Salinity

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Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2009. WOA09 sea-surf SAL AYool.png
Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2009.
International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater. IAPSO Standard Seawater.jpg
International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater.

Salinity ( /səˈlɪnɪti/ ) is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). This is usually measured in (note that this is technically dimensionless). Salinity is an important factor in determining many aspects of the chemistry of natural waters and of biological processes within it, and is a thermodynamic state variable that, along with temperature and pressure, governs physical characteristics like the density and heat capacity of the water.

Contents

A contour line of constant salinity is called an isohaline, or sometimes isohale.

Definitions

Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely. Conceptually the salinity is the quantity of dissolved salt content of the water. Salts are compounds like sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate which dissolve into ions. The concentration of dissolved chloride ions is sometimes referred to as chlorinity. Operationally, dissolved matter is defined as that which can pass through a very fine filter (historically a filter with a pore size of 0.45 μm, but nowadays usually 0.2 μm). [2] Salinity can be expressed in the form of a mass fraction, i.e. the mass of the dissolved material in a unit mass of solution.

Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical near coasts where rivers enter the ocean. Rivers and lakes can have a wide range of salinities, from less than 0.01 g/kg [3] to a few g/kg, although there are many places where higher salinities are found. The Dead Sea has a salinity of more than 200 g/kg. [4] Rainwater before touching the ground typically has a TDS of 20 mg/L or less. [5]

Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few percent (%). Physical oceanographers working in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five significant digits. [6] A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.

Composition

Measurement and definition difficulties arise because natural waters contain a complex mixture of many different elements from different sources (not all from dissolved salts) in different molecular forms. The chemical properties of some of these forms depend on temperature and pressure. Many of these forms are difficult to measure with high accuracy, and in any case complete chemical analysis is not practical when analyzing multiple samples. Different practical definitions of salinity result from different attempts to account for these problems, to different levels of precision, while still remaining reasonably easy to use.

For practical reasons salinity is usually related to the sum of masses of a subset of these dissolved chemical constituents (so-called solution salinity), rather than to the unknown mass of salts that gave rise to this composition (an exception is when artificial seawater is created). For many purposes this sum can be limited to a set of eight major ions in natural waters, [7] [8] although for seawater at highest precision an additional seven minor ions are also included. [6] The major ions dominate the inorganic composition of most (but by no means all) natural waters. Exceptions include some pit lakes and waters from some hydrothermal springs.

The concentrations of dissolved gases like oxygen and nitrogen are not usually included in descriptions of salinity. [2] However, carbon dioxide gas, which when dissolved is partially converted into carbonates and bicarbonates, is often included. Silicon in the form of silicic acid, which usually appears as a neutral molecule in the pH range of most natural waters, may also be included for some purposes (e.g., when salinity/density relationships are being investigated).

Seawater

Full 3 minute NASA video Feb 27,2013 The NASA Aquarius instrument aboard Argentina's SAC-D satellite is designed to measure global sea surface salinity. This movie shows salinity patterns as measured by Aquarius from December 2011 through December 2012. Red colors represent areas of high salinity, while blue shades represent areas of low salinity.

The term 'salinity' is, for oceanographers, usually associated with one of a set of specific measurement techniques. As the dominant techniques evolve, so do different descriptions of salinity. Salinities were largely measured using titration-based techniques before the 1980s. Titration with silver nitrate could be used to determine the concentration of halide ions (mainly chlorine and bromine) to give a chlorinity. The chlorinity was then multiplied by a factor to account for all other constituents. The resulting 'Knudsen salinities' are expressed in units of parts per thousand (ppt or ).

The use of electrical conductivity measurements to estimate the ionic content of seawater led to the development of the scale called the practical salinity scale 1978 (PSS-78). [9] [10] Salinities measured using PSS-78 do not have units. The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS-78 measurement values. [11] The addition of PSU as a unit after the value is "formally incorrect and strongly discouraged". [12]

In 2010 a new standard for the properties of seawater called the thermodynamic equation of seawater 2010 (TEOS-10) was introduced, advocating absolute salinity as a replacement for practical salinity, and conservative temperature as a replacement for potential temperature. [6] This standard includes a new scale called the reference composition salinity scale. Absolute salinities on this scale are expressed as a mass fraction, in grams per kilogram of solution. Salinities on this scale are determined by combining electrical conductivity measurements with other information that can account for regional changes in the composition of seawater. They can also be determined by making direct density measurements.

A sample of seawater from most locations with a chlorinity of 19.37 ppt will have a Knudsen salinity of 35.00 ppt, a PSS-78 practical salinity of about 35.0, and a TEOS-10 absolute salinity of about 35.2 g/kg. The electrical conductivity of this water at a temperature of 15 °C is 42.9 mS/cm. [6] [13]

Lakes and rivers

Limnologists and chemists often define salinity in terms of mass of salt per unit volume, expressed in units of mg per litre or g per litre. [7] It is implied, although often not stated, that this value applies accurately only at some reference temperature. Values presented in this way are typically accurate to the order of 1%. Limnologists also use electrical conductivity, or "reference conductivity", as a proxy for salinity. This measurement may be corrected for temperature effects, and is usually expressed in units of μS/cm.

A river or lake water with a salinity of around 70 mg/L will typically have a specific conductivity at 25 °C of between 80 and 130 μS/cm. The actual ratio depends on the ions present. [14] The actual conductivity usually changes by about 2% per degree Celsius, so the measured conductivity at 5 °C might only be in the range of 50–80 μS/cm.

Direct density measurements are also used to estimate salinities, particularly in highly saline lakes. [4] Sometimes density at a specific temperature is used as a proxy for salinity. At other times an empirical salinity/density relationship developed for a particular body of water is used to estimate the salinity of samples from a measured density.


Water salinity
Fresh water Brackish water Saline water Brine
< 0.05% 0.05 – 3% 3 – 5% > 5%
< 0.5 ‰ 0.5 – 30 ‰ 30 – 50 ‰ > 50 ‰

Classification of water bodies based upon salinity

Thalassic series
>300
hyperhaline
60–80
metahaline
40
mixoeuhaline
30
polyhaline
18
mesohaline
5
oligohaline
0.5

Marine waters are those of the ocean, another term for which is euhaline seas. The salinity of euhaline seas is 30 to 35. Brackish seas or waters have salinity in the range of 0.5 to 29 and metahaline seas from 36 to 40. These waters are all regarded as thalassic because their salinity is derived from the ocean and defined as homoiohaline if salinity does not vary much over time (essentially constant). The table on the right, modified from Por (1972), [15] follows the "Venice system" (1959). [16]

In contrast to homoiohaline environments are certain poikilohaline environments (which may also be thalassic) in which the salinity variation is biologically significant. [17] Poikilohaline water salinities may range anywhere from 0.5 to greater than 300. The important characteristic is that these waters tend to vary in salinity over some biologically meaningful range seasonally or on some other roughly comparable time scale. Put simply, these are bodies of water with quite variable salinity.

Highly saline water, from which salts crystallize (or are about to), is referred to as brine.

Environmental considerations

Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water. As well, salinity influences the kinds of plants that will grow either in a water body, or on land fed by a water (or by a groundwater). [18] A plant adapted to saline conditions is called a halophyte. A halophyte which is tolerant to residual sodium carbonate salinity are called glasswort or saltwort or barilla plants. Organisms (mostly bacteria) that can live in very salty conditions are classified as extremophiles, or halophiles specifically. An organism that can withstand a wide range of salinities is euryhaline.

Salt is expensive to remove from water, and salt content is an important factor in water use (such as potability). Increases in salinity have been observed in lakes and rivers in the United States, due to common road salt and other salt de-icers in runoff. [19]

The degree of salinity in oceans is a driver of the world's ocean circulation, where density changes due to both salinity changes and temperature changes at the surface of the ocean produce changes in buoyancy, which cause the sinking and rising of water masses. Changes in the salinity of the oceans are thought to contribute to global changes in carbon dioxide as more saline waters are less soluble to carbon dioxide. In addition, during glacial periods, the hydrography is such that a possible cause of reduced circulation is the production of stratified oceans. In such cases, it is more difficult to subduct water through the thermohaline circulation.

See also

Related Research Articles

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Solution A homogeneous mixture which assumes the phase of the solvent

In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution usually has the state of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. One important parameter of a solution is the concentration, which is a measure of the amount of solute in a given amount of solution or solvent. The term "aqueous solution" is used when one of the solvents is water.

Solubility Capacity of a substance to dissolve in a solvent in a homogeneous way

Solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.

Electrical resistivity and its inverse, electrical conductivity, is a fundamental property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-meter (Ω⋅m). For example, if a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

Brine A highly concentrated solution of a salt in water

Brine is a high-concentration solution of salt (NaCl) in water (H2O). In different contexts, brine may refer to salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of solutions used for brining foods) up to about 26% (a typical saturated solution, depending on temperature). Lower levels of concentration are called by different names: fresh water, brackish water, and saline water.

Sodium chloride Chemical compound with formula NaCl

Sodium chloride, commonly known as salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/mol respectively, 100 g of NaCl contains 39.34 g Na and 60.66 g Cl. Sodium chloride is the salt most responsible for the salinity of seawater and of the extracellular fluid of many multicellular organisms. In its edible form of table salt, it is commonly used as a condiment and food preservative. Large quantities of sodium chloride are used in many industrial processes, and it is a major source of sodium and chlorine compounds used as feedstocks for further chemical syntheses. A second major application of sodium chloride is de-icing of roadways in sub-freezing weather.

Seawater Water from a sea or ocean

Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram of seawater has approximately 35 grams (1.2 oz) of dissolved salts. Average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater ever recorded was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C (27.3 °F). Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.

In oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.

Alkalinity The capacity of water to resist changes in pH that would make the water more acidic

Alkalinity is the capacity of water to resist changes in pH that would make the water more acidic. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with a monoprotic acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of meq/L, which corresponds to the amount of monoprotic acid added as a titrant in millimoles per liter.

Saline water water that contains a significant concentration of dissolved salts

Saline water is water that contains a high concentration of dissolved salts. The salt concentration is usually expressed in parts per thousand or parts per million (ppm). The United States Geological Survey classifies saline water in three salinity categories. Salt concentration in slightly saline water is around 1,000 to 3,000 ppm (0.1–0.3%), in moderately saline water 3,000 to 10,000 ppm (0.3–1%) and in highly saline water 10,000 to 35,000 ppm (1–3.5%). Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 grams of salt per one liter of water. The saturation level is dependent on the temperature of the water. At 20 °C one liter of water can dissolve about 357 grams of salt, a concentration of 26.3% w/w. At boiling (100 °C) the amount that can be dissolved in one liter of water increases to about 391 grams, a concentration of 28.1% w/w.

Salt water chlorination is a process that uses dissolved salt as a store for the chlorination system primarily for swimming pools and hot tubs. The chlorine generator uses electrolysis in the presence of dissolved salt (NaCl) to produce chlorine gas (Cl2) or its dissolved forms -hypochlorous acid (HClO) / sodium hypochlorite (NaClO)- which is the sanitizing agent already commonly used in pools. Hydrogen is produced as byproduct too. As such, a saltwater pool or Jacuzzi is not actually chlorine-free; it simply utilizes added salt and a chlorine generator instead of direct addition of chlorine.

Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.

Total dissolved solids

Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular suspended form. TDS is sometimes referred to as parts per million (ppm). Water quality levels can be tested using a digital TDS PPM meter.

Aquarius (SAC-D instrument) NASA instrument aboard the Argentine SAC-D spacecraft

Aquarius was a NASA instrument aboard the Argentine SAC-D spacecraft. Its mission was to measure global sea surface salinity to better predict future climate conditions.

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Conductivity (electrolytic) measure of ability of an electrolyte solution to conduct electricity

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Paleosalinity is the salinity of the global ocean or of an ocean basin at a point in geological history.

A brinicle is a downward-growing hollow tube of ice enclosing a plume of descending brine that is formed beneath developing sea ice.

Brine rejection is a process that occurs when salty water freezes. The salts do not fit in the crystal structure of water ice, so the salt is expelled.

Brine mining is the extraction of useful materials which are naturally dissolved in brine. The brine may be seawater, other surface water, or groundwater. It differs from solution mining or in-situ leaching in that those methods inject water or chemicals to dissolve materials which are in a solid state; in brine mining, the materials are already dissolved.

References

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  18. Kalcic, Maria, Turowski, Mark; Hall, Callie (2010-12-22). "Stennis Space Center Salinity Drifter Project. A Collaborative Project with Hancock High School, Kiln, MS". Stennis Space Center Salinity Drifter Project. NTRS. Retrieved 2011-06-16.
  19. "Hopes To Hold The Salt, And Instead Break Out Beet Juice And Beer To Keep Roads Clear". www.wbur.org.

Further reading