Instrumental Measurements with Carbon Paste Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 140

pair.in.the.organic.phase.for.the.new.one..In.analytical.practice,.these.principles.may.be.used.in.
titrations.of.compounds.of.pharmaceutical.importance.[1972].and,.especially,.in.determinations.of.
surfactants.of.different.types,.thus.eliminating.the.health-unfriendly.or.even.hazardous.two-phase.
titrations.[1966].
Similar.explanations.can.be.offered.for.titrations.based.on.acid-base,.complex-forming,.or.oxida-
tion-reduction. equilibria. [1967].. Generally,. more. detailed. information. on. various. potentiometric.
techniques.can.be.found.in.specialized.literature.[1956-1961].
Carbon.paste-based.electrodes,.when.compared.with.traditional.and.marketed.ion-selective.elec-
trodes,.offer.several.advantages..One.of.them.is.signiicantly.lower.Ohmic.resistance.(less.than.10.Ω.
instead.of.up.to.MΩ.values.for.electrodes.equipped.with.polymeric.membranes)..Thus,.the.experi-
mental.work.with.CPEs.is.more.convenient,.and.simpler.potentiometers.not.allowing.measurements.
using.potentiometric.cells.with.such.a.high.inner.resistance.may.be.applied..Historically,.it.is.nec-
essary.to.mention.irst.the.universal.carbon-based.electrodes.called.“Selectrodes,”.introduced.by.
Ruzicka.et.al..[35,1962,1963]..These.were.made.of.Telon.tubing.machined.so.that.inside.the.lower.
end.a.thick.carbon.rod.could.be.itted.very.tightly..This.spectral-grade.porous.carbon.rod.was.irst.
hydrophobized.and.impregnated.with.pure.organic.solvent.(CCl 4 ,.CHCl 3 ,.xylene,.benzene,.etc.).and.
then.mounted.into.the.electrode.body.(for.more.details,.see.[1962])..Finally,.it.was.immersed.in.a.
solution.of.a.proper.ion.exchanger.in.the.organic.solvent.with.which.the.carbon.rod.was.originally.
impregnated..With.this.method,.a.nearly.invisible.layer.was.formed.at.the.carbon.surface,.which.
was.water.repellent.and.could.be.renewed.either.by.dipping.the.electrode.in.the.organic.phase,.by.
scratching.off.the.electrode,.or.by.a.combination.of.both.
Mesaric.and.Dahmen.were.probably.the.irst.who.used.electrodes.illed.with.carbon.pastes.[36]..
Later,. an. increasing. number. of. papers. dealing. with. similar. electrode. constructions. appeared.
[36,178,251,517,538,1964,1965,1974]..Evidently,.the.more.polar.solvents.(the.same.as.those.used.
as. plasticizers. of. PVC. membranes). can. be. used. in. preparing. carbon. pastes. [709].. Because. of.
their.good.extraction.ability.against.the.ion-associated.carbon.paste.composed.of.lipophilic.ionic.
species,.they.may.also.be.applied.to.monitor.titrations.based.on.ion-pair.formation.[253,709,712,.
1966-1968].. Mixing. of. carbon. powder. with. epoxy. resin,. the. ion. exchanger. of. choice,. plasti-
cizer,.and.hardener.then.led.to.the.construction.of. carbon paste -coated. ion-selective electrodes .
( CP-ISEs ;.[1969-1971]).
From.the.viewpoint.of.equilibrium.potentiometry,.the.composition.of.carbon.pastes.makes.it.pos-
sible.to.classify.the.CPEs.as.ion-selective.liquid.membrane.type.electrodes..Pasting.liquids.usually.
exhibit. good. extraction. ability. against. neutral. electroactive. species. of. the. type. of. nondissociated.
weak. acids,. neutral. metal. chelates,. or. ion. associates,. and. the. potential. of. the. electrode. contain-
ing.such.an.organic.solvent.extract.is.predominantly. governed.by.an.ionic.exchange.at.the.inter-
face.between.the.organic.phase.of.the.electrode.and.the.sample.solution,.resulting.in.the.so-called.
Donnan.potential:

RT
z F

a w
a o

( )
( )

i
i

∆ j D = ln . (6.27)

.

i

where
a i ( w ).is.the.activity.of.the.ionic.species.( i .=.H + ,.M z + ,.L z − ,.etc.).in.the.aqueous.sample.phase
a i ( o ).is.the.activity.of.these.ionic.species.present.in.an.extractable.form.(HL,.ML,.etc.).in.the.
organic.phase
z i .is.the.corresponding.ion.charge.(including.the.positive.or.negative.sign)
R ,. T ,. F .have.the.usual.meaning

If.the.discussion.is.conined.to.dilute.solution.of.ionic.species.in.the.aqueous.phase.as.well.as.to.dilute.
solutions.of.extractable.species.in.the.organic.phase,.activities.can.be.replaced.by.concentrations,.and.
the.theory.can.be.applied.for.evaluation.of.electrode.properties.by.means.of.extraction.data.