is used by microorganisms or reduced species in the water. A similar pattern has
been observed when monitoring the redox potential (Eh) after purging a dip-
well. Apart from biasing the oxygen measurements this instant oxygen uptake
will, of course, influence the concentrations of other species over time. One of
the most sensitive species is dissolved iron: at full oxygen saturation, 20°C and
neutral pH (7) it takes only 30 min before 50% of the dissolved Fe^2 is oxi-
dised and precipitated as iron(III)hydroxides.
To completely avoid these problems it is necessary to work in an inert atmos-
phere during purging and handling of samples. As for purging, a continuous
flow of nitrogen gas in the headspace of the dipwell can effectively hinder the
instant oxygen uptake, and handling of samples in glove boxes in an inert
atmosphere is a standard practice when working with marine sediments. This is
the ideal solution, but it must also be admitted that such procedures make the
sampling and analysis a lot more tedious. If the highest precision is not required,
fast sample handling after purging of the dipwells may reduce the error to an
acceptable level for all species but oxygen itself.
Following purging, a water sample must be retrieved with a syringe and tube
or suitable pumping system. PVC tubing should be used rather than silicon tub-
ing due to its lower oxygen permeability. Water samples should be filtered using
0.45m filters, as this by tradition is a standard filter size for distinguishing
between dissolved and particulate species in environmental science. All sample
handling should be performed as quickly as possible and with minimum con-
tact to atmospheric air – the most sensitive species should be analysed or
“fixed” within half an hour of sampling. Ideally samples should be analysed
in the field using suitable electrodes or spectrophotometric methods designed
for field analysis of water quality parameters. However, if such equipment is
not available samples can be fixed and sent away to the laboratory.
2.3 Redox Potential (Eh)
The extent of reducing ion species present can be collectively measured as the
Eh. Oxidation involves the loss of electrons, while the process of reduction can
be viewed as the gaining of electrons. In order to maintain electrical neutrality
overall, it is clear that the oxidation of one species must be accompanied by the
reduction of another somewhere in the system: hence, the concept of redox – the
simultaneous occurrence of reduction and oxidation. In practice, a chemically
inert electrode such as platinum or gold is used, which can transfer electrons to
or from the environment. The potential developed is measured relative to a stand-
ard cell which itself is calibrated relative to the standard hydrogen electrode.
Although measurement of Eh can be problematic, relatively stable measure-
ments can be obtained using permanently installed electrodes of platinum or
gold whereas “spot” measurements should be avoided due to unstable and
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