Carbon dioxide
Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and  (carbonic acid). The relative concentrations of , and the deprotonated forms  (bicarbonate) and (carbonate) depend on the acidity (pH). In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble. So, air-equilibrated ocean water (mildly alkaline with typical pH = 8.2–8.5) contains about 120 mg of bicarbonate per liter.