Chemistry Reference
In-Depth Information
dicted initially from thermodynamics, i.e. the rela-
tive potential of the species in the electrochemical
series. These data can indicate only the possibility of
selective metal recovery, because in practice this is
affected also by the cathode substrate and the
electrolyte composition. Thus, it is usual to carry
out experimental tests to obtain appropriate
current/voltage responses of the complete system.
These data (polarisation curves), when obtained
under well-defined hydrodynamics, enable the iden-
tification of conditions for selective (or otherwise)
metal deposition in the absence of other competing
reactions. A typical curve that is representative of a
system with a neutral electrolyte containing a
mixture of Cu(II), Ni(II) and Cd(II) ions is shown
schematically in Fig. 19.16. It is seen in Fig. 19.16
that selective Cu deposition is possible in the absence
of hydrogen evolution, but that for selective
cadmium deposition the copper ion concentration
will need to be reduced to very low values.
In many cases the system can be more complicated
than suggested by the curves of Fig. 19.16 due to
several factors, such as:
Fig. 19.16
Schematic diagram of polarisation curves for multi-
deposition reactions.
values of overpotential for competing reactions at
the electrode surface, e.g. the reduction of protons
to hydrogen gas.
In aqueous solution containing dissolved oxygen,
reduction of oxygen may occur. This is more signifi-
cant when dealing with low concentrations of the
metal ions with well-oxygenated solutions, espe-
cially those systems where the anodic reaction is the
evolution of oxygen gas. Here the concentration of
oxygen can be supersaturated, although the actual
value depends significantly on the pH, the electrolyte
composition, etc. Current efficiency also can be
lowered by other reactions in addition to the hydro-
gen evolution and oxygen reduction reactions, such
as:
(1)
The nature and actual material form of the metal
deposit will change during the course of the elec-
trode reactions.
(2)
The evolution of hydrogen (and of oxygen) and
the reduction of oxygen can cause pH changes
at the electrode surface, which can lead to
changes in the chemical form of the deposit.
This, for example, could be the formation of
metal hydroxides, which also could lead to
passivation of the cathode surface:
(1)
The presence of other ionic (non-metallic)
species that may be cathodically active in the
potential range of operation. For example,
Fe(III) ions will result in the cathodic reduction
of Fe(III) to Fe(II) at potentials more anodic than
for most metal ions, e.g. Cd(II), and thus gener-
ally it will be under mass-transport control.
(2)
The presence of redox species that may be
chemically reactive with other dissolved ionic
and solid species. For example, metals that have
been electrodeposited and are not cathodically
protected are free to corrode by reactions such
as oxygen reduction or Fe(III) ion reduction.
(3)
The presence of other metal ions that also can
be cathodically electrodeposited.
M
n
+
+
n
OH
-
→
M(OH)
n
(3)
The possibility of the deposition of metal alloys.
(4)
A complex metal deposition reaction involving
adsorbed intermediates, etc.
Many solution species, particularly metal ions,
exhibit a tendency to complex with other species in
solution. The metal then is not present as a free ion
but as a complex with an inorganic or organic ligand,
e.g. cyanide, chloride, EDTA. Changes in the stan-
dard reduction potential of a metal ion occur if the
metal ion is complexed. Metal deposition will occur
less easily when complexation occurs and this is seen
in a more negative reduction potential by as much
as 0.6-0.8 V for the complexed metal ion.
In the above cases, the performance of the system
with a mixture of metal and other cations can be pre-
