The accumulation of carbon dioxide in the ocean has led to ocean acidification. This term describes the movement of ocean pH from a slightly basic pH to a more acidic pH, just like the water in the experiment on the previous page. Surface ocean pH has dropped from a pre-industrial value of 8.2 to the current value of 8.1. This is about a 25% increase in hydronium ion concentration [H3O+(aq)]. Models suggest that the average surface ocean pH will drop to 7.8 by the end of this century - a 150% increase in [H3O+(aq)] from pre-industrial values.
Question: How does a pH change from 8.2 (pre-industrial) to 8.1 (current) translate into a 25% increase in hydronium ion concentration?
Use the pH Calculator learning tool to help you distinguish between logarithmic measurements, such as pH and [H3O+(aq)], and linear measurements.
A 25% increase in acidity comes from taking the ratio of the [H3O+(aq)] before and after industrial influence.
A pH of 8.2 translates into a [H3O+(aq)] of 6.31 x 10-9 mol L-1, whereas a pH of 8.1 translates into a [H3O+(aq)] of 7.94 x 10-9 mol L-1.
Using a formula to evaluate percent change, we find:
Since the pH scale is logarithmic, a solution of pH 1.0 has a [H3O+(aq)] that is 10 times higher than a solution with a pH of 2.0, and a hundred times higher than the concentration of a solution with a pH of 3.0. By definition, acidic solutions have a pH lower than 7, whereas basic solutions have a pH higher than 7. At a pH of 8.1, the surface of the ocean is still basic, but the pH is shifting slightly toward the acidic side, hence the term "Ocean Acidification".