Chemistry Reference
In-Depth Information
in.literature..Compared.with.adsorption,.the.resultant.effectiveness.of.the.accumulation.based.on.
ion.pairing.depends.primarily.on.the.reagent.involved,.whereas.the.“quality”.of.the.paste.used.is.of.
marginal.importance.
A.straightforward.approach.of.ion.exchange.accumulation.is.the.elegant.
in situ
.modiication.of.
the.electrode.surface.by.charged.amphiphile.moieties,.creating.thereby.an.ion.exchanger.directly.
during.the.measurement..Subsequently.(or.simultaneously),.ionic.analytes.of.opposite.charge.can.be.
immobilized.and.preconcentrated.by.ion.pairing,.if.they.show.suficient.afinity.toward.the.modi-
ier..Typical.amphiphile.moieties.used.for.that.purpose.are.long-chain.alkylsulfonic.acids.(e.g.,.sodium.
dodecyl. sulfate,. SDS;. [703]). or. bulky. organic. ammonium. salts. (e.g.,. CTAB;. [704-707]),. which.
are.lipophilic.enough.to.adsorb.strongly.at.the.carbon.paste..Since.the.resultant.ion.pairs.(or.ion.
associates,.respectively).show.hydrophobic.characteristics.again,.immobilization.is.often.not.only.
restricted.to.the.surface.layer.but.the.species.may.migrate.into.the.bulk.of.the.electrode.material;.
see.Section.3.4.2.
Other.strategies.concern.the.preparation.of.chemically.modiied.CPEs.displaying.ion.exchange.
or.ion-pairing.properties..This.can.be.achieved.by.various.ways:.(i).the.direct.incorporation.of.a.
solid. ion. exchanger. into. the. paste. (irst. example. in. early. 1984. by. Wang. et. al.. [707]);. (ii). carbon.
pastes.where.liquid.anion.exchangers.are.mixed.with.the.paste;.and.(iii).carbon.pastes.where.the.
liquid.binder.takes.the.role.of.ion.exchanger.(discovered.in.1993.[205])..The.irst.method.was.very.
popular,. and. many. types. of. organic,. inorganic,. or. organic-inorganic. hybrid. ion. exchangers. have.
been. added. in. carbon. paste. matrices. (a. detailed. description. is. given. in. Section. 4.3).. Liquid. ion.
exchangers.were.also.widespread,.and.they.were.often.described.as.preferable.to.solid.ones.because.
they.ensure.better.homogeneity.of.the.modiied.paste.and,.therefore,.easier.surface.renewing.and.
better.reproducibility.of.measurements.[82]..The.third.approach.demonstrates.some.new.functions.
of. the. carbon. paste. binder. that. was. mainly. made. of. TCP. or. dioctyl. phthalate. (DOP),. known. as.
plasticizers.in.polymeric.membrane-based.ion-selective.electrodes.[709]..Of.particular.interest.is.
the.more.polar.TCP.because.it.has.a.high.tendency.to.be.protonated.(Equation.3.9a).in.the.form.of.a.
large.(and.lipophilic).cation.ready.to.be.involved.in.the.ion-pair.formation.(Equation.3.9b):
+
+
{(
C H O
)
≡
}
CPE
P O
+ →
H
{(
C H O
)
≡
}
P OH
.
(3.9a)
.
7
7
3
7
7
3
CPE
+
−
+
−
{
(
C H
O)
≡
}
P OH
+ →
X
{(
C H
O)
≡
}
P OH ,
X
(3.9b)
.
.
7
7
3
CPE
7
7
3
CPE
The.symbol.“{(C
7
H
7
O)
3
≡}
CPE
”.sketches.the.highly.lipophilic.organic.moiety.of.the.TCP.molecules.
in.the.carbon.paste.surface.layer;.thereby,.the.hydrophilic.cationic.fragments.“P
=
OH
+
”.are.oriented.
into. the. solution. where. they. act. as. highly. effective. ion-pairing. sites. for. interaction. with. a. coun-
ter. anion.. For. example,. the. whole. mechanism. can. be. advantageously. exploited. in. electrochemi-
cal. stripping. analysis. to. accumulate. selectively. some. inorganic. anions,. such. as. I
−
. [254,639,638].
or.Au
X
−
.(where.“
X
”.….Cl,.Br,.SCN,.CN.[205,710,711]),.in.measurements.using.either.differential.
pulse.cathodic.stripping.analysis.[205,639,638].or.constant.current.potentiometric.stripping.analy-
sis. [254,710,711].. Other. anions. can. then. be. determined. with. the. aid. of. automated. potentiometric.
titration,.which.is.the.case.for.ClO
4
−
.and.BF
4
−
.[719].or.As(Mo
3
O
10
)
¾−
[712]..Besides.the.classical,.
commercially. available. organic. resins. exhibiting. ion-exchange. properties,. which. have. been. used.
in.CPEs.(see,.e.g.,.[707,713,714],.the.most.commonly.used.solid.ion.exchangers.in.CPEs.were.clays.
[715,716],.zeolites.[717,718],.and.porous.silica.materials.bearing.charged.moieties.[718,719]..In.all.
the.aforementioned.approaches,.however,.the.respective.accumulation.mechanisms.were.often.syn-
ergistic.processes,.also.involving.extraction.principles.or.even.adsorption.
3.4.1.4 Electrocatalysis-Assisted Detection
Electrocatalysis.can.be.deined.as.a.catalytic.process.involving.oxidation.or.reduction.through.the.
direct.transfer.of.electrons.(see.Chapter.4.for.a.detailed.presentation.of.this.concept)..This.usually.
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