Carbon Pastes and Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 17

. iii.. Minimal solubility in water .. Except. special. measurements,. both. CPEs. and. CMCPEs.

are. designed. as. sensors. for. aqueous. solutions. and. hence,. also. heterogeneous. carbon.
pastes. have. to. be. suficiently. stable. in. water,. without. undesirable. disintegration. and.
dissolution.in.consequence.of.miscibility.of.the.binder.with.water.and,.in.general,.any.
aqueous.solution.

Note: . This.obvious.aspect.is.of.particular.signiicance.in.measurements.with.bulk-modiied.car-
bon.pastes..There.are.indications.that.some.modifying.agents—despite.their.minimal.solubility.
in. water—may. tend. to. leach. out. of. the. paste.. Although. this. phenomenon. known. as. “bleeding”.
[55,75].concerns.usually.negligible.amounts.of.modiier,.it.may.have.a.considerable.effect,.when.
considering.“micro”relations.at.the.electrode.interface.and.the.function.of.modiier.itself.in.lipo-
philic. phase. of. binders. compared. to. that. in. aqueous. solutions.. Then,. even. essentially. insoluble.
modiiers.have.to.be.anchored.into.the.carbon.paste.bulk.either.mechanically.or,.better,.via.chemi-
cal.immobilization.[51].(for.details,.see.Chapters.4.and.5)..A.slight.solubility.of.the.binder.is.the.
case.of.ordinary.carbon.pastes.and,.for.instance,.even.extremely.hydrophobic.molecules.of.highly.
molecular.oils.are.capable.of.penetrating.to.the.aqueous.phase.[20].and.vice.versa,.when.a.little.
amount.of.water.is.entrapped.in.the.carbon.paste.bulk.[157,175]..Such.saturation.effects.can.be.
observed.after.recommended.storage.of.preilled.electrode.holders.in.a.beaker.with.distilled.water,.
which—after. a. longtime. exposure—may. give. rise. to. an. opalescent. appearance,. serving. as. an.
experimental.proof.for.such.a.leakage.[189]..The.whole.process.can.be.revealed.during.electro-
chemical.experiments.when.the.saturation-affected.carbon.paste.layer.suffers.from.the.markedly.
increased.background.[175,183].

. iv.. Controlled miscibility with organic solvents ..If.a.pasting.liquid.is.to.be.suficiently.hydro-

phobic.in.order.to.repel.molecules.of.water.in.aqueous.solutions,.such.a.binder.is.then.mis-
cible.with.all.the.solvents.of.the.same.character.[20,157]..It.means.that,.in.principle,.carbon.
pastes.are.inapplicable.in.nonpolar.solvents.and.in.mixed.solutions.containing.such.solvo-
lytic.systems.[2,3,83]..Unfortunately,.as.found,.common.CPEs.can.also.be.seriously.dam-
aged.in.some.polar.solvents,.such.as.MeOH,.EtOH,.ACN,.DMSO,.and.DMFA.[192,210]..In.
this.respect,.the.use.of.CPEs.in.the.presence.of.organic.solvents.(which.can.be.brought.into.
the.system.also.via.the.solution.of. in situ .modiier.spiked.into.the.sample).always.require.
particular.care.and—if.needed—even.special.precautions.and/or.maintenance.

Note: . The.irst.successful.attempt.to.keep.a.carbon.paste.stable.in.organic.solvents.was.based.on.
addition. of. highly. lipophilic. substance. into. the. paste. mixture. [16].. (Properly. chosen. surfactant.
and. its. substantial. amount. in. the. paste,. of. about. 30%. w/w,. had. then. been. capable. of. effective.
repelling.some.polar.solvents.like.methanol,.acetonitrile,.or.diethylcarbonate,.thus.hindering.their.
molecules.to.penetrate.onto.the.carbon.paste.interior.).The.second.possibility.of.how.to.stabilize.
carbon.pastes.in.polar.solvents.is.the.choice.of.highly.viscous.pasting.liquid.[211].or.even.resis-
tant.solid-like.binder.(see,.e.g.,.[66])..Finally,.there.is.yet.another.way.of.stabilizing.the.carbon.
paste.mixture.during.the.permanent.contact.with.organic.solvent—it.is.the.selection.of.a.special.
carbon.material,.such.as.the.GC.powder.of. Sigradur ® .type,.reportedly.withstanding.up.to.90%.
(v/v).MeOH.[114].

Although.none.of.pasting.liquids.obeys.all.the.criteria.for.an.ideal.binder,.there.is.a.relatively.
wide.offer.of.organic.compounds.or.mixtures.that.may—more.or.less—provide.carbon.pastes.with.
satisfactory.quality..Speciically,.one.can.quote.several.particular.substances.described.in.the.fol-
lowing.sections.

2.1.3.1 Parafin (Mineral) Oils
These.liquids.are.undoubtedly.the.most.frequently.chosen.carbon.paste.binders..A.typical.parafin.
oil.(PO).is.formed.by.a.mixture.of.liquid.aliphatic.hydrocarbons.and.usually.being.marketed.under.
trademark.