. electrode,.
the.original.functionality.of.the.modiier.must.be.restored.if.it.does.not.happen.spontaneously..The.
methods.for.doing.so.are.straightforward:.chemical,.electrochemical,.and.mechanical.
Chemical.regeneration.means.that.the.electrode.surface.is.exposed.to.a.solution.where.sorbed.
species.are.removed.or.the.modiier.is.reconstituted.by.a.chemical.reaction..Sometimes.these.effects.
can.be.achieved.by.the.application.of.a.potential.(electrochemical.regeneration)..In.tedious.cases,.
none.of.these.procedures.is.successful,.and.mechanical.removal.of.the.outmost.layer.of.the.bulk-
modiied.paste,.usually.done.by.wiping.off.with.a.ilter.paper,.remains.the.only.alternative..With.
respect. to. repeatability,. chemical. or. electrochemical. regeneration. is. preferable. to. mechanical,.
particularly.when.particulate.modiiers.are.used..An.ensuing.conditioning.step,.with.or.without.a.
potential,.is.recommended;.both.regeneration.and.conditioning.should.be.performed.also.before.the.
irst.measurement.to.provide.the.same.conditions.for.all.measurements.
4.2.3 i MMobilizaTion and p reConCenTraTion : C onCepTs and s TraTegies
Preconcentration.of.an.analyte.can.be.accomplished.in.many.ways..In.the.simplest.form,.the.target.
molecules.show.high.afinity.to.the.unmodiied.paste.already.and.accumulation.occurs.via. adsorp-
tion .[226]..If.the.afinity.of.an.analyte.toward.the.carbon.paste.material.is.not.high.enough.due.to.
lack. of. lipophilicity,. other. strategies. can. be. applied,. such. as. ion exchange . [252,522,719,734,800,.
805-809],. insoluble salt formation .[810,811],. complexation .[234,549,683,799,812-816],.or. ion-pair
formation .[182,704-707,817,818].
All.these.types.represent.some.form.of.chemical.accumulation,.where.the.driving.force.for.the.
preconcentration.of.the.analyte.comes.from.the.chemical.reactivity.of.the.target.species.with.the.
modiier.
Chemical.accumulation.is.often.an.essential.part.of.stripping.analysis.with.modiied.electrodes,.
where.a.chemical.equilibrium.reaction.replaces.the.electrochemical.deposition.step.(see.Figure.4.3)..
As. the. chemical. reaction. proceeds. spontaneously,. in. many. cases. the. application. of. a. deposition.
potential.is.obsolete.(open.circuit.preconcentration);.the.accumulated.amount.is.controlled.by.the.
exposure.time.of.the.modiied.electrode.surface.to.the.analyte.solution..Nevertheless,.an.optional.
potential.can.be.applied.if.it.has.a.positive.effect.on.the.migration.of.the.species.to.the.electrode,.
if.an.oxidation.state.of.the.analyte.that.is.not.present.in.the.sample.is.involved,.or.if.the.product.
should.undergo.electrochemical.changes.(e.g.,.electrochemical.deposition.of.the.analyte.in.modiier-
assisted.accumulation.[182]).
Very.common.is.a.medium.exchange.(approach).between.accumulation.and.actual.measurement.
(unusual.with.classical.stripping.analysis.with.electrochemical.deposition),.so.that.the.conditions.for.
preconcentration.and.determination.can.be.optimized.separately.and.independently..Additionally,.
interfering.species.that.do.not.have.an.afinity.to.the.electrode.material.are.separated.off.