Environmental Engineering Reference
In-Depth Information
METHOD 11.2.
How to Measure O
2
Concentration
The most commonly used method for measuring O
2
concentration in the
field is with an O
2
electrode. The electrode has a gold-plated cathode and
a reference electrode (anode). When a voltage is applied across the cath-
ode and the anode, the cathode reacts with O
2
and causes an electrical cur-
rent to flow. The more O
2
that reacts with the cathode, the higher the cur-
rent that registers on the meter. A higher current corresponds with a higher
O
2
concentration. This type of electrode must be calibrated regularly for
variations in temperature and atmospheric pressure. These electrodes are
useful because they can be constructed with long lead wires and lowered
into lakes or wells to assess
in situ
O
2
concentrations. Very small-scale
cathode tips have been constructed to allow for determination of O
2
with
0.1 mm spatial resolution (Revsbech and Jørgensen, 1986).
The other commonly used method of measurement of O
2
is Winkler
titration. This method has been used for many years and is very reliable.
The presence or absence of O
2
is an important aspect of aquatic ecosys-
tems because it determines whether and what type of organisms can live in
a given ecosystem. Habitats that have any O
2
in the water are referred to
as
oxic
or
aerobic,
and those without detectable O
2
are
anoxic
or
anaero-
bic
. The absolute concentration of O
2
is also important, and several meth-
ods have been developed to measure it (Method 11.2).
The main cause of biological consumption of O
2
in most environments is
aerobic respiration. All organisms must metabolize, and the oxidation of or-
ganic carbon with molecular O
2
is the most efficient. The general reaction is
CH
2
O
O
2
→
CO
2
H
2
O
chemical energy
where CH
2
O is a general formula for sugar, and chemical energy is in the
form of ATP. However, the ratios of C to H and O vary in different or-
ganic compounds (e.g., phospholipids have relatively low oxygen concen-
trations relative to sugars). The previous formula should be considered
only an approximate representation of respiratory metabolism.
In addition to the O
2
that dissolves in surface water from the atmos-
phere, a significant amount of O
2
in surface environments is contributed
by photosynthesis. A generalized equation for oxygenic photosynthesis is
O
2
The photosynthesis equation is essentially the reverse of the equation for
respiration. Again, the equation for photosynthesis is a general equation
because the exact composition of the organic molecules (represented as a
general stoichiometry for sugars, CH
2
O, in the equation) varies depending
on the biological molecules being synthesized.
Respiration and photosynthesis are primarily responsible for main-
taining a constant concentration of O
2
in the earth's atmosphere. At smaller
CO
2
H
2
O
light energy
→
CH
2
O
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