Environmental Engineering Reference
In-Depth Information
The DMT is a speciation technique based on the transport of the metal ions from the sample solution, through a negatively
charged semipermeable cation exchange membrane, to an acceptor solution with the same ionic strength as the sample solution.
This technique was first proposed by lampert [50] but gained wider acceptance only since its development by Temminghoff
et al. [51]. The membrane is initially in equilibrium with the chosen salt solution on the acceptor side. When the sample solution
is added, the cations will permeate the membrane until they attain equilibrium, whereas the rate of transport of anionic species
is impeded by the negative charge of the membrane. If the salt concentrations of the donor and acceptor solutions are equal, they
will result in equal concentrations of the free metal ions on both sides of the membrane, reaching what is called Donnan
equilibrium. If the free ion concentrations are equal, then a simple measurement of the total metal in the acceptor will yield the
free metal ion concentration in the sample. This technique has the advantage of being able to perform simultaneous measure-
ments of several elements; nevertheless, the equilibration time is very long (often 24 or 48 h), and the common membranes used
are calcium-based. This implies that the background electrolyte will necessarily have calcium ions, with the disadvantage of
modification of the ionic strength of the sample solution and the introduction of a significant amount of a divalent cation that
competes with our metal ions of interest for the binding ligands present in the solution. DMT is probably the best technique to
determine free metal ions in a solution as long as the system under study already has a medium to large concentration of calcium
and is stable over longer time windows.
AGNES is an electrochemical stripping technique designed to quantify low free metal ion concentrations (in particular
conditions, as low as 10
−10
mol l
−1
). This is a two-step technique: a deposition step where the metal ion is amalgamated in the
working electrode and a stripping step where the concentration of amalgamated metal is quantified. The essential concept in
this particular technique is that during the deposition step, the amalgamation of the metal in the working electrode is allowed
to proceed up until the equilibrium value between oxidized and reduced forms on both sides of the interface, which is set by the
applied potential and Nernst equation. When this equilibrium state is reached, there will be no concentration gradients, either
in solution or in the amalgam. The constant ratio is maintained between the concentrations of the electroactive couple due to
the Nernstian equilibrium (determined by the applied potential
E
1
) and is given by [52]
(
)
′
0
−
nF EE
−
*
c
c
1
0
Y
==
M
exp
(32.3)
*
RT
M
where
Y
is the concentration gain (or preconcentration factor),
c
M
*
is the metal concentration inside the mercury electrode, and
E
0
′
stands for the standard formal potential of the redox couple of the metal. The stripping signal is the characteristic measured
signal that depends on the electrochemical technique used in the stripping mode, and can be, for instance, the charge in a
stripping chronopotenciometry measurement [53]. One clear advantage of AGNES is its simple Nernstian response, similar to
an ISE. Effectively, the amalgamated metal is linearly related with the free metal ion concentration [52] with a proportionality
factor
h
(which can be obtained from the calibration), since, according to the following equation,
c
M
0
*
is
Yc
*
:
*
=
hc
Stripping signal
_
(32.4)
M
Although AGNES has very good selectivity and low detection limit, two main disadvantages need to be point out: (1) the
limitation to metal ions that can amalgamate reversibly on the working electrode, usually a mercury electrode (cadmium, lead,
zinc, indium, thalium, bismuth, tin, and antimony, although the last one has very low solubility in mercury); and (2) the time
involved in the preconcentration step to reach equilibrium (minutes to hours), which increases with the decrease of bulk
concentration.
32.2.2.5.2 Dynamic Speciation Techniques
Although in this chapter we focus on the free metal ion concentration, it is
worthwhile remembering that the physicochemical systems under study are seldom at equilibrium. Therefore, it is interesting
to briefly mention the main trace metal dynamic speciation techniques used: (i) stripping electrochemical techniques, among
which anodic stripping voltammetry (ASV) and scanned stripping chronopotentiometry (SSCP) are the most commonly used,
and (ii) diffusive gradients in thin films (DGT). The interpretation of the DGT signal for an NM system is quite complicated
and is currently under revision, so it will not be detailed here [54].
Among the electrochemical techniques, SSCP has many significant advantages over ASV, and, therefore, we will focus on
it. SSCP curves are constructed from a series of individual measurements made over an interval of deposition potentials,
E
d
,
covering the situations where no metal is reduced at the electrode, passing through the Nernstian regime to the region were all
the metal reaching the electrode is reduced (diffusion-limited situation). Each individual measurement consists of a two-step
