1. VoltammetryAIT

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

Basics

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

Applied potential

Reduction potential

Initial potentialToo 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

a measurable change in current as a consequence of a voltage change

this is known as a wave

whole scan is a voltammogram

an analogy between spectroscopy and voltammetry

Exercise 1.1

peak wave

wavelength/frequency voltage

intensity current

for both quantitative and qualitative analysis:◦ the wave position (voltage) is characteristic of a particular species

◦ the wave height (current) is proportional to concentration

Uses of voltammetry

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

Movement of ions

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

Removing problems

diffusionelectrostatic attraction

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

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

1.2 Polarography

Polarogram

Applied Potential

Cur

rent

limiting current

diffusioncurrent

residualcurrent

half-wave 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.

(a) -1.1 V (b) 7.5 uA

Exercise 1.2

Polarographic cell

Hg reservoir

N2 bubblerauxiliary electrode

DME

reference electrode

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

Cell components

one pair (DME & ref.) to control voltage one pair (DME & aux.) for current path and measurement

Why 3 electrodes?

Auxiliary DME Reference

Current Voltage

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

Why a DME?

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

Matrix effects

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

Improvements to polarography

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

1. Sampled DC

polarogram output

measurement point

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

2. Pulsed polarography

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

Solutions

time

Vcontinuous

voltage change

pulsedchange

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)

3. Differential pulse

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.

Advantages

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

Disadvantages

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 25C)

DO electrode

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

Anodic stripping voltammetry

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

ASV

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)

Exercise 1.4