18
chemists. Additionally, different strategies have been adopted and in-line pre-treatments prior to
potentiometric detection have been recently put forward. Specifically, the pre-treatment units aim
to eliminate and/or reduce the major interfering ions. For example, the presence of OH
–
anions in
natural water (pH~8, ~10
-6
M) with the excessively large amount of Cl
–
(~600 mM) in seawater
complicates the potentiometric detection of very low amounts (µM level or much lower) of other
anions, like nitrate, nitrite and phosphate. Therefore, in-line pre-treatments, such as acidification
and desalination, permits the detection of nitrate and nitrite, though the potentiometric detection of
phosphate (in the form of HPO
4
2
–
) seems more complicated owing to the very low limits of
detection required (see Table 1) in conjunction with the few, poor receptors available.[76, 117,
118]
Nitrite and nitrate cannot be potentiometrically detected in seawater as shown in Figure 6. No
response is observed with rising nitrate concentrations up to mM levels
and a very high limit of
detection is needed for nitrite (~3x10
-5
M) when potentiometric calibration curves are established
in 600 mM NaCl to mimic seawater. Once Cl
–
levels are reduced down to the mM level, the nitrate
calibration curve significantly improves by one order of magnitude (~3x10
-6
M).[118] However, for
nitrite, it was demonstrated that by decreasing Cl
–
and OH
–
levels in a sample, a sub-µM limit of
detection was accessible.[76, 117]
A microfluidic custom-fabricated thin-layer flat cell allows for
the reduction of the Cl
–
concentration
of seawater down to mM levels.[118] The cell operates by exhaustive electrochemical plating of
the Cl
–
(and to a minor extent, other halides) from the sample onto a Ag electrode as AgCl (Figure
6a,b). This process is coupled to the transfer of the corresponding counter cation across a
permaselective ion-exchange membrane to an outer solution, the fine design enabling its coupling
with a potentiometric flow cell containing an all-solid-state nitrate-selective electrode for nitrate
detection in the dechlorinated plug sample. Furthermore, the concentration of nitrate was
successfully determined in seawater samples using the described setup.[118] The miniaturized
dimensions of the total system facilitated its application for
in situ
detection of nutrients using an
adequately designed flow system.
19
Other elegant approaches for saline water desalination have also been proposed and rely on
basic electrochemistry, such as electroplating.[119, 120] A remarkable recent advance is the use
of bipolar electrodes for Cl
–
removal
by its oxidation to Cl
2
gas at a platinum electrode.[119, 121]
This electrochemical principle is simple, reversible and sufficiently selective for Cl
–
and the
custom-built electrochemical cell was miniaturized although its implementation for further ion
detection in seawater has not been accomplished to date. Despite the very promising efficiency of
this concept for desalination (~25%), it is not yet adequate for potentiometric measurements
compared to other electrochemical processes based on the deposition of Cl
–
as AgCl on the
working electrode (WE).[118, 120] In-line acidification not only of seawater, but also of any type of
water, is also feasible. The working principle of the reported acidification cell relies on the cation-
exchange process between the sample and an ion-exchange Donnan exclusion membrane in its
protonated form (Figure 6c).[117] The control of the flow rate of the sample allows for the
attainment of the desired modification of the sample pH through different contact times with the
membrane. For
instance, in-line acidification of natural water with mM NaCl levels (freshwater,
drinking water, aquarium water as well as dechlorinated seawater) at 30 µL min
-1
provides pH~5
that is appropriate for enhancing the limits of detection of an all-solid-state nitrite-selective
electrode by more than two orders of magnitude with respect to that observed at environmental
pH (pH~8; see Figure 6d).[117] Easy coupling with any detection technique, miniaturized
configuration and simple implementation for long-term monitoring with submersible probes are
relevant analytical features of the proposed approach. Finally, this device was successfully
applied for potentiometric nitrite detection in aquarium waters and dechlorinated seawater
samples.
Mendecki et al.[75] described lowering the limits of detection of an all-solid-state carbonate
selective electrode. The ISE was conditioned in a solution that contained a THF/water mixture of
the carbonate ionophore for a period of one to 24 h. This results in the
reduction of ion fluxes
across the membrane interface, consequently lowering
the LDL to pM levels.
The proposed
ISEs exhibited near-Nernstian
potentiometric responses to carbonate ions with a detection limit
20
of 10
-9
M in artificial seawater. Despite the levels of carbonate ions in fresh and seawater ranging
approximately between 0.1 to 5 mM and being sufficient for traditional protocols with this particular
application, such an elegant approach may open up new horizons for
nitrate and nitrite ion
detection and probably for phosphate ions, as well.
The reported desalination and acidification concepts together with the potentiometric detection of
nitrate and nitrite demonstrate that seawater monitoring by ISEs is even more challenging than in
freshwater. Therefore, the direct and partial embedding of the electrodes into a submersible
prototype, as in the case of ammonium detection in lakes, is not sufficient and additional efforts
are required for a flow-based final deployable prototype. In this respect, the configurations of the
developed cells for the desalination and acidification processes facilitate their incorporation into a
submersible prototype that utilizes a pump system to push the water from outside of the
submersible housing first to the cell(s) for pretreatment and then through the potentiometric cell
for the detection of the anions.
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