Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 181

Nevertheless,.various.strategies.based.on.indirect.amperometric.detection.of.these.electroinac-
tive.cations.have.appeared.in.the.literature..They.include.(1).the.resort.to.electrodes.modiied.with.
intercalation. materials. (i.e.,. metal. oxides). for. which. cation. insertion-ejection. phenomena. result.
in.change.in.their.electrochemical.behavior,.which.can.be.related.quantitatively.to.the.cation.con-
centration.in.solution,.(2).the.exploitation.of.the.electrochemical.activity.of.an.electroactive.ligand.
likely.to.interact.with.the.metal.ion.(making.the.resulting.complex.likely.to.undergo.electron.trans-
fer.via.its.ligand.part),.or.(3).the.use.of.CPEs.containing.organic.or.inorganic.ion.exchangers.doped.
with.an.electroactive.cation.which.is.likely.to.be.released.from.the.solid.(and.thereby.detected.at.the.
electrode.surface).by.ion.exchange.with.the.cationic.analyte..The.latter.approach.was.particularly.
developed. using. zeolites. exchanged. with. redox-active. cations. [1115],. as. these. solids. feature. both.
ion.exchange.and.size.selectivity.properties,.enabling.the.indirect.amperometric.detection.of.non-
size-excluded.cations.while.using.a.supporting.electrolyte.containing.cations.larger.than.the.zeolite.
pore.aperture.(size-excluded.cations),.which.did.not.contribute.to.the.amperometric.response.of.the.
electrode.(see.method.principles.in.Figure.4.7,.in.Section.4.3).
An.attractive.extension.of.this.approach.concerns.its.possible.use.in.lowing.streams.(batch.or.low.
injection.conditions). in the absence .of.supporting.electrolyte,.the.cationic.analyte.playing.the.role.of.
analyte.by.liberating.the.redox.probe.from.the.zeolite.and.the.role.of.(transient).electrolyte,.with.a.
resulting.amperometric.response.being.the.sum.of.two.contributions:.a.faradic.one.arising.from.the.
probe. reduction. and. a. capacitive. one. resulting. from. conductivity. changes. at. the. electrode/solution.
interface.[1116]..In.such.conditions,.any.kind.of.ion.exchanger.(not.only.molecular.sieves).doped.with.
a.redox.active.ion.(either.cation.or.anion).can.be.used,.and.this.has.opened.the.door.to.applications.as.
amperometric.detectors.in.ion.chromatography.operating.with.the.suppressor.technology.with.analyti-
cal.performance.of.the.same.order.as.the.classical.conductivity.detector—see.illustration.in.Figure.
8.10,.depicting.the.analysis.of.some.alkali.metal.and.alkaline.earths.cations,.together.with.ammonium.
(according.to.[1736].
8.1.8 n on -M eTalliC i ons , C oMplexes , and n euTral M oleCules
This.last.category.dealing.with.non-metallic.species.is.characterized.by.a.wide.diversity.of.physi-
cochemical.and.electrochemical.properties.of.the.target.analytes..They.have.been.classiied,.more.
or. less. arbitrarily,. on. the. basis. of. their. charge.. Data. relative. to. non-metal. cations. and. inorganic.
molecules.are.gathered.in.Table.8.10.while.inorganic.anions,.such.as.single.and.oxo-anions,.plus.
some.anionic.complexes,.dwell.in.Table.8.11..The.latter.ones.include.halide.species.(Cl − ,.Br − ,.I − ,.
and.I n − ),.halo-anions.(ClO 3 − ,.ClO 4 − ,.BrO 3 − ,.and.IO 3 − ),.or.pseudo-halides.(CN − ,.SCN − ,.SeCN − ,.and.
N 3 − ),.as.well.as.other.anionic.species.formed.from.nitrogen.(i.e.,.NO 2 − .and.NO 3 − ),.sulfur-containing.
anions.(S 2− ,.SO 3 2 ,.SO 4 2 ,.and.S 2 O 8 2 ),.phosphate.anions,.or.even.some.other,.that.is:.[Si(Mo 3 O 10 ) 4 ] 4− .
and.B(O 2 )(OH) 2 − .
Figure. 8.11. shows. the. initial. study. on. an. ion-exchanger-modiied. carbon. paste. developed. for.
sensitive. detection. of. nitrite,.NO 2 − .. As.documented. by. the. set. of. CV.cycles. given,. such. CMCPE.
had.exhibited.strong.sorption.capabilities.toward.nitrite—the.CVs.themselves.having.illustrated.the.
opposite.situation:.its.rapid.desorption—enabling.the.determination.of.this.toxic.anion.down.to.the.
low.ng.mL −1 .level.[734].
The. second. category. comprises. the. proton,. H + ,. and. protonated. amino-derivatives. (NH 4 + ,.
NH 3 + OH,.N 2 H 5 + ).as.cations,.and.several.molecules.as.oxygen,.hydrogen.peroxide,.nitrogen.oxide.
and.dioxide,.hydroxylamine,.and.hydrazine..As.a.result.of.such.diversity,.a.wide.variety.of.meth-
odological.approaches,.modiiers,.and.measuring.techniques.has.been.considered..Among.detec-
tion. modes,. one. can. mention. direct. potentiometry,. automated. potentiometric. titrations,. a. series.
of. modiications. of. electrochemical. preconcentration. techniques. with. various. stripping. modes,.
voltammetry. associated. or. not. to. electrocatalysis,. amperometric. detection. in. lowing. streams,. or.
electrochemiluminescence.measurements..The.selection.of.the.modiier.was.of.course.the.function.
of.the.target.analyte,.as.briely.discussed.hereafter.