Sea water

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Most of the earth's surface is covered by sea ​​water . From a chemical point of view, seawater is an aqueous solution , mainly of various salts ( salt water ). However, natural seawater also contains a large number of other components (see below).


Chemical composition of sea salts
Salinity at the sea surface over the oceans in PSU

Sea water has an average salt content ( salinity ) of 3.5% by mass . This corresponds to a salt content of 35 grams per kilogram of seawater. The total salt content varies depending on the sea. The Baltic Sea has a salinity of 0.2 to 2%. Some inland lakes without a runoff have much higher levels of salt in the water; the Dead Sea is known for its salt content of 28%.

The mean value therefore applies above all to the main volume of the oceans and also to most of the secondary seas such as the North Sea . In 1819 Alexander Marcet discovered that the ratio of the main ions in seawater is exactly the same in all oceans. This principle of constant proportions applies regardless of the total salt content of the respective sea.

The salt is dissociated in sea water, i.e. split into ions . The latter only become salts when the water evaporates, which form according to their solubility and are deposited in layers. The main part of the anions is the chloride ion , followed by the sulfate ion . In the cations , the sodium ion predominates, which is why the majority of the crystallized sea salts consist of sodium chloride ( table salt ). Magnesium , calcium and potassium ions are represented in smaller proportions. Traces also contain other ions, of which the trace element iodine is worth mentioning, because as a result in earlier times fewer people near the coast suffered from iodine deficiency than inland.

The density of seawater increases roughly proportionally to the salinity, which also changes with temperature in accordance with the ( anomalous ) expansion behavior of water. This is important for swimming and diving, the carrying capacity of a ship (compare loading mark ), a pontoon bridge and the laying of pipelines .

Salt balance

The freezing point of sea water is -1.9 ° C with an average salinity of 3.5%. The salts are washed out of the soils and rock layers of the mainland by rain and meltwater and carried into the oceans by running waters . The originally diluted salt solution is further concentrated through evaporation , and salty sea water is produced. This effect would cause the salt content of the oceans to rise slowly but continuously if salt was not withdrawn from the ocean at the same time. Firstly, this happens through the drying up of the seas, which causes the salt to be deposited back on the mainland. This salt is then found later z. B. in salt domes again. Second, seawater is trapped in the pores of the sediments on the sea floor, thereby drawing the salt out of the water. The second process is the more important.

Saline inland lakes

In addition to the accumulation of salt in the world's oceans, there is an accumulation of salt in all bodies of water with high evaporation and little or no runoff.

In addition to lakes with extremely high salinity, which are known as salt lakes (e.g. Dead Sea , Great Salt Lake in Utah with a salt content of over 25%), there is also this effect in a moderate form, e.g. B. in Neusiedler See with a salt content of 0.2%. Salt lakes are mostly lakes with a low average water depth. This leads to the following effects: On the one hand, the salt content varies depending on the location (flat areas are too salty), on the other hand, it also varies over time (the salt content rises in the dry season).

The composition of the salt in salt lakes differs e.g. T. considerably different from those in the world's oceans. Calcium ions, which can only be found in low concentrations in the world's oceans, can accumulate in waters that are low in sulphate (Dead Sea, Don Juan Lake ). Carbonate-rich lakes have a high pH and are known as soda lakes .

In addition to the formation of salt lakes through evaporation accumulation, there are also rare cases of the direct formation of salt lakes on a salty subsoil, e.g. B. at Ocna Sibiului (former salt production in open-cast mining).

Other ingredients

In addition to the salts, carbon dioxide (CO 2 ), oxygen (O 2 ) and other atmospheric gases are dissolved in sea water (as in other surface waters ) . The storage capacity for the greenhouse gas CO 2 depends, among other things, on the water temperature and is an important factor for the global climate ; it decreases with increasing temperature. Dissolved oxygen is the basis for the breathing of aquatic organisms, e.g. B. for fish that manage their gas exchange via gills .

After all, organic compounds from “natural” sources and pollution are found in seawater .

Unfiltered sea ​​water contains suspended fine particles , microorganisms and plankton .

The density of sea water is (again depending on the salinity) between 1020 and 1030 kg · m −3 . The pH value is slightly alkaline and lies between 7.5 and 8.4. Due to the increasing concentration of carbon dioxide in the earth's atmosphere , it dissolves in the form of carbonic acid in the world's oceans and the pH value slowly decreases, which results in the acidification of the oceans .

Seawater desalination

With different methods of seawater desalination , the proportion of dissolved salts can be reduced to such an extent that potable water is obtained. Such systems are operated in many regions with little precipitation. However, these processes are so energy-consuming and expensive in operation or construction that they are only used in tourist regions or wealthy settlements.

Medical application

Natural seawater is offered as a purified isotonic and hypertonic saline solution for medical use (aqua maris) as a non- pharmacy- only nasal spray or medical product from various pharmaceutical companies. Occasionally a small amount of dexpanthenol is added to form a protective film on the nasal mucosa or eucalyptus .


Injuries in seawater can lead to infections with special pathogens ( Aeromonas hydrophila, Edwardsiella tarda, Mycobacterium marinum , Vibrio vulnificus ), which may have to be treated with antibiotics.

See also


  • Oliver Wurl: Practical guidelines for the analysis of seawater. CRC Press, Boca Raton 2009, ISBN 978-1-4200-7306-5 .
  • Klaus Graßhoff, et al .: Methods of seawater analysis. Wiley-VCH, Weinheim 1999, ISBN 3-527-29589-5 .

Web links

Commons : Sea Water  - Collection of pictures, videos and audio files
Wiktionary: Sea water  - explanations of meanings, word origins, synonyms, translations
Wiktionary: Seewasser  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Nasal spray: Good against colds , Stiftung Warentest , accessed on July 21, 2016.
  2. WDR's common cold remedy dictionary , accessed on July 21, 2016.
  3. ^ Marianne Abele-Horn: Antimicrobial Therapy. Decision support for the treatment and prophylaxis of infectious diseases. With the collaboration of Werner Heinz, Hartwig Klinker, Johann Schurz and August Stich, 2nd, revised and expanded edition. Peter Wiehl, Marburg 2009, ISBN 978-3-927219-14-4 , p. 158 ( infections after injuries in sea water ).