The Chemistry Behind Ocean Acidification
We all have a basic understanding of what happens when carbon dioxide reacts with seawater. An acid is formed. But what’s the fun in just having a basic understanding of the chemistry behind ocean acidification when we can go in depth and explore every bond formation, ion transfer, and other cool science concepts involving the chemical reactions that occur? Let’s dive right in!
The amount of hydrogen ions [H+], or hydronium ions, are used to determine whether a solution is basic or acidic using the pH scale (pH = -log[H+]). Note how the pH scale is logarithmic. This means that the change in pH changes the concentration of hydrogen ions by a tenfold. For example, a pH of 5 is ten times more acidic than a pH of 6, and a pH of 5 is one hundred times more acidic than a pH of 7. Similarly a pH of 8 is ten times more basic than a pH of 7, 7 being neutral. The first important thing to note is that the ocean is not acidic in nature, it’s basic. For the past 300 million years, the ocean’s pH has averaged about 8.2. Today the pH is about 8.1. This is about a 25% increase in acidity and this increase has occurred, remarkably, over the past 2 centuries. Now a pH drop of about .1 may not seem like much, but it is. Species can take thousands to millions of years to adapt to their surroundings, and at the rate this change in the ocean is occurring, many animals will not be able to adapt. Another article will be coming out shortly on this, but back to chemistry.
This picture from the University of Maryland depicts the chemistry involved in ocean acidification very well.
Credit: University of Maryland
Carbon dioxide from the air first dissolves into the ocean. This dissolved carbon dioxide then reacts with water and forms carbonic acid. Carbonic acid dissolves rapidly to form H+ ions (acid) and bicarbonate, HCO3 (base), which further breaks down into H+ and CO3 2- ions. More H+ ions makes seawater more acidic, but scientists do not think the seas will become truly acidic (with a pH less than 7.0), but rather less alkaline. Also, CO3 (2-) is naturally present in seawater and serves sort of like an antacid to neutralize the H+, as a result forming more bicarbonate. Marine organisms need carbonate ions to build their shells, but even though the total amount of carbon in solution increases as more carbon dioxide dissolves in seawater, the concentration CO3 (2-) ions actually decreases. This happens because more carbon dioxide means more hydrogen ions in seawater. Those additional hydrogen ions consume carbonate ions in a reaction to form bicarbonate ions.
Another important thing to note is that carbon stays in the ocean for a very long time. To return CO2 levels to those that existed before the Industrial Revolution, “we’re going to have to reduce CO2 emissions as soon as possible, and then wait a few hundred years for the oceans to adjust,” says Justin Ries, former postdoctoral scholar in the Ocean and Climate Change Institute at Woods Hole Oceanographic Institution.
