Measuring Salinity of Seawater

Bring along the Refractometer

Seawater in the world's oceans has a salinity of about 35 parts per thousand.  Although the vast majority of seawater has a salinity of between 31 and 38 ppt, seawater is not uniformly saline throughout the world.  Where mixing occurs with fresh water runoff from the mouths of rivers or near melting glaciers, seawater can be substantially diluted. The most saline water is located at areas where high rates of evaporation and low levels of precipitation, river inflow, and circulation result in salty water (e.g. the Mediterranean and Red Seas).  Seawater is primarily composed of the positive ions Sodium (Na⁺), Magnesium (Mg⁺), Calcium (Ca⁺⁺), and Potassium (K⁺), and the negative ions Chloride (Cl^-) and Sulfate (SO₄^-2).  

Table of the Relative Amounts of Salts in Seawater

The salinity of seawater is determined with a refractometer, a laboratory or field device for the measurement of any aqueous solution using its index of refraction.  They are commonly used in science, medicine, brewing, and beverage production.  A different kind of refractometer is needed for each application, including seawater salinity.  In clinical medicine, a refractometer is used to measure protein concentration in human fluids such as urine, and in the food industry, a brix refractometer is used to measure the concentration of sugar in beverages.  In marine aquarium keeping, a refractometer is used to measure the salinity and specific gravity of aquarium water.   A seawater refractometer can be obtained online from a biological research laboratory supply company or retail stores that sell salt water aquarium supplies.

Hand-Held Refractometer

This refractometer measures salinity from 0 to 100 parts per thousand.  Left, held like a telescope, a sample of seawater is placed in the chamber and salinity is determined on a scale seen through the refractometer eyepiece.  Right, view through the eyepiece.  Salinity is expressed as optical density on the left scale and parts per thousand (ppt) on the right. 

Salinity in marinas on monitoring trips during summers of 2011-2012 ranged from 32-35 ppt in Provincetown and 30-34 pp in Wellfleet. These readings are not unexpected since the outer Cape Cod is continually replenished by currents in the Gulf of Maine and by high tides that penetrate up into streams.  The Cape also has a limited supply of fresh water from run-off and streams in comparison to Boston Harbor (Wellfleet Marina is at the mouth of a small stream, Duck Creek, and several streams empty into other areas of the harbor).   

Salinity in the Outer Cape

LINKS:
Holmes-Farley, R.  Refractometers and Salinity Measurement:  Reefkeeping: An online magazine for the marine aquarist. 
Wiki Refractometer
Seawater Refractometer Model I, Model II, Model III 
ATC Natural Seawater Refractometer