Transcript 4. Voltammetry

AIT
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a redox reaction transfers electrons between the reactant species and the
electrode
produces a measurable current
the greater concentration of reactive species, the greater the current
measurement of currents can be used to determine concentrations
voltammetry - an electrical current is measured as a function of applied
potential
used to identify and quantify
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M+ + e  M (s)
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reaction will only occur if both the following conditions apply:
◦ the ion is close enough to the electrode
◦ the voltage applied at the electrode is enough to allow the reaction to
occur (the reduction potential)
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some ions will always be close to the electrode by sheer chance
voltage as the controlling factor for whether reaction will occur
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Current
Reduction potential
Applied potential
Initial potential
As potential
Too low, so no reaction approaches redn
can occur
V, some ions react
As potential pass
redn V, all ions near
electrode react
After redn V, ions
newly arrived near
electrode react
Current is zero
Current is high
Current is high and
constant
Current is low and
increasing
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a measurable change in current as a consequence of a voltage change
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this is known as a wave
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whole scan is a voltammogram
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an analogy between spectroscopy and voltammetry
peak
wave
wavelength/frequency
voltage
intensity
current
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for both quantitative and qualitative analysis:
◦ the wave position (voltage) is characteristic of a particular species
◦ the wave height (current) is proportional to concentration
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diffusion (simple random motion),
electrostatic attraction, and
convection
current-concentration only linear, if diffusion is the only mechanism
minimise the other two processes as much as possible
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not stirring the solution controls convection
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not possible to prevent electrostatic attraction between the positive ions
and the negative electrode
reduced by addition of a high concentration of non-reactive ions, known
as the supporting electrolyte
KCl or KNO3 at concentrations around 0.1 M
the very high level of other ions masks attraction to the electrode
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electrostatic
attraction
diffusion
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supporting electrolyte has two other functions:
◦ masks matrix interference due to different levels of background ions in
different samples
◦ ensures that the solution will have enough electrical conductivity
voltammetry only ever uses up a tiny fraction of the reducible species in
the sample
multiple scans can be run on the one sample without changing its overall
concentration
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the most commonly used form of voltammetry
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one of the electrodes is made from a capillary of mercury, forming a drop
at the end
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known as a dropping mercury electrode (DME)
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scan is called a polarogram
Current
limiting
current
diffusion
current
residual
current
half-wave potential
Applied Potential
Measure the half-wave potential and diffusion current
 Applied potential: each scale division is equal to 0.5 V,
becoming more negative from 0 V
 Current: each scale division is equal to 1 uA starting from 0.
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(a) -1.1 V
(b) 7.5 uA
Hg reservoir
DME
auxiliary
electrode
reference
electrode
N2 bubbler
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Dropping mercury electrode – the electrode at which the analyte reaction
occurs
Reference electrode –an electrode which maintains a constant voltage
regardless of the solution and reactions occurring
Auxiliary electrode – provides a path through which current can flow and
be measured; usually a platinum wire
Nitrogen bubbler – dissolved oxygen produces two visible polarographic
waves, at around –0.1 and –0.9 V bubbling nitrogen through the solution
for 5 minutes removes the oxygen
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one pair (DME & ref.) to control voltage
one pair (DME & aux.) for current path and measurement
Current
Auxiliary
Voltage
DME
Reference
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does not seem like the most obvious choice
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one significant advantage: it presents a fresh surface to the solution
every second or so
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allows a much more reproducible control of potential than a fixed
electrode, where the reduced metal (for example) becomes coated to it
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Hg oxidised >+0.4 V, so a Pt or graphite working electrode must be used
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presence of complexing agents (ligands) shifts E½
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working voltage range of +0.4 to –1.8V
◦ >+0.4: the mercury drop will be oxidised
◦ < –1.8V (varies with pH) water is reduced to hydrogen gas
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limited sensitivity – DC polarography is limited to about 5 mg/L for most
species
difficulty in measurement – due to the waveform shape and the
oscillations
improve the former and get rid of the latter by changing the way that:
◦ the voltage changes
◦ the current is measured
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most obvious problem is the oscillations
“digitise” the current measurement, so that a single measure
per drop was taken
measurement is timed at just before the drop falls off
(knocker)
slightly improved sensitivity
polarogram
output
measurement
point
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sensitivity is limited by the relatively high level of background current
it “hides” analyte response
three causes:
◦ other species – apart from oxygen, not solvable
◦ voltage changes – the drop charges like a capacitor as the V changes
◦ drop growth – high bkgd current at start of drop growth
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V changes – apply V increases in steps (pulses), since capacitor
behaviour fades if V is constant
drop growth – measure at end of drop life: bkgd current has faded
away
10 x improvement in sensitivity
pulsed
change
V
time
continuous
voltage change
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pulse still gives difficult to measure wave
DP measures at two points (start and end of drop)
current plotted is difference
20 x increase in sensitivity
change of shape to peak (like 1st derivative titration curve)
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sensitivity – realistic detection limits for differential pulse polarography
are around 50 ug/L,
multi-component analysis – provided the half-wave potentials are at least
100 mV apart,
equipment that is relatively simple and not particularly expensive –
typically $40,000 for a computer-controlled device capable of
polarography and voltammetry,
a wide range of analytes - metallic ions, non-metallic ions and organic
species.
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contaminated mercury – which can be purified by distillation with special
apparatus,
relatively slow – due to purging time
matrix interference – due to complex formation, which can make a species
not analysable because the half-wave potential is outside the measurable
range
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an electrode which measures dissolved oxygen (DO)
not an ion-selective electrode
relies on current, not potential, measurement
the oxygen is not an interference but the analyte
non- scanning: V held at -0.8V
current is proportional to the oxygen concentration
calibrated using a saturated solution (9 mg/L at 25C)
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the most sensitive form
analyses much more of the sample than normal polarography
requires stirring & longer reaction period
cannot do a very slow scan
Hg drop electrode still used
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Step 1 (slow) - fixed voltage, with stirring for 90s to 10 minutes
◦ M+ + e => M (Hg amalgam)
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Step 2 (normal speed) – scan
◦ M(Hg amalgam) => M+ + e
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not all analyte is reduced – time dependent
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can measure at ng/L (not ug/L) level
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limited to those which form an amalgam with Hg
◦ copper, lead, cadmium, zinc, indium and bismuth
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What is the problem with using a DME for this analysis?
reduced analyte in step 1 falls to the bottom of the cell and is lost
What could be done to get around this problem, still using a mercury drop
as the electrode?
both steps are done with a single drop
called Hanging Drop Mercury Electrode (HDME)