230
Chemical Oceanography, 4th Edition
The concentration of a gas at equilibrium is given by
[i] = P i (gas)/k i
(6.10)
The concentration of a gas in solution can be expressed by a variety of scales. Most physical chemists express the concentration as molality (mol [kgH 2 O] −1 ) or mole fraction. These
scales are useful since the magnitude is not affected by the composition of the gas phase
and is independent of temperature. Much of the earlier solubility of gases in seawater was
expressed in terms of the Bunsen coefficient, which is the value of [i] expressed in cubic
centimeters of a gas at STP per cubic centimeters of solution at the temperature in question when the P i = 1.0 atm. Since the volume of each gas is different at STP, this has led to
confusion. For practical reasons, the most convenient scale to use is moles per kilogram
of seawater when P i is the partial pressure and the total pressure is 1 standard atmosphere (1.013 bar). The correction for P i for different total pressures (in atm) and percentage
humidity (h) from standard values is given by
P i′ = P i (P T – P S h/100)/(1 – P S )
(6.11)
where P i′ is the corrected partial pressure of the gas, and P S is the vapor pressure of water
in seawater at a given temperature and salinity (S). It can be calculated from
P S = P 0 + A S + B S 3/2 + C S 2
(6.12)
where P 0 is the vapor pressure of water at S = 0 (Equation 6.6):
A = –3.7433 × 10 −3 + 1.6537 × 10 −4 t – 1.9667 × 10 –6 t 2 – 2.435 × 10 –7 t 3
(6.13)
B = 5.2556 × 10 –4 – 7.72660 × 10 –6 t
(6.14)
C = –4.9535 × 10 –5
(6.15)
At t = 25°C and S = 35, this equation gives P S = 2.7251 kPa or 0.027251 bar. The appropriate
humidity (h) and pressure (P S ) can be obtained from the conditions of the surface waters.
Care must be taken, however, since solubility is slow to respond to changes in P T , and
the humidity near the water–air interface may be different from that observed on a ship
(~5 m above the interface). For a water parcel that has left the surface, it is reasonable to
assume P T = 1 atm and h = 100%. In recent years, more accurate data have become available for the solubility of gases in seawater. These more accurate data allow examination
of the departure of waters from equilibria. The supersaturation of N 2 and Ar, for example,
can be related to entrapped bubbles. The super- or undersaturation of O 2 can be related to
photosynthesis and respiration.
The solubility of gases (C is in μmol kg −1 ) in seawater has been fit by Weiss (1971) to equations of the form
ln C = B 1 + B 2 S
(6.16)
which conforms to the Setchenow salting out equation and
ln C = A 1 + A 2 /T + A 3 ln T
(6.17)
Chemical Oceanography, 4th Edition
The concentration of a gas at equilibrium is given by
[i] = P i (gas)/k i
(6.10)
The concentration of a gas in solution can be expressed by a variety of scales. Most physical chemists express the concentration as molality (mol [kgH 2 O] −1 ) or mole fraction. These
scales are useful since the magnitude is not affected by the composition of the gas phase
and is independent of temperature. Much of the earlier solubility of gases in seawater was
expressed in terms of the Bunsen coefficient, which is the value of [i] expressed in cubic
centimeters of a gas at STP per cubic centimeters of solution at the temperature in question when the P i = 1.0 atm. Since the volume of each gas is different at STP, this has led to
confusion. For practical reasons, the most convenient scale to use is moles per kilogram
of seawater when P i is the partial pressure and the total pressure is 1 standard atmosphere (1.013 bar). The correction for P i for different total pressures (in atm) and percentage
humidity (h) from standard values is given by
P i′ = P i (P T – P S h/100)/(1 – P S )
(6.11)
where P i′ is the corrected partial pressure of the gas, and P S is the vapor pressure of water
in seawater at a given temperature and salinity (S). It can be calculated from
P S = P 0 + A S + B S 3/2 + C S 2
(6.12)
where P 0 is the vapor pressure of water at S = 0 (Equation 6.6):
A = –3.7433 × 10 −3 + 1.6537 × 10 −4 t – 1.9667 × 10 –6 t 2 – 2.435 × 10 –7 t 3
(6.13)
B = 5.2556 × 10 –4 – 7.72660 × 10 –6 t
(6.14)
C = –4.9535 × 10 –5
(6.15)
At t = 25°C and S = 35, this equation gives P S = 2.7251 kPa or 0.027251 bar. The appropriate
humidity (h) and pressure (P S ) can be obtained from the conditions of the surface waters.
Care must be taken, however, since solubility is slow to respond to changes in P T , and
the humidity near the water–air interface may be different from that observed on a ship
(~5 m above the interface). For a water parcel that has left the surface, it is reasonable to
assume P T = 1 atm and h = 100%. In recent years, more accurate data have become available for the solubility of gases in seawater. These more accurate data allow examination
of the departure of waters from equilibria. The supersaturation of N 2 and Ar, for example,
can be related to entrapped bubbles. The super- or undersaturation of O 2 can be related to
photosynthesis and respiration.
The solubility of gases (C is in μmol kg −1 ) in seawater has been fit by Weiss (1971) to equations of the form
ln C = B 1 + B 2 S
(6.16)
which conforms to the Setchenow salting out equation and
ln C = A 1 + A 2 /T + A 3 ln T
(6.17)