Forchhammer's Principle: Why the Sea's Salts Always Come in the Same Proportions

Seawater is saltier in some places than others, but the mix of salts in it is the same everywhere. That was the discovery of Johan Georg Forchhammer, and it told scientists that differences in saltiness come from fresh water being added or removed — not from different amounts of salt minerals in the water.

A CTD, a package of instruments that measures conductivity, temperature and depth. Conductivity is directly related to salinity, one of the most basic measurements in ocean science. Credit: NOAA.

The samples

In 1865, Forchhammer — a Danish geologist and mineralogist — worked with naval and civilian collaborators to gather many samples of seawater from the North Atlantic and the Arctic Ocean. He wanted to know why the salinity, or saltiness, of seawater varies from one part of the ocean to another.

The discovery

After a detailed series of chemical analyses, Forchhammer found that the proportions of the major salts in seawater stay about the same everywhere. That constant ratio is known as Forchhammer's Principle, or the Principle of Constant Proportions. He is also credited with defining salinity as the concentration of major salts in seawater.

Still in use

The principle is still applied in marine research today. It gives scientists a simple way to estimate salinity and to trace how masses of water mix in the global ocean. And salinity matters well beyond chemistry: combined with temperature, it determines seawater density — a primary force behind the major ocean currents.

Sources

Based on "What is the Forchhammer's Principle?" from NOAA's National Ocean Service, a work of the United States government in the public domain.