The constant composition of seawater means that the proportion of constant ions is always constant regardless of the concentration of salt dissolved in seawater. The constant composition of seawater provides favorable conditions for studying the physical and chemical properties of seawater.
Regardless of the concentration of salt dissolved in seawater, the proportion of constant ions is always constant. This relationship is called "constancy of seawater composition". The principle of invariability of seawater composition is also called the principle of invariability of principal component ratio, the principle of invariability of seawater composition, Marset principle and dietmar law.
As early as18th century, Boyle and others studied the chemical composition of seawater. 1823- 1826, the composition of seawater at different depths in the Atlantic Ocean, the Indian Ocean and the Pacific Ocean was investigated in the Earth-Ocean Survey led by Lengci and Kezibu.
1875- 1876, the British research ship Challenger made an expedition around the ocean, collected 77 representative samples in different sea areas of the world, made accurate analysis, and obtained widely representative results.