Project
The suspense of the mystery behind the title of our page must be huge by now, but it is about to be revealed. Your project is to find the DOC (dissolved oxygen content) necessary to support marine life, such as Nemo.
Brief:
Use Henry's Law to estimate the DOC of a body of water and use engineering and biological methods to alter this to a suitable level for marine life.
Motivation:
The DOC in a body of water is one the major indicators of the health of the ecosystem within that body of water. If it is too low, then the water will not support flora or fauna and if it is too high, then fauna living in the water can develop gas bubble disease. In the construction of a reservoir or a tank for an aquarium it is essential to consider whether the built environment will be able to support life.
Technical Justification:
Provided that the body of water is fairly still, it should be possible to estimate the DOC in in the liquid due to diffusion from the atmosphere using Henry's Law.
Henry's Law states that concentration of gas dissolved in a liquid is directly proportional to the partial pressure of that gas in the atmosphere above the liquid and that the coefficient of proportionality is the reciprocal of the Henry's constant of that gas.
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Henry's constant also varies with temperature.
To make these estimates, a few variables that would need to be defined. These are the partial pressure of the oxygen above the water, Henry’s constant for the combination of water and oxygen combination, how this varies with temperature and how the temperature above the body of water varies.
Finding the partial pressure of oxygen above the body of water, which could be estimated by finding the atmospheric pressure and molar percentage of oxygen in the air through on site testing. The temperature above the body of water could also be found through onsite monitoring in the case of a unheated body of water or could be controlled in the case of a heated body.
The Henry’s Law constants and their variation with temperature could be found from literature values as in this journal entry: https://www.sciencedirect.com/science/article/pii/S0021961499905815 [8].
The other major considerations would be the impact of bacteria or plants on the oxygen content of the water. Plants release oxygen through photosynthesis and both plants and bacteria use oxygen through respiration. Water will also have more oxygen dissolved in it if it is aerated through movement.
The above could also be used to alter the DOC in the water. It might be possible to aerate the water, either through movement or by bubbling water through the body. Alternatively, adding plants to the water might mean an increase in the DOC whilst they're exposed to light.