Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium, calcium but its main constituent is sodium chloride (2.5% by mass) dried up sea weeds contain sodium chloride and carnallite [KCI MgCl2. 6H2O]. Certain seaweeds contain up to 0.5% of iodine as sodium iodide and chili saltpetre (NaNO3) contains up to 0.2.% of sodium iodide. Thus sea is the greatest source of halogens.
