Measuring how salty seawater is turns out to be surprisingly hard
Seawater holds about 35 grams of salt per kilogram, and the Dead Sea over 200. Yet pinning down salinity precisely has required three successive measurement scales and a bottled reference seawater, so that deep-ocean researchers working years apart can compare results to almost five significant digits.
In principle salinity is just the amount of dissolved salt in water. In practice natural water is a messy soup of ions from many sources, some changing form with temperature and pressure, and a full chemical analysis of every sample is impractical. Even the definition of dissolved is operational: whatever slips through a very fine filter, historically with pores of 0.45 micrometres and now usually 0.2.
Before the 1980s, chemists titrated seawater with silver nitrate to measure chlorine and bromine, then multiplied by a factor to estimate everything else, giving salinity in parts per thousand. The 1978 practical salinity scale switched to electrical conductivity and, confusingly, has no units at all; tacking psu onto the number is formally discouraged. In 2010 a new standard, TEOS-10, introduced absolute salinity in grams per kilogram, adjusted for regional differences in composition. One typical sample reads 35.00 on the old scale, about 35.0 on the practical scale and about 35.2 grams per kilogram on the newest.
The stakes are large. Salinity, together with temperature and pressure, sets water's density, so saltier, colder water sinks and helps drive global ocean circulation. Saltier water also absorbs less carbon dioxide. Since the 1950s climate change has altered salinity patterns, and projections suggest fresh regions will get fresher while salty ones get saltier. In the subpolar North Atlantic, extra Greenland meltwater has freshened the water since the mid-2010s.
Scientists sort waters by salt. Brackish water spans 0.5 to 29 parts per thousand, open ocean 30 to 35, and brine is water so salty that crystals form. Plants adapted to salty ground are halophytes, and microbes thriving in brine are halophiles. Closer to home, road salt washing off streets has made some American lakes and rivers measurably saltier.
Source: Salinity
