37
while others in more demanding environments allowed extended operation for days
to weeks. Some protected versions failed well before the unprotected version but
others showed marked performance improvement.
In general, results were encouraging for month-long deployments in certain low
productivity environments such as the coral reef waters of Kaneohe Bay, Hawaii, or
those of the test area in the US Great Lakes. On the other hand, deployments in
environments with very high biological fouling rates cannot be relied upon for more
than a few days. One instrument that otherwise showed acceptable performance
reported a drop of 8.6 mg.l
−1
or 268 μmole.l
−1
in a high biological fouling environment of the Chesapeake Bay.
Both instrument types are subject to biological fouling, be it of the polarograph
membrane or the optode foil. Biological fouling interferes with gas exchange and
can even constitute an oxygen sink in sufficient biomass. Oil slicks are not encountered as often but oxygen absorption is similarly impeded when it coats the active
element.
Despite the advances related, performance of these instruments leaves space for
improvement. Response times of even the faster amperometric instruments are still
unsuitable for rapid profiling and neither those nor the currently available optodes
can provide accuracy much greater than about 8 μmole.kg
−1
. Carefully executed
Winkler titrations using automatic titrators provide accuracy about four times
greater at around 2 μmole.kg
−1
. Winkler calibration using discrete samples, judiciously timed and spaced, is recommended for applications requiring high accuracy
and precision. Coppola et al. (2013) have provided an in-depth review of sensor
accuracies and scientific need.
2.3.3 The Inorganic Carbon System and pH in Seawater
The pH scale provides a shortcut for expressing the acidity of liquids which can
vary over 14 orders of magnitude. This acidity is caused by ionized hydrogen atoms
or free protons in solution (H
+
) which combine with a water molecule to form the
ionic species H 3 O
+
. Since this concentration can effectively range from as little as
10
−14
mol.kg
−1
(strong lye) to as much as 1 mol.kg
−1
(battery acid), an inverse log
scale is used with resulting pH values of 14 and 0. Distilled water is by definition
neutral (neither acidic nor alkaline) at pH 7, while seawater is slightly alkaline with
over ten times less protons in solution.
Anthropogenic release of carbon dioxide (CO 2 ) into the atmosphere, primarily
due to combustion of fossil carbon, is causing ocean acidification (Caldeira and
Wicket 2003; Zeebe 2012) because CO 2 reacts with seawater upon dissolution to
form carbonic acid, which rapidly dissociates to carbonate, bicarbonate, and H
+
ions. Historical data indicate that a net pH decrease from 8.2 to the current 8.1 has
occurred since the dawn of the industrial era, a time span of less than 300 years. It
is expected that continued fossil C emissions will result in a net decrease of 0.2–0.4
pH by the end of this century (Caldeira 2007) with eventual stabilization at about
2.3 Electrochemical Sensors for Coastal Ocean Observing
while others in more demanding environments allowed extended operation for days
to weeks. Some protected versions failed well before the unprotected version but
others showed marked performance improvement.
In general, results were encouraging for month-long deployments in certain low
productivity environments such as the coral reef waters of Kaneohe Bay, Hawaii, or
those of the test area in the US Great Lakes. On the other hand, deployments in
environments with very high biological fouling rates cannot be relied upon for more
than a few days. One instrument that otherwise showed acceptable performance
reported a drop of 8.6 mg.l
−1
or 268 μmole.l
−1
in a high biological fouling environment of the Chesapeake Bay.
Both instrument types are subject to biological fouling, be it of the polarograph
membrane or the optode foil. Biological fouling interferes with gas exchange and
can even constitute an oxygen sink in sufficient biomass. Oil slicks are not encountered as often but oxygen absorption is similarly impeded when it coats the active
element.
Despite the advances related, performance of these instruments leaves space for
improvement. Response times of even the faster amperometric instruments are still
unsuitable for rapid profiling and neither those nor the currently available optodes
can provide accuracy much greater than about 8 μmole.kg
−1
. Carefully executed
Winkler titrations using automatic titrators provide accuracy about four times
greater at around 2 μmole.kg
−1
. Winkler calibration using discrete samples, judiciously timed and spaced, is recommended for applications requiring high accuracy
and precision. Coppola et al. (2013) have provided an in-depth review of sensor
accuracies and scientific need.
2.3.3 The Inorganic Carbon System and pH in Seawater
The pH scale provides a shortcut for expressing the acidity of liquids which can
vary over 14 orders of magnitude. This acidity is caused by ionized hydrogen atoms
or free protons in solution (H
+
) which combine with a water molecule to form the
ionic species H 3 O
+
. Since this concentration can effectively range from as little as
10
−14
mol.kg
−1
(strong lye) to as much as 1 mol.kg
−1
(battery acid), an inverse log
scale is used with resulting pH values of 14 and 0. Distilled water is by definition
neutral (neither acidic nor alkaline) at pH 7, while seawater is slightly alkaline with
over ten times less protons in solution.
Anthropogenic release of carbon dioxide (CO 2 ) into the atmosphere, primarily
due to combustion of fossil carbon, is causing ocean acidification (Caldeira and
Wicket 2003; Zeebe 2012) because CO 2 reacts with seawater upon dissolution to
form carbonic acid, which rapidly dissociates to carbonate, bicarbonate, and H
+
ions. Historical data indicate that a net pH decrease from 8.2 to the current 8.1 has
occurred since the dawn of the industrial era, a time span of less than 300 years. It
is expected that continued fossil C emissions will result in a net decrease of 0.2–0.4
pH by the end of this century (Caldeira 2007) with eventual stabilization at about
2.3 Electrochemical Sensors for Coastal Ocean Observing