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Answer:

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:

At an atmospheric pressure of 760 mm Hg (NTP) the partial pressure of oxygen (pO2) will be 159 mm Hg i.e.

At 100% air saturation solubility of oxygen at 30°C is 7.54 mg/l (see Table 1). If one records 6 mg/l at the same temperature then the corresponding pO2 value will be:

or 126.5/159 (6.0/7.54) = 79.6% saturation.

For presentation of values in different units of expression please see Fig. 2. To give an example, for conversion mg/l of oxygen to ml/1, multiply by 0.7 (i.e. 22.4/32 or mole volume/mole weight).

The sources of dissolved oxygen (D.O.) in natural waters is from atmosphere and photosynthesis of plants.

Explanation:Oxygen is absorbed in water by direct diffusion and by surface-water agitation. Solubility of oxygen in water is so small and by diffusion process alone in still water, it was culculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water. Absorption of water is very minor, that almost all the oxygen enrichment of natural waters takes place by agitation of water.

Photosynthesis of plants supply oxygen considerably - but this limited to the zones to which light penetration takes place. Photoperiod (day light duration), diurnal and seasonal, would decide the amount of oxygen produced by photosynthesis. Daily pulses of oxygen could be seen clearly in waters with high concentration of plankton - with an afternoon maximum and early morning drop, in some cases taking D.O. values close to zero. This can be seen in our tropical waters, where phytoplankton blooms occur. This will be referred to again in our discussion.

Answer:

Oxygen is absorbed in water by direct diffusion and by surface-water agitation. Solubility of oxygen in water is so small and by diffusion process alone in still water, it was culculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water.

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