Electrochemistry and Solution Kinetics
On his appointment to a permanent post in
1964, John Albery decided to develop the rotating disc electrode for the study
of electrode kinetics. He spent a most productive summer at the University of
Minnesota with Professor Stanley Bruckenstein. In three months they developed
the analytical theory for the ring-disc electrode and its application to the
study of both heterogeneous and homogeneous kinetics. This has been a
profitable line, and it continues today. Two early students were Michael
Hitchman, now a Professor at Strathclyde and the distinguished theoretical chemist
Jens Ulstrup. At the start of his project Jens had to build for himself a
potentiostat. After toiling for three months he finished one evening and the
next morning was to be the great switch on. And so it was. A flick of the
switch, and Family Favorites filled the room; a small radio had been connected
inside.
Among many successful applications of the
ring-disc electrode one of the most significant was the collaboration with Dr
Allen Hill in which Robert Hillman and Mark Eddowes used a.c. to find the
complete free energy profile for the reaction of cytochrome c on a gold
electrode modified with 4-4' bipyridyl. This has led to the development of a
range of biosensors using enzyme electrodes.
In 1976 a collaborative programme was
started with Dr Marianne Fillenz of the Department of Physiology on the
application of in vivo electrodes to neurophysiology. Early work
established the identity of the three compounds measured on implanted carbon
paste electrodes as being ascorbic acid, uric acid and 3-homovanillic acid. The
collaboration continues and in 1991 Peter Galley has developed the first enzyme
electrode that can measure in vivo changes in the important
neurotransmitter, glutamate. Ascorbate can now be measured on a time scale of
half a second and changes in the level of ascorbate as a result of mild
behavioural stimuli such as pinching the rat's tail can now be followed.
The study of photoelectrochemistry was
started in 1971 with the development by Dr Russ Egdell of the transparent
rotating disc electrode. With Dr Mary Archer this technique was applied to the
development of photogalvanic cells for solar energy conversion. Differential
electrode kinetics were shown to be vital and to be present in the iron
thionine cell. Unfortunately experiment and analysis subsequently showed that
these cells could never be more than one percent efficient. The thionine coated
electrode was one of the first examples of a modified electrode and led to the
study of polymer modified electrodes.
In 1974, John Albery persuaded Barry Coles
that it was safe to carry out electrochemistry in his esr spectrometer. The
theory for the signal to be observed when a radical is generated on an
electrode in a flow cell was developed at first by Albery and later by his
student Richard Compton. Electrochemistry came to an end in the PCL when John
Albery was appointed Professor of Physical Chemistry at Imperial College. It
started up again when Richard Compton returned from the University of Liverpool
in 1985.
Richard Compton, following his D.Phil. with
John Albery, was appointed Lecturer at the University of Liverpool in 1981, and
returned to the PCL in 1985. The work of his group lies in the general field of
electro- and interfacial chemistry, and an important aspect is that of the
measurement and interpretation of rate processes at the insulator/liquid
interface. The experiment involves the use of the Channel Flow System, whereby
a reacting solution is pumped along a rectangular duct, and over the substrate
which forms part of one wall of the channel: changes in concentration of
reactant are measured downstream by an appropriate electrochemical sensor. Such
a system is susceptible of mathematical analysis, provided that the type of
chemical kinetics is specified, so that comparison of theory and experiment
allows selection between candidate rate equations, thus giving mechanistic
information. In recent work, the technique has been applied successfully to
problems such as the kinetics of dissolution of calcite in aqueous solutions,
the adsorption of a range of species on calcite and the kinetics of bleaching
of a coloured fabric. There are significant industrial applications.
|
Channel flow cell |
|
|
|
|
While much is known about the separate
use of photochemical and electrochemical activation of organic molecules to
bring about reaction, not much is known about their simultaneous use,
where new chemistry can be expected. The group has developed, partly in
collaboration with Barry Coles, a channel electrode technique for the
quantitative study of photo-electrochemical reactions at metallic electrodes,
based on photocurrent/solution flow rate measurements, combined with simultaneous
EPR or other spectroscopic observations. This has enabled reaction mechanisms
for processes taking place at illuminated electrodes to be established. The
electro-reductions of systems such as crystal violet in acetonitrile and
fluorescein in aqueous base are indeed found to differ according as the
processes are carried out in light or dark, and correspondingly different
products are formed.
The group is also active in developing new
techniques for the study of electrode processes with the aim of clarifying the
relative contributions of the individual steps - mass transport, electron
transfers, homogeneous chemical reactions of intermediates and
adsorption/desorption phenomena - which go to make up the overall electrode
reaction.
In 1989 John Albery returned to the PCL as
Master of University College. Work in the PCL had two themes, of which the
first was the study of the kinetics of charge transfer in polymer modified
electrodes. In situ FTIR and ac impedence studies provided experimental
data which were successsfully interpreted using a new model involving a
two-resistance transmission line. Secondly, the kinetics of the deposition of
colloidal particles under electrochemical control were investigated as a
function of ionic strength and electrode potential. It was shown that particles
can be trapped hundreds of ångströms from the surface in the secondary minimum.
Reducing the ionic strength destroys the secondary minimum and releases the
particles back into solution.
