Scientists tell us that the oceans are becoming more acidic as a result of increasing levels of atmospheric carbon dioxide (written as CO2.) But how does what’s in the air affect the oceans, and why should CO2 make them acidic?
To answer these questions, it’s important to begin with a review of acids, bases (sometimes called alkalis,) and the pH scale. This will lead in to a discussion of the chemical processes that take place when CO2 comes in to contact with seawater, and will explain why the oceans are becoming more acidic.
Acidity is a function of the number of hydrogen ions in a solution. An ion of hydrogen is an atom with its electron missing, written as H+. The more ions that are present in a solution, the more acidic it is. Conversely, a solution with relatively few hydrogen ions is said to be a base or alkali. Acidity is measured on the fourteen-point pH scale, with a pH of zero being the most acidic. In other words, an acid with this pH level has as many hydrogen ions as is possible to cram in to a given volume. Battery acid is an example of something with a pH of zero. At the other end of the scale, with a pH of 14, is liquid drain cleaner.
Pure water falls in the middle of the pH scale with a value of 7. This means it is neither acidic nor basic. Seawater however has a pH level of around 8.2, meaning that it is slightly basic or alkali. This is believed to have been its natural state for thousands, if not millions, of years, and is a result of minerals being washed off the land and dissolving in the water.
The surface of the oceans is in constant contact with the air above, and this allows a proportion of airborne CO2 to dissolve in the water. (The extent to which this happens is governed by Henry’s Law and is the same process as that which allows carbonated drinks to fizz when the bottle is first opened: releasing the pressure in the bottle reduces the quantity of CO2 that can be held in the liquid.)
Water is of course composed of hydrogen atoms bonded to oxygen, forming the molecule H2O. When CO2 molecules come into contact with H2O they combine to form H2CO3, better known as carbonic acid. According to Henry’s Law, the quantity of CO2 that dissolves in seawater is directly proportional to the quantity of CO2 in the atmosphere. This means that as atmospheric CO2 rises, so too will the amount of carbonic acid in the oceans.
Carbonic acid tends to split into its various constituents, which means that it turns into bicarbonate and releases a hydrogen ion. (The chemical expression for this is [H2CO3] -> [H+] + [HCO3-].) This shows that the consequence of CO2 dissolving in seawater is to increase the number of hydrogen ions, which, as discussed previously, increases the acidity of the oceans.
It is important to note though that the term “ocean acidification” is somewhat misleading. Since seawater normally has a pH of 8.2, increased levels of atmospheric CO2 are actually making the oceans slightly less alkaline. This may not sound especially serious, but for plants and animals living in the ocean the effects can be quite devastating as this change in chemistry affects their growth.