Seawater


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seawater

[′sē‚wȯd·ər]
(oceanography)
Water of the seas, distinguished by high salinity. Also known as salt water.

Seawater

 

water on the earth’s surface, concentrated in the seas and oceans. The total volume of water in the world’s oceans is 1.37 billion cu km. Seawater contains dissolved mineral salts, gases (primarily oxygen, nitrogen, and carbon dioxide; in some regions, hydrogen sulfide), and a small amount of organic matter (1-5 milligrams per liter [mg/l]). It also contains small quantities of organic and inorganic suspensions. Seawater is characterized by constancy of the quantitative ratios of the concentrations of its principal ions, which make up 99.9 percent of the substances dissolved in it (called Dittmar’s law; see Table 1). The ratio among ions is different in some seas (the Baltic, Black, and Caspian seas and the Sea of Azov).

Table 1. Concentration of most important ions in seawater at S = 35‰ (after S. V. Bruevich)
 Concentration
 g/kg% equiv
Anions
CI......................19.353445.09
SO42−.....................2.70074.64
HCO3...................0.14270.19
Br.....................0.06590.07
F.......................0.00130.01
H3BO3.....................0.0265
Cations
Na+.......................10.763838.66
Mg2+......................1.29708.81
Ca2+......................0.40801.68
K+.......................0.38750.82
Sr2+......................0.01360.03

The quantity of solid matter (in grams) dissolved in 1 kg of seawater is called the salinity S, in parts per thousand (%c), on the condition that all halogens are replaced by equivalent quantities of chlorine, all carbonates are converted to oxides, and organic matter is burned. The chlorine content is the amount of chlorine, expressed in grams per kilogram or parts per thousand, that is equivalent to the total sum of halogens in the seawater. The average salinity of seawater in the ocean is close to 35%<?. In practice salinity is always found indirectly, from either chlorine content or relative electrical conductivity, which are determined directly. In the former case the Knudsen tables are used, and in the latter case the 1966 UNESCO International Oceanographic Tables are used. In both cases the international standard is “normal” water, whose chlorine content has been precisely determined.

The density of seawater depends on the salinity S, the temperature T, and the pressure. Oceanography uses a standard density σt = (p − 1) × 103, where p is the ratio of the water density at the given temperature and salinity to the density of distilled water at 4°C (both at atmospheric pressure). In the ocean, σt, ranges between 23 and 30. When salinity increases by 1, σt, increases by approximately 0.8—that is, density increases by 0.0008. When temperature drops by 1°C, σt, increases by 0.02-0.35. The freezing point of seawater depends on salinity; when S = 35‰ it is - 1.91°C. For S = 24.1‰ the freezing point and point of highest density coincide at — 1.33°C. If S is less than 24.7‰ the process of freezing is the same for seawater as for fresh water. If 5 is greater than 24.7‰ the density of seawater increases until the moment of freezing; as a result, powerful convection develops and the process of freezing takes longer than for fresh water.

The compressibility of seawater is low. With an increase in pressure of 1,000 decibars (in oceanography, pressure is conventionally measured in decibars), which corresponds to an increase in depth of approximately 1 km, density increases by 0.004. According to the findings of the British scientists R. Cox and N. Smith (1959), the specific heat capacity of seawater cp decreases both with an increase in salinity from 4,217 joules (J) per (kg.°K) at 0°C and 0‰ to 3,985 J/(kg.°K) at 0°C and 35%c‰, and at oceanic salinity with a decrease in temperature, from 3,999 J/(kg.°K) at 30°C and 35‰ to 3,985 J/(kg.°K) at 0°C and 35‰.

The speed of sound in seawater is greater than in fresh water, and it increases with an increase in salinity (from 1,399 m/sec for S = 0‰ to 1,445 m/sec for S = 35‰ at 0°C) and temperature (from 1,445 m/sec for 0°C to 1,543 m/sec for 30°C at 35‰). The index of refraction of light in seawater increases slightly with an increase in salinity and a drop in temperature (the Rossby tables, which are part of the International Oceanographic Tables). The light absorption factor is greatest in the infrared part of the spectrum.

The relative electrical conductivity Rt of seawater is used extensively in oceanography. It is defined as the ratio of the conductivity of a given sample to the conductivity of seawater with a salinity of 35‰ at identical temperatures and atmospheric pressure. The value of Rt increases with an increase in salinity from 0.105 at S = 3‰ to 1.126 at S = 40‰? (for a temperature of 20°C).

REFERENCES

Bruevich, S. V. “Elementarnyi sostav vody Mirovogo okeana.” Tr. In-ta okeanologii AN SSSR, 1948, vol. 2.
Zubov, N. N. Okeanologicheskie tablitsy, 3rd ed. Leningrad, 1957.
Shuleikin, V. V. Fizika moria, 4th ed. Moscow, 1968.
Vinogradov, A. P. Vvedenie v geokhimiiu okeana. Moscow, 1967.
Alekin, O. A. Khimiia okeana. Leningrad, 1966.
Mamaev, O. I. T, Sanaliz vod Mirovogo okeana. Leningrad, 1970.
Khorn, R. Morskaia khimiia. Moscow, 1972.
Mezhdunarodnye okeanologicheskie tablitsy, no. 1. [Moscow] 1969.
Fofonoff, N. P. “Physical Properties of Seawater.” In M. Hill (ed.), The Sea, vol. 1. London-New York, 1962.
Pytkowicz, R. M., and D. R. Kester. “The Physical Chemistry of Seawater.” Oceanography and Marine Biology, 1971, vol. 9.

G. N. IVANOV-FRANTSKEVICH

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