1. 1. Voltammetry
AIT

2. Basics
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

3. M+ + e  M (s)
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)
some ions will always be close to the electrode by sheer chance
voltage as the controlling factor for whether reaction will occur

4. Applied potential
Reduction potential
Initial potential
Too low, so no reaction can occur
Current is zero
As potential approaches redn V, some ions react
Current is low and increasing
Current
As potential pass redn V, all ions near electrode react
Current is high
After redn V, ions newly arrived near electrode react
Current is high and constant

5. a measurable change in current as a consequence of a voltage change
this is known as a wave
whole scan is a voltammogram

6. Exercise 1.1 an analogy between spectroscopy and voltammetry peak wave
wavelength/frequency
voltage
intensity
current

7. Uses of voltammetry for both quantitative and qualitative analysis:
the wave position (voltage) is characteristic of a particular species
the wave height (current) is proportional to concentration

8. Movement of ions 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

9. Removing problems not stirring the solution controls convection
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

10. diffusion
electrostatic attraction

11. 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

12. 1.2 Polarography the most commonly used form of voltammetry
one of the electrodes is made from a capillary of mercury, forming a drop at the end
known as a dropping mercury electrode (DME)
scan is called a polarogram

13. Polarogram

14. Exercise 1.2 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.
(a) -1.1 V
(b) 7.5 uA

15. Polarographic cell Hg reservoir DME auxiliary electrode
N2 bubbler
auxiliary electrode
DME
reference electrode

16. Cell components
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

17. Why 3 electrodes? one pair (DME & ref.) to control voltage
one pair (DME & aux.) for current path and measurement
Auxiliary
DME
Reference
Current
Voltage

18. Why a DME? does not seem like the most obvious choice
one significant advantage: it presents a fresh surface to the solution every second or so
allows a much more reproducible control of potential than a fixed electrode, where the reduced metal (for example) becomes coated to it
Hg oxidised >+0.4 V, so a Pt or graphite working electrode must be used

19. Matrix effects presence of complexing agents (ligands) shifts E½
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

20. Improvements to polarography
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

21. 1. Sampled DC 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

22. 2. Pulsed polarography
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

23. Solutions
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
time V
continuous
voltage change
pulsed
change

24. 3. Differential pulse 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)

25. Advantages
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.

26. Disadvantages
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

27. DO electrode 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)

28. Anodic stripping voltammetry
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
Step 1 (slow) - fixed voltage, with stirring for 90s to 10 minutes
M+ + e => M (Hg amalgam)
Step 2 (normal speed) – scan
M(Hg amalgam) => M+ + e

29. ASV not all analyte is reduced – time dependent
can measure at ng/L (not ug/L) level
limited to those which form an amalgam with Hg
copper, lead, cadmium, zinc, indium and bismuth

30. Exercise 1.4 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)