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A student from the HIGH TIDE project turns on the CTD instrument. High school students use the CTD recorder to measure salinity, temperature and depth of the water in the Lafayette River which is a part of Chesapeake Bay.
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Image courtesy of the HIGH TIDE project

Salinity - Dissolved Salts, Measuring Salinity

One way to measure how much dissolved salt is in water is to look at the concentration of salt in the water. Concentration is the amount (by weight) of salt in water and can be expressed in parts per million (ppm). Here are the classes of water:
  • Fresh water - less than 1,000 ppm
  • Slightly saline water - From 1,000 ppm to 3,000 ppm
  • Moderately saline water - From 3,000 ppm to 10,000 ppm
  • Highly saline water - From 10,000 ppm to 35,000 ppm

Ocean water has a salinity that is approximately 35,000 ppm. If you take 35,000/1,000,000 then you end up with 3.5%. So, ocean water is about 3.5% salt. Scientists used to report salinity in ppt (parts per thousand), but reporting salinity in ppt is obsolete now as the method of determining ocean salinity in ppt is not used anymore. Instead psu (practical salinity units) are used. Scientists measure salinity by measuring the conductivity of the water as would be measured by a CTD instrument (CTD = conductivity, temperature, depth). Using this method ocean salinity is approximately 35 psu.

Ocean water is about 3.5% salt. That means that if the oceans dried up completely, enough salt would be left behind to build a 180-mile-tall, one- mile-thick wall around the equator. About 90 percent of that salt would be sodium chloride, or ordinary table salt. What other dissolved salts are found in ocean water? The major dissolved salts of the ocean are listed in the following table:

Dissolved salts in
sea water (atoms):
55.3 % Chlorine
30.8 % Sodium
3.7 % Magnesium
2.6 % Sulfur
1.2 % Calcium
1.1 % Potassium


Last modified August 30, 2001 by Jennifer Bergman.

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