Editing Salinity (section)

===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,<ref name=wetzel>{{cite book|last=Wetzel|first=R. G.|title=Limnology: Lake and River Ecosystems, 3rd ed.|year=2001|publisher=Academic Press|isbn=978-0-12-744760-5}}</ref><ref name=limteos>{{cite journal|last1=Pawlowicz|first1=R.|first2=R.|last2=Feistel|title=Limnological applications of the Thermodynamic Equation of Seawater 2010 (TEOS-10)|journal=Limnology and Oceanography: Methods|year=2012|volume=10|issue=11|pages=853–867| doi=10.4319/lom.2012.10.853|bibcode=2012LimOM..10..853P |s2cid=93210746 |doi-access=free}}</ref> although for seawater at highest precision an additional seven minor ions are also included.<ref name=teos10>{{cite book|last=IOC, SCOR, and IAPSO|title=The international thermodynamic equation of seawater – 2010: Calculation and use of thermodynamic properties |year=2010 |publisher=Intergovernmental Oceanographic Commission, UNESCO (English) |pages=196pp |url=http://www.TEOS-10.org}}</ref>  The major ions dominate the inorganic composition of most (but by no means all) natural waters. Exceptions include some [[Cenote|pit lake]]s and waters from some [[hydrothermal spring]]s.

The concentrations of dissolved gases like [[oxygen]] and [[nitrogen]] are not usually included in descriptions of salinity.<ref name=key /> 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).