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.

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