Electrochemical Investigation with Carbon Paste Electrodes and Sensors - Electroanalysis with Carbon Paste Electrodes - page 156

H 2 Q Q+ 2H + + 2e -

“Dry” CPE
“Wet” CPE

5 μA

Q+ 2H + + 2e -

H 2 Q

-0.2 0 +0.4

E [V] vs. SCE

+0.8 +1.2

FIGURE 7.1 Cyclic.voltammetry.of.the.quinone/hydroquinone.(Q/H 2 Q).redox.pair.in.acidic.media.and.its.
behavior.at.two.different.CPEs..dry.CPE:.0.5.g.graphite.powder.(C).+.0.2.mL.Nujol.oil,.wet.CPE:.0.5.g.C.+.
0.4.mL.Nujol.. Experimental conditions :.CV:.potential.scan,.+0.2.V.→.+1.2.V.and.reversely.up.to.−0.2.V.vs..
SCE,.scan.rate:.50.mV.·.s −1 ;.model.solution:.0.1.M.H 2 SO 4 .(pH.≈.0.5),. c (H 2 Q).=.5.×.10 −3 .mol.·.L −1 ..(Unpublished.
data,.an.outtake.from.Švancara,.I..and.Schachl,.K.,. Chem. Listy ,.93,.490,.1999.)

behaved.in.a.similar.way,.but.the.degree.of.irreversibility.was.incomparable..If.this.parameter.
is.expressed.as.the.difference.of.both.anodic.and.cathodic.peak.potentials,.Δ E .=. E P ( A ) .−. E P ( C ) .
(theoretically.59.mV.for.ideally.reversible.reaction.with.one-electron.transfer),.the.experiment.
with.“dry”.CPE.has.given.Δ E .=.340.mV,.whereas.CPE ( W ) .resulted.in.a.Δ E .=.590.mV..Although.
the.Q/H 2 Q.redox.pair.is.not.an.ideal.model.(due.for.some.anomalies.in.the.dependence.of.pH.
[3]),.the.whole.assay.has.demonstrated.that.the.increasing.content.of.binder—represented.by.
“wet”.CPE—is.mirrored.in.higher.irreversibility.and.vice.versa.
Compared.with.hydroquinone.and.its.substituted.homologues,.the.anodic.oxidation.of.
catechols. leads. to. the. o -quinones,. which,. in. aqueous. solutions,. are. known. as. unstable.
intermediates.transforming.themselves.into.the.corresponding.hydrolytic.products.such.as.
3-alkyl-4-hydroxy- o -hydroquinone:

R

HO R

O

-2e -
+2e -

+ 2H +

(7.6)

.

HO O

.

Oxidative formation of unstable o -quinone

O

R

R

HO

(7.7)

+ H 2 O

HO

O

.

OH .

Reaction rate-controlled hydrolysis

The.anodic.hydroxylation.reaction.(7.7).is.completed.solely.after.a.prolonged.electrolytic.
oxidation,.again,.giving.rise.to.a.peak.whose.shape.is.markedly.broader.than.that.of.the.
previous.oxidation.signal.(corresponding.to.the. o -quinone.formation).
Apart.from.a.potentially.slowed.reaction.rate.for.these.electrode.systems.at.CPEs,.the.
measurements. with. carbon. pastes. can. also. result. in. some. irregularities,. including. the.
so-called.quasi-reversibility.(i.e.,.reversibility.at.higher.scan.rates.[2,3]),.associated.with.
the.already.mentioned.anomalies.in.the.electrode.behavior.of.the.native.hydroquinone.