41
industry, operates a digital ocean conditions dash board online for commercially
important Humboldt Bay with real-time pH data and an upwelling index and
forecast. http://www.cencoos.org/data/humboldt/oyster (accessed Sept 15 2017).
Similar efforts are underway in Alaska (http://www.aoos.org/alaska-ocean-acidification-network. accessed Sept 15 2017).
Sensitivity of reef corals to ocean acidification has been largely attributed to
decreases in the aragonite saturation index rather than directly to the increased acidity (Langdon et al. 2000; Caldeira 2007). Nevertheless, this concept has lately come
under increased scrutiny since intracellular fluids are actively controlled by the
organism. An alternative proton flux limitation model (Cyronak et al. 2015) addresses
the problem in terms of environmental limitations upon biochemical proton pumps
that favor precipitation by altering carbonate chemistry at active intracellular
precipitation sites.
Regardless of the biochemical mechanism, increased acidification is known to be
detrimental to a wide range of marine organisms. The adverse reactions of pteropods, a planktonic mollusk, to ocean acidification is of special concern since this
organism constitutes a prime prey for various Pacific salmon, species of great commercial importance. Profits from shellfish culture and ecosystem services provided
by coral reefs are other examples of the losses that may accrue and detrimental
economic effect of ocean acidification. Another, less obvious effect is the noisier
ocean resulting from changes in boron acid-base chemistry reducing its sound
attenuation capacity.
The complexity of environmental pH control by the carbon system requires the
measurement of at least two variables for its full characterization that is the ability
to estimate the value of all remaining variables. Those that can be measured in the
laboratory are pH, pCO 2 (g) , pCO 2 (a) , total inorganic carbon (C T – see below), and
total alkalinity (A T ). Together with concurrent measurements of S, T, and P, these
data pairs provide an estimate of the remaining and derived variables. Different
analytical pairs however result in different precision of the various estimates.
Autonomous in situ measurement instruments are at time of writing only available
for pCO 2 and pH. This constraint significantly reduces the options for autonomous
instrumental measurement. Since pCO 2 and pH tend to covary, precision of derived
values is poor. Development of autonomous instruments for measurement of C T and
AC will allow greater precision. An overview of the theory, principles, and practice
of operation of the instruments available for autonomous measurement of the two
components of the inorganic carbon equilibrium system, pH and pCO 2 , follows.
pH Sensors for Ocean Observing
The current IUPAC definition of pH is an empirical one and, for historical reasons,
is based on the NBS definition (after the former US National Bureau of Standards)
where it was developed. Measurement is based on determination of the potential
electromagnetic force (EMF) developed at an ion selective electrode half cell with
reference to a known standard half-cell. The glass membrane electrode, more properly a half cell, familiar to most who have taken a basic chemistry course, is the
2.3 Electrochemical Sensors for Coastal Ocean Observing
industry, operates a digital ocean conditions dash board online for commercially
important Humboldt Bay with real-time pH data and an upwelling index and
forecast. http://www.cencoos.org/data/humboldt/oyster (accessed Sept 15 2017).
Similar efforts are underway in Alaska (http://www.aoos.org/alaska-ocean-acidification-network. accessed Sept 15 2017).
Sensitivity of reef corals to ocean acidification has been largely attributed to
decreases in the aragonite saturation index rather than directly to the increased acidity (Langdon et al. 2000; Caldeira 2007). Nevertheless, this concept has lately come
under increased scrutiny since intracellular fluids are actively controlled by the
organism. An alternative proton flux limitation model (Cyronak et al. 2015) addresses
the problem in terms of environmental limitations upon biochemical proton pumps
that favor precipitation by altering carbonate chemistry at active intracellular
precipitation sites.
Regardless of the biochemical mechanism, increased acidification is known to be
detrimental to a wide range of marine organisms. The adverse reactions of pteropods, a planktonic mollusk, to ocean acidification is of special concern since this
organism constitutes a prime prey for various Pacific salmon, species of great commercial importance. Profits from shellfish culture and ecosystem services provided
by coral reefs are other examples of the losses that may accrue and detrimental
economic effect of ocean acidification. Another, less obvious effect is the noisier
ocean resulting from changes in boron acid-base chemistry reducing its sound
attenuation capacity.
The complexity of environmental pH control by the carbon system requires the
measurement of at least two variables for its full characterization that is the ability
to estimate the value of all remaining variables. Those that can be measured in the
laboratory are pH, pCO 2 (g) , pCO 2 (a) , total inorganic carbon (C T – see below), and
total alkalinity (A T ). Together with concurrent measurements of S, T, and P, these
data pairs provide an estimate of the remaining and derived variables. Different
analytical pairs however result in different precision of the various estimates.
Autonomous in situ measurement instruments are at time of writing only available
for pCO 2 and pH. This constraint significantly reduces the options for autonomous
instrumental measurement. Since pCO 2 and pH tend to covary, precision of derived
values is poor. Development of autonomous instruments for measurement of C T and
AC will allow greater precision. An overview of the theory, principles, and practice
of operation of the instruments available for autonomous measurement of the two
components of the inorganic carbon equilibrium system, pH and pCO 2 , follows.
pH Sensors for Ocean Observing
The current IUPAC definition of pH is an empirical one and, for historical reasons,
is based on the NBS definition (after the former US National Bureau of Standards)
where it was developed. Measurement is based on determination of the potential
electromagnetic force (EMF) developed at an ion selective electrode half cell with
reference to a known standard half-cell. The glass membrane electrode, more properly a half cell, familiar to most who have taken a basic chemistry course, is the
2.3 Electrochemical Sensors for Coastal Ocean Observing