Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 64

stripping.techniques.[674],.which.has.also.inluenced.electrochemistry.with.CPEs..Several.kinds.
of.adsorption.processes.at.CPEs.can.be.distinguished,.and,.depending.on.the.accumulation.mecha-
nism,.various.experimental.parameters.are.likely.to.affect.the.effectiveness.of.the.preconcentration.
event..The.accumulation.mechanisms.at.CPEs.have.been.classiied.in.several.categories.(Equations.
3.4.through.3.8;.according.to.[675]):

Direct adsorption ORG → (ORG) . (3.4a)

.

CPE

Me X → (Me X ) . (3.4b)

.

m m CPE

m − m − m −

Competitive adsorption (Me X ) + ( X ) → (Me X ) + X . (3.5a)

.

m CPE m CPE

m − m − m −

(Me X ) + ( Y ) → (Me Y ) + X . (3.5b)

.

m CPE m CPE

m +

Synergistic adsorption Me X + ( ) L → ( Me XL ) . (3.6)

.

CPE CPE

m + n + −

Electrosorption Me + ( ) L → ( Me L ) + ( n m e − ) . (3.7)

.

CPE CPE

m +

m +

Chemisorption Me + − ( Z ) → (Me − Z )

CPE .

(3.8)

.

CPE

where
the.subscript.“ CPE ”.means.the.carbon.paste.surface
other.abbreviations.are.explained.in.the.following

A.straightforward.way.to. direct adsorption .is.the. physical sorption .or. chemisorption .of.organic.mol-
ecules.(“ORG”;.[676-681])..It.can.also.be.the.case.of.numerous.neutral.metal.complexes.(“Me X m ,”.
e.g.,. Be II − o -(2-hydroxy-5-methyl-phenylazo). benzoate. [682],. Ni II -. or. Co II -dimethylglyoximates.
[683,684],.a.series.of.metal.ions—Alizarin.Red.complexes.(with.metals.=.Ce.[685],.Zr.[397,407,686],.
Cu. [393,687],. V. [688],. or. Ga. [689]),. Ag I -Alizarin. violet. [690]. or. Y III (Sc III ,. Sm III ,. Eu III ,. etc.)—
Alizarin ates. [691],. and. several. metal. ions-bromopyrogallol. (with. metals. =. Bi. [692],. Sb. [693],. or.
Sn. [694]).. Voluminous. inorganic. anions. or. anionic. complexes. (“(Me X m ) m − ,”. e.g.,. HCrO 4 − . [695],.
P(Mo 3 O 10 ) 4 3− .[696],.and.I − .or.I 3 − .[639,697]).can.also.enter.into.competitive.adsorption.schemes..One.
can.speak.about. synergistic.adsorption. when.an.uncharged. complex.Me X .formed.in.the.solution.
interacts.with.another.ligand.(“ L ”).strongly.adsorbed.at.the.CPE.surface.to.form.an.Me XL .adduct.
tending.to.be.adsorbed.as.well.(e.g.,.[Hg II (CN)L 3 ] 2− .[698]).
One.can.also.observe.an. electrosorption .mechanism.in.which.immobilization.of.the.analyte.is.
driven.by.an.electron.transfer.reaction.(e.g.,.I − .precipitation.in.the.form.of.CuI.at.Cu 2 O-CPE.under.
cathodic.conditions.[699])..Chemisorption.of.cationic.species.can.also.occur.at.CPEs.modiied.with.
minerals. or. organic-inorganic. hybrids. containing. functional. groups. (“− Z ”). that. are. likely. to. be.
complex.metal.ions,.“Me m + ”.[700-702],.but.this.relies.primarily.on.chemically.modiied.electrodes.
(see.Chapter.4).

3.4.1.3 Ion Exchange and Ion-Pair Formation
Both. processes. are. based. on. electrostatic interactions . between. charged. analytes. and. a. suitable.
charged.reagent,.either.dissolved.in.solution.(and.with.some.afinity.for.the.CPE.surface).or.intro-
duced.into.the.carbon.paste.as.an.electrode.modiier.
Often,.ion.exchange.and.ion.pairing.act.together,.when.one.may.be.substituted.by.the.other,.or.
they.mean.the.same.process,.which.is.also.evident.from.various.mechanisms.based.on.and.reported.