e mërkurë, 20 qershor 2007

The Sea Water

The Geochemistry of Sea Water

The complex mixture of minerals and liquid that make up sea water consists of 96.5 per cent water, 3.4 per cent salts, and smaller amounts of other substances including dissolved inorganic and organic materials, particulates, and a few atmospheric gases. It is the salt that gives sea water its taste. Salt is a chemical compound of elemental atoms that act as molecules and ions when in solution. Eighty-four of the earth's basic elements have been identified in sea water as either macro or trace mineral ions.

Ocean water is naturally alkaline and its bicarbonate buffering results in a normal pH of 8.0-8.3. Metal bicarbonates such as magnesium bicarbonate and calcium bicarbonate will only form in the natural aqueous, alkaline state of the ocean. These mineral combinations do not occur in solid or dry form.

Water is compatible with more substances than any known solvent and the minerals in sea water are fully dissolved, which means they exist in a free-flowing, non-bound, ionic form; they are not tiny suspended metallic particles in water. Water molecules align in such a way as to shield each ion in sea water and thereby maintain dissolution. This property helps to explain the abundance of ions in the ocean that give it its salty character. Sea water cannot be artificially created in the laboratory.

The complex spectrum of macro and trace minerals found in sea water is the result of the interaction of natural forces over millions of years; a process not yet fully understood by scientists. The trace elements of sea water should not be underestimated. For example, there is more gold in a ton of sea water than in a ton of good to average gold ore.

The chemical composition of sea water is influenced by a wide variety of chemical transport mechanisms. Rivers add dissolved and particulate chemicals to the oceanic margins, and windborne particulates are carried to mid-ocean regions thousands of kilometres from their continental source. Hydrothermal solutions that have circulated through crustal materials beneath the sea floor add both dissolved and particulate materials to the deep ocean. Organisms in the upper ocean convert dissolved materials to solids, which eventually settle at greater depths. Particulates in transit to the sea floor, as well as materials both on and within the sea floor, undergo chemical exchange with the surrounding solutions. Through these local and regional chemical input and removal mechanisms, each element in the oceans tends to exhibit spatial and temporal concentration variations. Physical mixing in the oceans (thermohaline and wind-driven circulation) tends to homogenise the chemical composition of sea water. The opposing influences of physical mixing and of biogeochemical input and removal mechanisms result in a substantial variety of chemical distributions in the oceans.
Encyclopedia Brittanica

In the following table the 84 readily identifiable elements in sea water are listed:

click here to download Seawater Analysis Table

A significant feature of sea water is that while the total concentration of dissolved salts varies from place to place, the ratios of the more abundant components remain almost constant. This 'law' of constant proportions was discovered by Dittmar (1884) based on 77 samples of sea water collected from around the world by the Challenger Expedition, and confirmed a hypothesis first conceived by Forchhammer (1865).

Salinity has been defined as the mass in grams of solid material in a kilogram of sea water after evaporating the water away. For example, the average salinity of ocean water is about 35 grams of salts per kilogram of sea water (g/kg), previously written as 'S = 358/oo' or as 'S = 35ppt' and read as 'thirty-five parts per thousand'.
Pickard and Emery, op. cit. (Revised ed.)

Many of the characteristics of sea water correspond to those of water in general, owing to their common chemical and physical properties. For example, the molecular structure of sea water, like that of fresh water, favours the formation of bonds among molecules.

However, some of the distinctive qualities of sea water are attributable to its salt content. The internal resistance to flow is higher than that of fresh water because of its higher salinity. The density of sea water also is higher for the same reason. Sea water's freezing point is lower than that of pure water and its boiling point is higher. It has excellent electrical conductivity in comparison to distilled water, which is a poor conductor. Apart from its stores of sodium chloride, sea water also constitutes a rich source of other commercially-important chemical elements such as magnesium and bromine.

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