In
chemistry, Dittmar's Principle (or rule of
constant proportion) can be used to calculate the
concentration of certain
constituents of sea water, by measuring the
concentration of another
component.
Oceanographers use this principle to e.g. find the concentration of
salt by measuring the amount
chloride ions.
The principle is named after Wilhelm Dittmar, who first discovered the constant proportions of the seven major components of sea water (besides hydrogen and oxygen, that is); sodium, calcium, magnesium, potassium, chloride, bromide and sulfate.