Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 183

If.excluding.the.few.examples.of.CPE-based.potentiometric.sensor.applicable.to.pH-measure-
ments. (because. they. are. not. expected. to. compete. with. the. conventional. glass. electrodes),. most.
cations.of.non-metallic.character.which.have.been.determined.using.CPEs.are.nitrogen-containing.
species. such. as. ammonium,. NH 4 + ,. hydrazinium,. N 2 H 5 + ,. and. hydroxylammonium,. NH 3 OH + .. Ion-
pairing.and.amperometric.detection.at.electrodes.modiied.with.ion-exchangers.was.especially.used.
for.ammonium,.while.the.detection.schemes.for.hydrazinium.and.hydroxylammonium.were.always.
based. on. the. use. of. oxidation. electrocatalysts. such. as. phthalocyanines,. porphyrins,. and. quinone.
derivatives,.among.others,.as.applied.to.basically.native.unprotonated.compounds.(i.e.,.NH 3 ,.N 2 H 4 ,.
and.NH 2 OH).
Other. molecular. analytes. were. essentially. oxygen,. O 2 ,. hydrogen. peroxide,. H 2 O 2 ,. and. nitro-
gen.oxides,.NO X ..Data.presented.in.Table.8.10.for.these.species.are.related.only.to.their.direct.
detection.while.omitting.methods.in.which.they.are.reagents.or.products.of.enzymatic.reactions.
at. CPE-based. biosensors. (i.e.,. the. analyte. of. interest. being. another. substance).. The. most. com-
monly. used. modiiers. were. electrocatalysts,. most. preferably. phthalocyanines,. porphyrins,. and.
quinone. derivatives,. but. also. methylviologens,. phenothiazines,. or. hexacyanoferrates. (see. again.
Table.8.10).
The. effect. of. H 2 O 2 . concentration. in. enhancing. or. suppressing. enzymatic. activities. was. also.
exploited. for. sensing. this. molecule. and. one. example. of. electrochemiluminescence. detection. was.
also. reported. [2366].. At. present,. most. applications. remained. restricted. to. model. solutions. (and.
biological/pharmaceutical.samples)..Gaseous.samples.were.sometimes.analyzed.on.their.content.of.
NO X .(see.[547,568,2372]).
Dealing. with. non-metallic. anions. and. oxy-anions,. almost. each. of. them. requires. rather. speciic.
conditions.for.sensing.at.CPEs,.so.it.is.rather.dificult.to.draw.a.general.trend.or.to.get.some.“rare”.
procedures.. A. basic. distinction. between. the. analytes. reported. in. Table. 8.11. can. however. be. made.
on.the.basis.of.their.intrinsic.or.mediated.electroactivity.or.their.non-electroactive.character..When.
looking.across.the.table.rows,.it.cannot.be.overlooked.that.iodide,.I − ,.was.one.of.the.most.frequently.
determined.anions,.which.would.not.be.so.surprising,.however,.if.one.considers.its.easy.transforma-
tion.to.elemental.iodine,.as.well.as.a.ine.electroactivity.in.higher.valencies,.or.its.tendency.to.form.
stable.ion-pairs.and.precipitates.
Figure. 8.12. shows. the. fundamentals. of. one. exemplary. method. that. could. be. used. in. several.
adaptations.for.ESA.[210,254,638,639,697,737].without.losing.its.outstanding.selectivity.over.other.
halides. (expressed,. e.g.,. as. I − :. Cl − . =. 1:. 2,000,000).being. superior. to. commercially.marketed. ion-
selective.electrodes.for.iodides.
Voltammograms.included.in.the.scheme.of.Figure.8.12.mirror.some.results.from.the.recent.stud-
ies.focused.on.the.method.performance.for.particularly.high.concentrations.of.iodide.[296]..It.has.
been.revealed—conirming.also.some.results.from.the.“prehistory”.of.the.ield.[1,8,13,40,217]—
that. the. 2I − /I 2 . redox. system. may. participate. in. both. extraction. and. adsorption. processes,. when.
iodide-iodine.or.chloride-iodine.adducts.exhibit.particularly.strong.sorption.capabilities..Otherwise,.
the. iodide. species. could. also. be. detected. directly. via. anodic. oxidation. without. the. preceding.
accumulation.step.or.by.potentiometric.indication.of.the.changes.in.chemical.equilibria.involving.
the.I − .anion.or.even.triiodide,. I 3 − ..Most.of.other.anionic.species.have.then.been.determined.via.
the.amperometric/voltammetric.detection,.which.is.the.special.case.of.oxy-anions,.requiring.the.
assistance. of. electrocatalysts. such. as. polyoxometallates,. Schiff. bases,. Me I/II -bipyridines,. Me II -
phenan-throline.complexes,.and.macromolecular.myoglobin..In.many.procedures,.special.efforts.
were. directed. to. synthesize. irst. the. new. modiiers. whose. improved. performance. has. enabled.
the.desired.determination..Examples.of.potentiometric.or.indirect.amperometric.detections.are.
also.available.(see.throughout.the.tables)..Among.the.analytes.that.are.rarely.determined.by.elec-
trochemical. methods,. one. can. cite. secondary. ortho -phosphate. or. ortho -silicate,. persulfate. and.
perborate,.or.azide.moieties,. S O

2 −

, BH O

−

,.and. N 3 − ,.the.latter.directly.in.solid.lead.azide,.after.

2 8 2 4

its.stabilization.with.dextrin.[2385].