Seawater is a mixed solution with very complicated components. The substances it contains can be roughly divided into three categories: ① dissolved substances, including various salts, organic compounds and dissolved gases; 2 bubbles; ③ Solid substances, including organic solids, inorganic solids and colloidal particles. In all ocean water, 96% ~ 97% is water, and 3% ~ 4% is various chemical elements and other substances dissolved in water. These elements can be divided into many types.

Up to now, more than 80 chemical elements have been found in seawater, but their contents vary greatly. The main chemical elements are chlorine, sodium, magnesium, sulfur, calcium, potassium, bromine, carbon, strontium, boron, silicon, and fluorine *** 12, accounting for about 99.8% ~ 99.9% of the total chemical elements in seawater, so they are called constant elements in seawater. The proportion of other elements in seawater is very small, all below1mg/L. They are called trace elements in seawater. One of the biggest characteristics of chemical elements in seawater is that the ratio between the above-mentioned 12 main ion concentrations is almost unchanged, so it is called the constancy of seawater composition. It can play a great role in the calculation of seawater salinity. Most elements dissolved in seawater exist in the form of salt ions. The content of main salts in seawater varies greatly. The chloride content is the highest, accounting for 88.6% of the total salt content, followed by sulfate, accounting for 10.8% of the total salt content.