Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 52

3.1.3 i nsTabiliTy oF C arbon p asTes in o rganiC s olVenTs
Tendencies.to.disintegrate.in.contact.with.most.of.the.organic.solvents—apart.from.if.being.used.
alone. or. in. mixed. media. with. water—were. already. commented. on. in. the. previous. sections—in.
context. with. stabilization. of. carbon. pastes. by. additional. agents. [16]. or. through. choice. of. more.
resistant.CP.constituents.such.as.highly.viscous.silicone.oils.[211],.wax-like.chlorotriluoroethylene.
polymer.( Kel-F ® ;.[66,318]).or.related.solid-like.materials.[308,310],.and.powdered.glassy.carbons.
[82,111,114].

3.1.4 a ging oF C arbon p asTes
According.to.some.special.studies.(e.g.,.[192,254]),.carbon.paste.mixtures.exhibit.a.series.of. typical
periods of changes in time .that.can.be.attributed.to.the.aging.effect..Namely,.(i).unstable.or.almost.
irreproducible. response. of. freshly made . carbon. paste. mixtures,. (ii). relative. stable. signal. in. the.
course.of.2-3.weeks.(up.to.several.months.[8,9,11]),.and.(iii).gradual.deterioration.of.the.response.
due.to. starting desiccation ..These.individual.stages.indicate.a.certain. self-homogenization .process.
in.a.newly.prepared.paste.(ca..24-48.h.[175,176,192]).
And.the.lifetime.of.a.CPE.ends.by.reaching.the.critical.limit,.when.the.mixture.turns.into.a.desic-
cated.and.incompact.mixture..The.overall.duration.of.such.a.lifetime.is.closely.connected.with.the.
overall.quality.of.both.main.constituents.[175].and,.mainly,.with.the.stability.of.the.liquid.binder..
Then,.in.extreme.cases,.the.lifespan.of.a.CPE.may.reach.several.months.[8,11].or.even.years.(see.
one.curiosity.in.[7]).

3.1.5 h ydrophobiCiTy oF C arbon p asTes
Lipophilic.character.of.the.carbon.paste.material.is.indisputably.the.most.important.physicochemi-
cal. feature,. having. a. principal. signiicance. for. the. resultant. electrochemical. and. electroanalyti-
cal.characteristics.of.CPEs.[2,3,68,83,175]..Hydrophobicity.is.relected.in.the.so-called.“repelling.
effect”. [55,69]. at. the. carbon. paste. surface. toward. hydrophilic. ions. and. molecules. as. well. as. in.
speciic interactions .with.some.substances.in.the.carbon.paste.bulk.(interior).
It. can. be. stated. that. the. hydrophobic character . of. carbon. pastes. codetermines. practically.
all. electrochemical. characteristics. of. CPEs. and. related. sensors. and. their. resultant. behavior. in.
practical.measurements..Thus,.the. lipophilicity of carbon pastes given.by the presence of liquid
binder .is.of.continuing.interest.in.the.following.sections,.including.the.discussion.on.the.indi-
vidual.interactions,.potentially.occurring.at.various.types.of.carbon.pastes,.and.is.summarized.
in.Section.3.4.

3.2 ELECTROCHEMICAL CHARACTERISTICS OF CARBON PASTES
A.unique.constellation.of.physicochemical.and.mechanical.properties.of.the.carbon.paste.electrode.
material.is.largely.relected.in.various.speciic.features.that.are.surveyed.and.briely.commented.on.
in.the.following.paragraphs:

3.2.1 V ery l oW b aCkground
In. faradic. measurements,. satisfactorily. low. background. currents. (residual. current,. parasitic.
signal,. noise). are. of. principal. importance. as. they. contribute. directly—in. positive. or. negative.
sense—to.the.analytical.signal.of.interest,.which,.for. current intensity .( I i ),.can.be.expressed.by.
the.following.relations:

I

∑ =

I ± I (3.1)

.

ANAL RES

.