The concentration of dissolved gases in water is of prime importance in considering the quality of water along with the other physico-chemical characteristics.
Ambient concentration of dissolved oxygen, carbon dioxide, nitrogen and ammonia are of greater importance, eventhough depending on the premises hydrogen sulfide, sulfur dioxide, chlorine etc, can also be of significance in aquaculture.
In view of the importance of gas content (in solution) of a mass of water we shall recall the gas laws, which you must be familiar with: Henry’s law, which states that “the concentration of saturated solution of a gas is proportional to the pressure at which the gas is supplied” and Dalton’s law of partial pressure, which states that, “the pressure exercised by a gas is proportional to the component of its concentration in the mixture, and the total pressure of the gas is equal to sum of its components”.
Thus the amount of consituent gases in atmosphere that will dissolve in water depends on the proportion of each gas in the air and the solubility of each constituent gas. A given volume of atmospheric air contains 78.084% nitrogen, 20.946% oxygen, and 0.934% argon. The remaining 0.036% contains carbon dioxide, noble gases and trace gases such as helium, krypton, neon and xenon plus water vapour. The partial pressure in the liquid phase that would be in equilibrium with measured concentration is called the dissolved gas pressure (TGP) (EIFAC, 1986) under natural conditions any of the three states can occur in water:
| TGP | = | BP (water is supersaturated – BP Barometer pressure in mm) |
| TGP | = | BP (In equilibrium) |
| TGP | = | BP (water is undersaturated) |
Percentage saturation can be indicated in terms of the constituent gases, or in terms of the total dissolved gases:
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