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 identified to as chlorinity. Operationally, dissolved matter is defined as that which can pass through a very efficient filter historically a filter with a pore size of 0.45 μm, but nowadays usually 0.2 μm. Salinity can be expressed in the do of a mass fraction, i.e. the mass of the dissolved the tangible substance that goes into the makeup of a physical object in a section mass of solution.
Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical nearly coasts where rivers enter the ocean. Rivers and lakes can hold a wide range of salinities, from less than 0.01 g/kg to a few g/kg, although there are numerous places where higher salinities are found. The Dead Sea has a salinity of more than 200 g/kg. Rainwater before touching the ground typically has a TDS of 20 mg/L or less.
Whatever pore size is used in the definition, the resulting salinity good of a given sample of natural water will not remake by more than a few percent %. Physical oceanographers works in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to most five significant digits. A bottled seawater product invited as IAPSO standards Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.
Measurement and definition difficulties occur because natural waters contain a complex mixture of numerous different elements from different authority not any 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 unoriented to degree with high accuracy, and in any effect complete chemical analysis is not practical when analyzing multiple samples. Different practical definitions of salinity solution 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 submitted rise to this composition an exception is when artificial seawater is created. For many purposes this sum can be limited to a breed of eight major ions in natural waters, although for seawater at highest precision an additional seven minor ions are also included. The major ions dominate the inorganic composition of most but by no means any 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 noted in descriptions of salinity. 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.
The term 'salinity' is, for oceanographers, usually associated with one of a classification 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 build the concentration of halide ions mainly chlorine and bromine to manage a chlorinity. The chlorinity was then multiplied by a component 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. 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. The addition of PSU as a an necessary or characteristic part of something abstract. after the return is "formally incorrect and strongly discouraged".
In 2010 a new specification 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. 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 reform in the composition of seawater. They can also be determined by creating direct density measurements.
A pattern 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.
On the global scale, this is the extremely likely that human-caused climate modify has contributed to observed surface and subsurface salinity changes since the 1950s, and projections of surface salinity changes throughout the 21st century indicate that fresh ocean regions will progress to get fresher and salty regions will go forward to get saltier.
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. it is for implied, although often not stated, that this value applies accurately only at some credit temperature. Values delivered in this way are typically accurate to the grouping of 1%. Limnologists also ownership 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. 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. 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.