Chemistry Reference
In-Depth Information
As
III
As
V
5.0
4.0
3
3
4.0
2
2
1
1
3.0
3.0
b
2.0
b
2.0
0
+0.2
+0.4
+0.6
0
+0.1
+0.2
+0.3
+0.4
+0.5
+0.6
(A)
E
[V] vs. SCE
(B)
E
[V] vs. SCE
FIGURE 8.6
Stripping.potentiograms.used.to.estimate.detection.limits.for.arsenic.analyzed.in.the.form.of.
As(III).(A).and.chemically.reduced.As(V).(B)..(b).base-line.(blank),.1-3.5.ppb.As
III.*
.or.2.ppb.As
V.**
.(three.
replicates)..
Experimental conditions
:.Gold.ilm-plated.carbon.paste.electrode,.AuF-CPE;.CCSA,.accumula-
tion.conditions:.
E
ACC
.=.−0.4.V.vs..SCE,.
t
ACC
.=.15.s.(A),.300.s.(B),.
t
EQ
.=.10.s;.stripping.limits:.
E
INIT
.=.−0.3.V.
and.
E
FIN
.=.+0.6.V.vs..ref.,.constant.current,.+5.μA;.s.e.:.1.M.HClO
4
.+.0.2.M.HCl..
Note
:.the.analyte.added.as.
aliquiot.of.(
*
).As
3+
,.(
**
).H
2
AsO
4
−
.pre-reduced.with.solid.α-cysteine..(Taken.and.rearranged.from.Švancara,.I..
et.al.,.
Talanta
,.58,.45,.2002.)
ligands.in.the.sample.is.the.use.of.CPEs.modiied.with.such.ligands.(reagentless.detection),.offering.
also.the.advantage.of.further.preconcentration.
Contrary.to.iron,.Co
II
.and.Ni
II
.species.can.be.directly.reduced.to.their.elemental.state,.but.this.
transformation.was.often.performed.in.complexing.media.to.improve.the.analytical.performance.
of.the.method.(by.enhancing.charge.transfer.kinetics.and/or.accumulation.of.the.target.analytes,.or.
even.to.avoid.precipitation.of.the.analyte)..Especially.dimethylglyoxime.(DMG),.and.also.cyclam.
derivatives.and.some.other.ligands,.have.been.used.for.these.purposes,.DMG.being.moreover.likely.
to.determine.both.Co
II
.and.Ni
II
.in.mixture..Due.to.a.certain.solubility.of.DMG.in.aqueous.solutions,.
CPEs.prepared.by.simple.admixing.of.the.modiier.into.the.paste.can.suffer.from.poor.long-term.
stability,.which.can.be.somewhat.got.round.by.
in situ
.approaches.(i.e.,.DMG.in.solution.to.form.
the.metal-ligand.complex.which.is.then.accumulated.by.adsorption.on.CPE)..These.elements.have.
been.analyzed.in.various.natural.water.samples,.ethanol.fuel,.electroplating.baths,.or.even.biologi-
cal.luids.
Strategies.exploiting.CPEs.to.detect.other.metal.species,.that.is,.of.groups.V,.VI,.and.VII,.have.
also.been.developed,.for.Mn,.Tc,.Re,.Cr,.Mo,.W,.and.V.(Table.8.8)..Vanadium.was.mainly.detected.
in.the.V
V
.form.after.the.accumulation.at.modiied.CPEs,.which.is.also.the.case.of.a.method.depicted.
in. Figure. 8.7. by. means. of. a. set. of. calibration. voltammograms. and. operated. via. a. complex. with.
oxalate.
A. huge. amount. of. work. has. been. performed. to. determine. chromium. at. both. Cr
III
. and. Cr
VI
.
oxidation.states,.using.various.detection.techniques.(anodic.and.cathodic.stripping.voltammetry.
with.linear.scanning,.in.differential.pulse.mode,.or.with.potentiometric.indication),.usually.asso-
ciated. to. accumulation. procedures. involving. complexation. or. ion-pairing. with. suitable. modi-
iers..One.has.to.confess,.however,.that.the.proposed.methods.to.date.do.not.reach.the.analytical.
performance.of.those.employing.mercury.electrodes..Other.elements.of.the.same.group.VI.can.
also. be. detected,. including. molybdenum. (either. Mo
IV
O
2+
. or. Mo
VI
O
4
2−
forms). and. tungsten. (the.
only.example.to.date.[2324])..Finally,.dealing.with.elements.of.group.VII,.some.methods.have.
been.developed.for.manganese.species.and,.to.less.extent,.for.technetium,.which.were.likely.to.
distinguish.between.various.redox.states..The.usual.way.to.determine.Mn
II
.species.relies.on.their.
oxidation. at. a. carbon. paste. anode. to. form. insoluble. MnO
2
. which. can. then. be. reduced. quanti-
tatively.(reduction.of.Mn
II
.into.metallic.manganese.is.also.an.alternative,.yet.much.less.used)..
To. date,. direct. detection. of. chromate. species. has. not. been. reported. (although. this. should. be.
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