SEAWATER CHEMISTRY
The ocean covers about 72% of the earth's surface. The ocean is 96.5% water, and 3.5% disolved substances. Each water molecule is made up of two atoms of hydrogen and one atom of oxygen. The angle formed between the two hydrogen atoms and the central oxygen atom is about 105°. This angular shape of the water molecule makes it electrically unbalanced, or polar.
Hydrogen Bonds in water. The attraction between adjacent
polar water molecules forms a webwork of hydrogen bonds. These bonds are
responsible for cohesion and adhesion. Hydrogen bonds between water molecules
make it difficult for individual molecules to escape the surface.
Each water molecule can be thought of as having a positive and negative side. When water comes in contact with compounds whose elements are held together by the attraction of opposite electrical charges (most salts), the polar molecule will seperate that compound's component elements from each other. This explains why water can dissolve so many compounds. Water is a powerful solvent, it will eventually dissolve almost any substance.
| Salinity | Chemical Equilibrium | Disolved Gases |
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The total quantity of dissolved organic solids in water is its salinity. The ocean's salinity varies from about 3.5% to 3.7% by weight. Most of the dissolved solids in seawater are salts that have been seperated into ions.
In order to determine salinity, you can measure the chlorinity of the water. Chlorinity is the measure of the total weight of chloride, bromide, and iodide ions in seawater. Because the proportion of chlorinity to salinity is constant, marine chemists have devised this formula to determine salinity.
Salinity in parts per thousand = (1.80655)(Chlorinity in parts per thousand)
In 1865, Johann Georg Forchhammer found that the ratio of major salts in samples of seawater from various locations was constant. This constant ratio is known as Forchhammer's Principle, or the Principle of Constant Proportions.
The ocean appears in chemical equilibrium. This means the proportion and amounts of dissolved salts per unit volume of ocean are nearly constant. The theory that is used to suggest that ions are added to the ocean at the same rate they are removed is the concept of residence time. Residence time is the average length of time an element spends in the ocean.
Residence Time = Amount of element in ocean / Rate element is added
Gases in the air readily dissolve in seawater at the ocean's surface. Plants and animals in the ocean depend on these dissolved gases to survive.
About 48% of the gas dissolved in the ocean is Nitrogen. Living organisms need nitrogen to build proteins and other important biochemicals, but they can not use the nitrogen directly. The nitrogen must first be fixed into a usable chemical form by specialized organisms. Most of the nitrogen compounds must be recycled among the organisms themselves.
About 36% of the gas dissolved in the ocean is oxygen. Most of the ocean's oxygen comes from photosynthetic plants. Since the plants require sunlight, most of the available oxygen lies near the surface.
The amount of carbon dioxide in the atmosphere is very small because it is used up by photosynthetic plants as a source for growth. Much more carbon dioxide moves from the atmosphere to the ocean than vice-versa.
Carbon dioxide concentrations increase with increasing depth, but oxygen concentrations usually decrease through the mid-depths and then rise again towards lower depths. Low concentrations of carbon dioxide at the surface are due to the photosynthetic plants. The amount of carbon dioxide increases with depth because there are less photosynthetic plants. There is a large amount of oxygen at the surface because it is produced by the photosynthetic plants. The oxygen decreases with depth because of the respiration of bacteria and marine animals. Oxygen levels increase slightly in deeper water because there are less animals to take it.
A Graph of Relative Proportions of Dissolved Gases
Red- Nitrogen
Yellow- Rare Gases
Blue- Bicarbonate's
Green- Oxygen
The Major Constituents of Seawater
| Constituents | Concentration in g/kg | Percent by Weight |
| Oxygen | 857.8 | 85.8% |
| Hydrogen | 107.2 | 10.7% |
| Chloride | 18.98 | 1.9% |
| Sodium | 10.556 | 1.1% |
As you can see, the two must abundent constituents are dissolved oxygen and hydrogen. The next two, however, are dissolved ions, Chlorine and Sodium.
This page has been designed entirely by Aaron Krause.
If you would like to find out more about seawater chemistry, try either of these web sites.
If you like Oceanography, go to the homepage for my High School Oceanography Class! It's Super!
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