Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 175

In.addition,.other.metallic.species.(Tl,.Sn,.In,.Bi,.and.Sb;.see.Table.8.6).have.been.successfully.
determined. at. chemically. modiied. CPEs. by. exploiting. the. selective. recognition. and/or. electro-
catalytic.properties.of.the.modiier..Bi III .and.Sb III .species.can.be.accumulated.and.then.detected.by.
anodic.stripping.voltammetry.because.their.electrochemistry.on.CPE.is.compatible.with.the.“elec-
trolytic. deposition—anodic. reoxidation”. scheme,. whereas. the. application. of. this. scheme. for. In III .
was.only.possible.on.metal-plated.CPEs.through.alloy.formation.in.the.electrolytic.deposition.step..
Studied.samples.were.water.solutions,.soils,.human.hairs,.and.pharmaceuticals.
Two.main.strategies.were.usually.used.for.the.analysis.of.Tl III .and.Tl I .as.well.as.Sn II .and.Sn IV .species,.
the.irst.one.similar.as.the.one.mentioned.earlier.(but.requiring.metal-plated.CPE.and.alloy.formation.
in.the.electrolytic.deposition.step).and.the.second.one.based.on.direct.electron.transfer.between.soluble.
forms.of.these.elements..A.speciicity.of.thallium.determination.at.CPEs.is.that.a.conversion.of.Tl I .into.
the.Tl III .state.offers.an.attractive.eventuality.to.pair.thallate.anions.with.a.suitable.counter-ion,.offering.
improved.performance.with.respect.to.the.“simple”.Tl I .determination.via.electrolytic.reduction.
Otherwise,.in.mixtures.with.other.heavy.metal.ions,.there.is.also.a.possibility.to.utilize.a.well-
known.approach.from.the.electrochemistry.of.thallium—its.determination.in.the.presence.of.EDTA.
that. irmly. masks. all. the. interfering. ions. whereas. the. Tl(I). themselves. remain. uncomplexed. and.
thus.available.for.detection..A.study.on.such.a.masking.is.depicted.in.Figure.8.5,.in.this.case,.per-
formed.atypically.in.basic.media.of.ammonia.buffer.
The.individual.combinations.(A-D).document.that.the.chelating.of.both.Cd II .and.Pb II .has.been.
effective.enough,.at.least,.for.concentrations.and.their.mutual.ratios.chosen..Less.evident.from.these.
sets.of.voltammograms.was.an.observation.that,.at.higher.concentrations,.stripping.analysis.of.thal-
lium.at.the.Bi-CPE.usually.led.to.a.couple.of.peaks,.Tl(1).and.Tl(2).[672],.whose.occurrence.might.
be.due.to.mutual.alloying.between.the.metallic.thallium.and.bismuth.
Finally,.one.has.to.note.that.most.of.the.elements.presented.in.Table.8.6.are.sensitive.to.hydroly-
sis,.requiring.to.be.analyzed.in.the.complexed.form.(either.by.adding.a.suitable.ligand.in.the.solution.
or.by.exploiting.the.ligand.properties.of.the.electrode.modiier).
8.1.3 M eTalloids
Only. few. applications. of. CPEs. for. electroanalysis. of. arsenic. and. selenium. species. have. been.
reported.(see.the.bottom.of.Table.8.6),.which.contrast.with.the.tremendous.works.that.appeared.in.
the.analysis.of.highly.toxic.arsenic.compounds.in.many.kinds.of.polluted.media.
Already.in.the.irst.reported.method.for.the.determination.of.arsenic.at.a.CPE,.the.differentia-
tion.of.both.As III .and.As V .forms.was.successful.[261].and,.despite.serious.interferences.from.the.
Cu II .ions,.the.corresponding.method.could.operate.at.satisfactorily.low.concentrations,.especially,.
when. coupled. with. CCSA. [108].. This. is. also. documented. in. Figure. 8.6,. presenting. the. experi-
mental.records.used.for.estimating.the.respective.limits.of.detection.(LODs).having.been.found.
at.the.low.ppb.(μg.L −1 ).level.for.both.As III .and.As V ..Notable.was.also.rather.different.character.
of. the. corresponding. potentiograms. with. a. considerable. shift. in. the. peak. potentials,. relecting.
maybe.some.effect.of.solid.α-cysteine.that.had.to.be.added.directly.in.sample.solutions.in.order.
to. accomplish. the. inevitable. chemical. reduction. of. electrochemically. almost. inert. As V . to. more.
reactive.atoms.of.As III .
Among.commonly.occurring.selenium.species,.only.tetravalent.Se IV .(selenite).can.be.detected.
since.other.soluble.forms.of.selenium.are.electroinactive.on.carbon.electrodes..Otherwise,.this.inter-
esting. element. (bioessential. in. traces. and. highly. toxic. at. slightly. higher. concentrations). is. also.
determinable.in.the.form.of.SeCN − .as.shown.in.a.lone.report.[652].
8.1.4 M eTals oF The i ron , M anganese , C hroMiuM , and V anadiuM g roups
In.contrast.to.metal.species.discussed.in.Sections.8.1.1.and.8.1.2,.the.determination.of.metals.Fe,.Co,.
and.Ni.(Table.8.7),.and.Mn,.Tc,.Cr,.Mo,.W,.and.V.(see.the.top.of.Table.8.8),.is.not.straightforward.