Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 78

Stirring
On
Off

Stirring
On
Off

Medium exchange
(optional, rare)

Medium exchange
(optional, frequent)

Potential Potential

Deposition potential
(optional)

Measurement Measurement

E dep

E dep

Pre-electrolysis

Chemical
accumulation

(A) Time (B) Time

FIGURE 4.3 Comparison.of.“classical”.stripping.analysis.including.electrolytic.deposition.of.the.analyte (A).
with.stripping.analysis.employing.CMCPEs.and.chemical.accumulation.(B).

Preconcentrations.are.referred.to.as. in situ .accumulations.if.the.surface.of.an.unmodiied.elec-
trode. is. altered. during. the. measurement. itself. by. adsorption. or. electrodeposition. of. a. modiier.
from.the.measurement.solution.[819,820]..In.fact,.if.the.reagents.are.suficiently.water.insoluble.or.
adsorbing.strongly.to.the.paste,.they.may.also.be.added.directly.to.the.paste.
Ion. exchangers. can. be. used. not. only. as. solid. organic. resins. [252,522,719,805-808]. but. also.
in. liquid. form. [734,800,802,809].. Most. popular. types. are. Amberlite LA2 ,. Aliquat 33 ,. Dowex
50 B ,. Triton X100 ,. or. tricresyl. phosphate,. which. can. function. as. binder. as. well.. Counterions.
immobilized.with.ion.exchangers.can.also.be.used.for.potentiometric.determinations.(see,.e.g.,.
[569,821,822]).
Organic.ligands.for.complexation.of.the.analyte.are.quite.frequently.employed.as.modiiers.for.
CPEs,.such.as.dimethyl.glyoxime.for.the.determination.of.Co,.Ni,.and.Pd.[234,549,683,812,813];.
dithizone.for.gold.and.lead.[809,814];.or.thioridazine.for.Pd.[816].
Formation.of.insoluble.salts.can.be.a.successful.strategy.for.accumulating.ions.forming.sparingly.
soluble.agglomerates.with.corresponding.counter-ions.immobilized.on.the.electrode.surface..In.this.
way,.bismuth.can.be.preconcentrated.with.alkyl.mercaptans.[810]..Nevertheless,.it.is.possible.to.use.
insoluble.salts.as.modiiers.and.to.exploit.the.different.solubility.products.to.accumulate.the.analyte;.
thus,.oxalate.will.replace.sulfate.in.PbSO 4 .and.will.be.immobilized.at.the.electrode.surface.of.a.
correspondingly.modiied.electrode.[811]..The.concept.of.ion.pair.formation.comprises.the. in situ .
formation.of.an.ion.exchanger.at.the.electrode.surface.[182,704-706,817,818].
Adsorption-capable. species,. generally. sorbents,. that. show. high. afinity. toward. certain. mol-
ecules.are.another.frequently.used.group.of.modiiers.for.CPEs.to.accumulate.the.substrate..Such.
substances. can. be. sepiolites. [823-827],. zeolites. [828-830]. and. other. types. of. clay. minerals. of.
natural. or. synthetic. origin. as. well. as. silica. of. inorganic-organic. hybride. type. and. other. meso-
porous. materials. [717,718,831].. But. naturally. occurring. organic. polymers. may. also. be. suitable.
for.this.purpose..Typical.examples.for.the.latter.are.chitin.and.humic.acids;.the.former.is.able.to.
preconcentrate. aglutinin,. nitrite,. copper,. or. complexed. iron. [832-836],. the. latter. heavy. metals.
[837]..In.fact,.the.chemical.mechanism.for.accumulation.with.biopolymers.is.mostly.ion.exchange,.
complexation,.or.sorption.

4.2.4 e leCTroCaTalysis
The. term. “electrocatalysis”. is. often. used. in. combination. with. modiied. electrodes. if. effects. of. a.
modiier.are.involved.other.than.of.preconcentrative.nature..Most.cases.of.this.type.represent.medi-
ation.of.the.electron.transfer.to.the.substrate.provoked.by.a.modifying.agent.in.the.paste.because.of.
some.reasons.the.target.molecule.or.ion.is.not.electrochemically.converted.directly.at.the.electrode.
surface.at.the.operation.potential..Such.a.mediation.is.sketched.in.Figure.4.4.