Water Education - Water Quality


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When an ionic salt like NaCl is added to water, the ions from the salt introduced will attract the water molecules in an effort to "solvate" the ions. This has the tendency to decrease the weak affinity of non-polar oxygen molecules to water and drive the dissolved oxygen out of the polar water. In general, the solubility of a gas in a solvent is affected significantly by the presence of other solutes in the solution.

Pile of salt

The maximum amount of dissolved oxygen a body of water can hold (saturated solution) depends on several factors. Dissolved oxygen solubility is affected by water temperature, atmospheric pressure and salinity. Cold water can dissolve more oxygen than warm water. As the temperature goes up, water releases some of its oxygen into the air. Water also holds less dissolved oxygen at higher elevations because there is less pressure. Solubility of dissolved oxygen also decreases as salinity increases.

Dissolved oxygen refers to the amount of oxygen contained in water. Oxygen has limited solubility in water usually ranging from 6-14 mg/L. Dissolved oxygen reflect an equilibrium between oxygen producing processes and oxygen consuming processes. An example of an oxygen producing process is photosynthesis; and oxygen consuming process would be aerobic respiration, nitrification, chemical oxidation and aeration. Dissolved oxygen is significant because most aquatic organisms require oxygen in specified concentration ranges for respiration and efficient metabolism. The amount of dissolved oxygen may change during the day as the water begins to warm up. More light penetrating the water causes more photosynthesis to occur. This can also increase the amount of dissolved oxygen. For this reason it is important to measure the dissolved oxygen at the same time of day each week.

The solubility of gases in water usually decreased by the addition of other solutes, particularly electrolytes. Aerated drinks have carbon dioxide dissolved in it under pressure. Therefore, when salt is added to an aerated drink, the dissolved carbon dioxide is "salted out". The drink fizzes as many small bubbles of carbon dioxide are released from the drink. The extent of this "salting out effect" varies considerably with different salts, but with a given salt the relative decrease in solubility is nearly the same for different gases including dissolved oxygen.

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