The Saltiness of Aqueous Solutions
Salinity is a fundamental parameter of water chemistry. In environmental science, it describes the mass of dissolved salts (such as sodium chloride, magnesium, and calcium) per unit mass of water. While often thought of solely in terms of the ocean, salinity is critical in industry, agriculture, and high-precision laboratories.
Biological Importance
The salinity of an environment dictates which organisms can survive. Through a process called **Osmosis**, water naturally moves from low-salinity areas to high-salinity areas across biological membranes. Most organisms are "osmoconformers" or "osmoregulators," meaning they must expend energy to maintain the balance of salt in their cells relative to the water around them.
Specific Gravity vs. Salinity
While Salinity (PPT/PSU) is a measure of content, **Specific Gravity (SG)** is a measure of density relative to pure water. Hydrometers measure SG, but because they are affected by temperature, readings must be calibrated. Our converter uses standard marine science approximations for conversion between PSU and SG at a standard reference temperature of 25°C.
Standard Salinity Metrics
| Concentration | PSU / PPT | Specific Gravity (25°C) |
|---|---|---|
| Freshwater | 0.00 | 1.000 |
| Ocean Average | 35.00 | 1.026 |
| Brine (Typical) | 50+ | > 1.037 |
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Frequently Asked Questions
What is PSU in salinity?
PSU stands for Practical Salinity Unit. It is a dimensionless value based on the electrical conductivity of seawater relative to a standard potassium chloride solution.
How does temperature affect salinity readings?
Temperature significantly affects the density and conductivity of seawater. For accurate salinity measurement, specific gravity readings must be temperature-compensated to 20°C or 25°C.
What is the average salinity of the ocean?
The average salinity of the world ocean is approximately 35 ppt (parts per thousand) or 35 PSU.