Carbon dioxide plays a vital role in the chemistry of sea water. When atmospheric carbon dioxide is dissolved in seawater, carbonic acid (H2CO3) is formed. Carbonic acid is diprotic, which means in has two H+ ions to donate to solution.
When the first proton is donated, HCO3- otherwise known as bicarbonate, is formed. Most of the carbon (around 88%) of the carbon in the ocean is in this state.
If bicarbonate donates its second proton (H+), it becomes a carbonate (CO32-) ion. About 11% of the carbon in the ocean is carbonate. The other 1% is dissolved Carbon Dioxide.
The balanced equation is below:
As you probably noticed, the chemical reaction has multiple steps and can go in both directions. The bicarbonate acts like a buffer in the ocean. What this means is that this system is very resistant to changes in pH. If you were to add an acid to the ocean, the excess H+ ions would simply drive this reaction to the left and produce more products on the left side of the reaction. If you added a base, the deficiency in H+ ions would drive the reaction to the right. The system is resistant to net changes in the pH of the system.
Free Calcium (Ca2+) ions can sequester the Carbonate ions to produce Calcium Carbonate (CaCO3) This calcium carbonate is used to make shells and tests for many organisms in the ocean. It also accumulates above the Carbonate Compensation Depth as Calcerous ooze.
The Carbonate/Bicarbonate buffer system is an important way for the ocean to maintain chemical equilibrium. What would happen if the amount of CO2 in the atmosphere were to sharply rise?
The increased atmospheric CO2 as a result of burning fossil fuels has driven this entire reaction to far to the right. This means that there is an excess of H+ ions in the ocean and the pH of the ocean has been driven down. This is called ocean acidification. This is just one more negative effect of Global Climate Change.