99.9% of dissolved material in sea water can be accounted for by eleven constituents: Na, Mg, Ca, K, Sr, Cl, SO₄, HCO₃, Br, BO₃, and F, whose relative proportions are almost constant in all oceans regardless of salinity. The pH of sea water is maintained at 8.0–8.4 by a buffering system. Because the water is alkaline, most of the Ca²⁺ and HCO₃⁻ form insoluble CaCO₃. Aluminium and iron oxides coagulate to form colloids because of the high concentration of electrolytes, and sink to the sea bed, silica is taken up by organisms, and thus Cl, Na, SO₄, Mg, and K, for which no removal mechanism exists, account for almost all the dissolved material, at concentrations much higher than those found in rocks.