Quantcast Henry's Law of Solubility

 
  
 
Nitrogen gas also enters few reactions (nitrogen fixation is an exception) and under some
conditions may be treated as an inert gas. However, nitrogen combined in other compounds is not
conservative and enters a large number of biological and chemical interactions. Nitrogen is an important
element to biota. But as a gas it is available to biota only under the extraordinary circumstance of
nitrogen fixation by procaryotes. (Don't worry, that topic will come later). Otherwise Nitrogen gas is
approximately conservative and does not enter biological processes. As a gas nitrogen is important
mostly when there is too much of it (when it is supersaturated). Supersaturation can cause injury or
death to fish and other biota and this is a topic of tremendous interest in the Pacific Northwest or other
regions where the phenomenon impacts wildlife resources. This is not a common problem in the
Southeast but supersaturation has harmed fish at some hydroelectric facilities.
Oxygen is obviously non-conservative. However, under very small time scales oxygen can
occasionally be viewed as approximately conservative. At such short time scales, the reactions affecting
oxygen concentrations do not proceed quickly enough to significantly affect the process. Carbon
dioxide forms only approximately 0.033% (and increasing) of the atmosphere and is not a conservative
material in water. Instead it enters chemical reactions that allow it to take forms not typical of gases.
Oxygen is one of the most important gases in a lake. Its presence or absence determines not only
the type or abundance of fisheries and other biota, but it also determines indirectly the type of chemical
processes that may take place in the lake. Oxygen is extremely chemically and biologically reactive and
is not conservative. Its distribution in a lake is determined by its solubility and lake conditions.
1.2.7.3 Henry's Law of Solubility
c=Kp
Where,
c
= the concentration of gas absorbed at equilibrium
K
= the solubility factor
p
= the partial pressure of the gas.
This simple relationship controls the solubility of gases in water. It is an equilibrium expression and
as such makes an assumption that equilibrium has been or will be attained. The solubility factor is
difficult because not only is it unique for each gas, but it varies with temperature and the presence of
other dissolved substances in the water. The partial pressure also varies with the environment. Water
at equilibrium with one atmosphere of air pressure (the major source for dissolved gases in lakes) will
have dissolved oxygen with a partial pressure of approximately 159 mm Hg (20.9% of atmosphere
times 760 mm Hg atmospheric pressure). Pure oxygen delivered to water from a diffuser head
submerged to a depth equivalent to 4 atmospheres of pressure (approximately 40 meters) will have a
partial pressure of 3040 mm Hg at equilibrium (don't be frightened by this stuff).
1.2-22

 


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