Environmental Protection Agency Narragansett Bay Commission University of Rhode Island Office of Marine Programs

Estuarine Science

Salinity

1. What is salinity?
2. How do we measure Salinity?
3. What is the salinity of Narragansett Bay?
4. What can we learn about Narragansett Bay by measuring salinity?
5. Why is the ocean salty?


2. How do we measure salinity?

Salinity is often measured by measuring how well electricity travels through the water. This property of water is called conductivity. Water that has dissolved salt in it will conduct electricity better than water with no dissolved salt. The more salt that is dissolved in the water, the better the water conducts electricity. The salt content of the water can be measured very precisely using the conductivity method.

Salinity can also be measured with a hand held refractometer. A refractometer measures the change of direction or bending of the light as it passes from air to water. Light moves slower in water than air. The more salt in the water, the slower the light moves.

         

You can see the effect of light bending (refracting) as it passes from the water into the air by placing a pencil in a glass of water.

The pencil appears to bend where it enters the water. The pencil is not really bent. The light we see has bent as it moves from water to air.

A refractometer is the best choice for measuring salinity when only approximate values are needed. Refractometers are easy to use in the field and relatively inexpensive.


Method of Salinity Measurement  Precision
 Hand-held refractometer   1 part in 70
 Laboratory titration   1 part in 350
 Modern salinometer (conductivity)   1 part in 40,000
  (From Pilson, 1998)


1. What is salinity?
2. How do we measure Salinity?
3. What is the salinity of Narragansett Bay?
4. What can we learn about Narragansett Bay by measuring salinity?
5. Why is the ocean salty?

       


(Image credits are listed in Image Index)

Please address comments and questions to webmaster.

Home | Estuarine Science | Policy and Management | History and Culture | Virtual Field Trip | Biota Gallery | Maps | Glossary | Teacher Resources | Image Index | Site Info