CHEM465/865, 2006-3, Lecture 8, 22nd Sep.
, 2006
Electrode configurations and reference electrodes
Nernst equation: in principle possible to calculate and
measure EMFs for half reactions and electrochemical
cells (examples in problem sets)
In general, the EMF
νC νD
E = E0 −
RT
ln Q with Q=
[C ] [ D ]
νe F
νA νB
[ A] [ C ]
has two parts:
Standard EMF E 0 (tabulated values)
standard conditions:
25°C
gases: 1 bar, solutions: 1M, unit activities
composition-dependent part, deviations from
standard conditions (involving T-variation)!
Known standard potentials and composition of system
→ in principle EMFs of all systems can be determined!
However: there is no absolute scale, only differences in
electrode potentials can be determined. Is that bad?
Electrode potential of single electrode configuration:
Metal|solution interface
Electrode potential: E el = ϕ − ϕ (cathodic)
M S
M S Remember: Eel proportional to the amount
of work required to move a test charge
ϕM ϕS across the metal|solution interface
Measurement: two electrodes needed
Electrochemical Cell
EMF: Ecell = ϕ − ϕ
C A
Electrical work performed by system
in bringing electron from anode to
cathode.
ϕA ϕC
Can be calculated for known Eel at the
ϕS two distinct M|S interfaces
Anode Cathode Ecell = EelC − EelA
Measurable! What do we need?
metal connection between electrodes
voltmeter with high electronic resistance
(“close to” electrochemical equilibrium).
Classification of electrodes
Based on nature of species involved in electrochemical
equilibria: electrode material, components of electrolyte,
other substances (gases, liquids, solids)
Conventionally, four types of electrodes distinguished:
1) Electrodes of the first type
Metal electrode in contact with solution of its
ions: Mz+ + ze- →M (ion transfer)
Eel = E 0 +
RT
zF
(
ln aMz+ )
e.g. Zn2+ |Zn or Au|[Au(CN)2]-
Nonmetal in contact with its ions on the
surface of an inert metal electrode, e.g. the
HYDROGEN ELECTRODE: H+|H2(g)|Pt
1/2
RT pH2
Eel = E −
0
ln
F a + (here: cathodic)
H
or Pt,Cl2|Cl- (these are gaseous electrodes)
e- transfer between neutral species and its ion
Hydrogen electrode
2) Metal electrodes in contact with solution containing
anions that form a poorly soluble salt with the metal
ions, e.g. the
CALOMEL ELECTRODE: Hg|Hg2Cl2|Cl-
E = E0 −
RT
F
( )
ln aCl-
Salt: almost entirely in solid form (~ unit activity).
Electrode potential: function anion activity only.
Low solubility of the salt: electrodes are very stable
– good reference electrodes.
Another example: Ag|AgCl|Cl-
3) Redox or inert electrodes: simultaneous equilibria
with respect to anions and cations, no surface
reaction occurring
4) Other, e.g. pH sensitive glass electrode.
Most important reference electrode:
standard hydrogen electrode (SHE)
Potential fixed (at all T) at ESHE = 0 .
0
Tabulated standard electrode potentials:
EMF of an electrochemical cell
standard hydrogen electrode on the left (ANODE)
considered electrode on the right (CATHODE)
all components of the system are at unit activity
very reproducible scale
Another important reference electrode:
standard calomel electrode (SCE), Hg|Hg2Cl2|Cl-
________________________________________________
Recommended literature (electrochemical equilibria, electrode
potentials, EMF, measurements of potentials):
Encyclopedia of Electrochemistry, Edited by A.J. Bard and
M. Stratmann, vol. 1, ch. 1, Weinheim, Wiley-VCH, 2002-...
From R.A. Silbey and R.J. Alberty, Physical Chemistry, Wiley, NY, 2001.
From: Encyclopedia of Electrochemistry, Edited by A.J. Bard and M.
Stratmann, vol. 1, ch. 1, Weinheim, Wiley-VCH, 2002-...
The Electrical Double layer
This part is about charged solid surfaces in liquids:
the most important liquid is water
high dielectric constant – good solvent for ions
most surfaces in water are charged
Here: consider case, when external electric potential
between the considered metal|electrolyte interface and a
counterelectrode is applied
(see previous topic: electrochemical cell and EMF).
Excess charges on metal surface →
internal electric field at metal|electrolyte interface
Electric field attracts counterions, i.e. ions of
opposite charge
Surface charge and counterions form the so-called
“electric double layer” (EDL)
We will see some characteristic experimental data for
the capacitance at the interface. Employing some ideas
about the distributions of ions in solution will help to
rationalize these data and understand in more detail the
nature of the interfacial region.
Whenever two dissimilar phases are in direct contact
with each other, forces become active:
Short-range forces (chemical forces, dispersion
forces)
Long-range forces (coulombic interactions)
They following phenomena can be observed:
Orientation of solvent molecules
Accumulation of ionic species
Specific adsorption of ionic/non-ionic species
Experimental techniques in surface electrochemistry:
Traditional: cyclic voltammetry, impedance spectroscopy,
electrocapillary measurements – no spatial resolution of structures
and processes, indirect information
Modern techniques: spectroscopy (IR reflection, X-ray diffraction and
absorption, second harmonic generation) and microscopy (e.g.
scanning tunneling microscopy) – study processes with
molecular/atomistic resolution
In any electrochemical experiment, one applies a potential to the electrode
and measures the charge flowing to the electrode surface. This charge
generates an electric field of 107-108 V/m that drives ions and dipolar
molecules. We want to understand in more detail the effects of this electric
field.
We start first with the purely electrostatic effects (due to coulomb
potential). Later on, we will consider specific adsorption and then charge
transfer processes.
For the studies of the pure electrostatic effects (i.e. no specific adsorption,
no charge transfer for the time being), we need suitable model systems.
These are called...
