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.
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.
Salinity is important in coastal waterways for the following reasons:
- Salinity is a dynamic indicator of the nature of the exchange system.
- The salinity of the water within the estuary tells us how much fresh water has mixed with sea water.
- Plots that show the relationship between salinity and other soluble substances (e.g. nutrients) can be used to demonstrate the dynamic or conservative nature of those substances in 'mixing plots';
- Salinity is an important determinant of the mixing regime - because of the density variation associated with salinity variation, salinity stratification tends to inhibit vertical mixing in an estuary; which can have important implications for dissolved oxygen concentrations.
So it can be said that oxygen solubility decreases slightly as salinity increases, but oxygen solubility decreases more as temperature goes up regardless of salinity. There is however, a sizable difference in oxygen solubility in freshwater and seawater. Solubility of oxygen in seawater is 21% less than that of freshwater at 32 degrees Fahrenheit and 17 % less than that of freshwater at 100 degrees Fahrenheit. Oxygen solubility in freshwater decreases from 14.6 to 8.24 mg/L as temperature rises from 32 to 100 degrees. This is a 46.3% decrease. On the other hand, oxygen solubility in seawater decreases from 11.5 to 6.75 mg/L for this same temperature increase, a decreased oxygen solubility of 41.3%.
The salt concentration directly affects the salinity which impacts circulation with estuaries and coastal regions can derive from or be strongly influenced by the density variation associated with salinity. In effect, dense saline water tends to flow under fresh water. Salinity is an important ecological parameter in its own right; and it is important in some chemical processes.
We are changing the environment every day by the choices we make. The salt and chemicals that are released from industries and residential water softeners do have an effect on our natural water sources. For our homes, there are better alternatives including salt-free water conditioners which can reduce scale buildup without the use of sodium. With proper knowledge, caring awareness and a positive attitude we can reduce our ecological footprint and make a big difference to the world we all share. We encourage you to learn more about how to protect our natural water resources, one drop at a time.
