Chemistry Reference
In-Depth Information
The.
k
i j
pot
,
.can.be.experimentally.determined;.one.of.the.methods.is.demonstrated.in.Figure.6.8..When.
calibration. dependence. is. measured. in. the. presence. of. the. known. and. constant. activity.
a
j
. of. the.
interfering.j.ion.(curve.2),.both.measuring.range.and.limit.of.detection.are.limited.correspondingly..
Moreover,.response.
E
j
.is.obtained.for.
−
*
pot
zi zj
.from.curve.1;.thus,.this.can.also.rep-
/
log
a
= −
log
,
k
a
i
i j
j
= /
..Various.methods.were.rec-
ommended.in.practice.to.establish.selectivity.coeficients,.but.for.many.reasons,.the.values.obtained.
by.different.methods.can.also.differ.but.never.by.orders.of.magnitude..For.many.electrodes,.espe-
cially.liquid-membrane.systems,.selectivity.coeficients.are.also.not.highly.reproducible.or.precise.
quantities.because.they.are.time.dependent..Nevertheless,.they.can.serve.to.estimate.measurement.
errors.in.the.presence.of.interfering.species.
In.contrast.to.direct.potentiometry,.potentiometric.titrations.offer.the.advantage.of.high.accu-
racy.and.precision,.although.at.the.cost.of.increased.time.and.increased.consumption.of.titrants.
(it should.be.mentioned.that.using.modern.automatic.titrators,.this.is.solved.by.using.precise.mea-
suring.microburettes)..A.further.advantage.is.that.the.potential.break.at.the.titration.end.point.must.
be.well.deined,.but.the.slope.of.the.sensing.electrode.response.need.be.neither.reproducible.nor.
Nernstian,.and.the.actual.potential.value.at.the.end.point.is.of.secondary.interest..Only.in.case.of.
titrations.to.a.ixed.potential,.the.correct.voltage.value.must.be.chosen.by.some.means.of.calibra-
tion..The.errors.involved.are.usually.much.smaller.than.in.direct.potentiometry..This.allows.the.
use.of.simpliied.sensors.[1967]..In.the.case.of.CPEs,.it.means.that.depending.on.the.chemical.reac-
tion.on.which.the.determination.is.based..In.precipitation.titrations,.for.example,.it.is.well.known.
that.the.magnitude.of.the.potential.break.is.governed.by.the.solubility.product.of.the.precipitate.
formed..However,.when.the.precipitate.is.extractable.into.the.membrane.mediator.(concretely,.the.
pasting.liquid),.the.role.of.the.extraction.parameters.must.also.be.taken.into.account..A quantita-
tive.expression.for.the.precipitation.equilibrium.is.given.by.the.solubility.product,.
K
s
(
QX
),.which.
is.deined.by
pot
zi zj
/
resent.one.method.of.the.evaluation.of.this.coeficient,.
k
a a
i j
,
i
j
K QX
(
)
=
[
Q
+
] [
X
−
]
.
(6.24a)
.
s
w
w
In. the. presence. of. another,. immiscible. organic. solvent. (a. pasting. liquid),. another. equilibrium.
attained.because.the.two.ions.will.form.an.associated.ion.pair.in.a.phase.of.lower.polarity,.charac-
terized.by.the.extraction.constant,.
K
ex
(
QX
):
K QX
(
)
=
[
QX Q
] [
+ −
] [
1
X
− −
]
1
.
(6.24b)
.
ex
o
w
w
The.distribution.ratio.of.
Q
+
.ion,.
D
(
Q
+
),.can.then.be.expressed.by
D Q
(
+
)
=
[
QX Q
] [
+ −
]
1
=
K QX X
(
)[
− −
]
1
.
(6.25)
.
o
w
ex
w
It. is. expected. that. during. conditioning. in. an. aqueous. suspension. of. the. appropriate. ion. pair,. an.
organic. solvent. (a. pasting. liquid,. analogous. to. plasticizers. of. polymeric. membrane. electrodes).
becomes. gradually. saturated. with. the. ion-pair.
QX
.. In. the. absence. of. side. reactions,. the. ion-pair.
concentration.in.the.organic.phase,.which.is.in.contact.with.such.an.aqueous.suspension,.is.deter-
mined.by.the.products.of.the.two.aforementioned.constants,.that.is,
[
QX
]
=
K QX K QX
(
)
⋅
(
)
.
(6.26)
.
o
ex
s
This.concentration.can.be.considered.as.that.of.ion-exchanging.sites.in.such.a.
Q
+
-.or.
X
−
-selective.
electrode..It.follows.that.this.concentration.increases.with.increasing.values.of.both.the.extraction.
constant.and.the.solubility.product.of.the.ion.pair.formed.[1967,1973]..When.the.titration.system.
(titrant,.determinand,.or.both).is.changed,.usually.two.titrations.are.needed.to.recombine.the.ion.
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