Chemistry Reference
In-Depth Information
•.
Normal pulse voltammetry (NPV)
.switches.the.potential.off.between.the.measuring.inter-
vals,. leading. to. a. pulse-like. form. of. the. excitation. potential;. the. corresponding. limiting.
current.is.given.according.to.the.Cottrell.equation.(6.16):
1
*
1 2
/
i
= ⋅
n F A D
⋅
⋅
⋅
C
⋅
.
(6.16)
d NP
,
⋅
t
p
p
.
. . where
i
d,NP
.designates.the.diffusion.limiting.current.of.this.technique
t
p
.designates.the.pulse.time.after.which.the.measurement.of.current.is.launched
•.
Differential pulse voltammetry (DPV)
.maintains.still.the.basic.linear.voltage.ramp,.but.a.
potential.pulse.is.superimposed.to.the.ramp.and.the.current.is.measured.before.and.after.
the.application.of.the.pulse.(usual.values.are.some.20-100.mV).and.the.decay.of.the.capaci-
tive.current.as.a.difference;.thus,.the.current.response.somehow.corresponds.to.the.irst.
derivative.of.the.fundamental.sigmoid.response.curve.and.consequently.resembles.a.peak..
The.peak.height.is.again.proportional.to.the.bulk.concentration.of.the.electroactive.species..
If.the.pulse.height.is.less.than.59/n.mV,.then.the.peak.current.can.be.estimated.according.
to.Equation.6.17:
n F
RT
A D
2
2
1
*
∆
1 2
/
i
=
⋅
⋅
C
⋅
E
⋅
.
(6.17)
p
p
4
⋅
t
p
p
.
•.
Semiderivative and semi-integral voltammetry
..Occasionally.2.5th.derivative.voltamme-
try.is.used.by.some.Chinese.electrochemical.groups.[1926-1928]..Semidifferential.elec-
troanalysis. was. introduced. by. Goto. and. Ishii. [1929]. based. on. the. idea. of. semi-integral.
electroanalysis. as. developed. by. Oldham. [1930,1931].. 2.5th. derivative. voltammetry. was.
irst.described.in.1978.again.by.Goto.et.al..[1932].
Semiderivative.or.semi-integral.response.curves.can.be.done.by.measuring.with.semi-
differential.electronic.circuits.[1929,1932].or.by.mathematical.treatment.of.linear.sweep.
(scan). voltammetries. (LSVs). [1933-1935].. Compared. with. linear. sweep. voltammetry,.
semiderivative.methods.show.lower.detection.limits.and.better.resolution.capabilities.for.
overlapping.peaks.[1932].
•.
Elimination voltammetry (ELV)
..This.type.of.voltammetry.tries.to.eliminate.kinetic.and.
charging.currents.by.recording.a.voltammogram.at.the.reference.scan.rate.
v
.as.well.as.at.
the.half.and.the.double.scan.rate.
v
1/2
.and.
v
2
,.respectively..The.voltammogram.in.linear.scan.
mode.is.reconstituted.by.the.relation.(Equation.6.18.after.[1936,1937]):
f i
( )
= −
11 657
.
i
+
17 485
.
i
−
5 8284
.
i
2
.
(6.18)
.
1 2
/
The.subscript.of.
i
.refers.to.the.scan.rate.with.which.they.were.recorded..ELV.was.applied.
in. connection. with. the. determination. of. deoxyribonucleic. acid. and. its. components.
[1938-1945]..ELV.should.be.applied.with.care.in.order.to.avoid.artefacts.in.the.results.
6.2.1.2 Voltammetry at Faster Scan Rates
If.the.scan.rate.is.increased.(>10.to.1000.mV.s
−1
),.peaks.are.formed.rather.than.sigmoidal.curves.due.
to.the.fact.that.the.potential.is.changed.already.before.the.current.characteristic.for.slow.scan.rates.
is.reached..For.reversible.electrochemical.reactions,.the.
Randles-Sevcik equation
.can.be.used.to.
describe.the.dependence.of.the.peak.current.“
i
p
”.on.the.scan.rate.“ν”.(Equation.6.4)..Thus,.a.plot.of.
i
p
.vs..the.square.root.of.the.scan.rate.should.give.a.linear.relation.
Search WWH ::
Custom Search