The current drumbeat is that (anthropogenic) CO2 has caused the oceans to be 30% more acidic. Acid oceans certainly sound worse than just reporting the pH.Acids dissolve carbonates, so the shells of shell fish and corals are supposed to dissolve or not form in acidified oceans. Some of the explanations I’ve read give a technical explanation of acids being protons (the hydrogen atom nucleus) or sometimes just H+. And if water has H+ in it, it is acidic. At least in some explanations. Carbonated water is made by dissolving CO2 in water (under pressure) and it has a pH of 3, so dissolving CO2 in water can make it acidic. Hydrogen ion concentration and pH are certainly inseparable, but reporting one without the other doesn’t mean much.
The pH scale, developed in 1909 is a measure of the hydrogen ion activity. It runs from 1 (0.1 molar H+) to 14 (0.1 molar OH-). 7 is neutral. Above 7 is considered basic and below 7 is considered acidic. Ocean pH’s are generally taken as 8.2-8.3, so where does this ocean acidification with 30% more acid come from? The simple strong acid/strong base calculation is pH = -log[H+], where H+ is the molar hydrogen ion concentration. (Yes, I know about H3O+). So using this simple approach, any decrease of 0.155 pH units is a 30% increase in H+ or acidity. So, a decrease from pH 14 to pH 13.845 is a 30% increase in the hydrogen ion concentration. I’m not I’d call pH 13.8, which is so high that the hydroxyl ion interferes with the normal glass electrode, acidification. Ocean acidification appears to be a rounded number assuming a decrease from about 8.3 to about 8.15. When we see the concerns about ocean acidification we rarely see actual pH values. pH is a better indicator, but not nearly as sexy as a claim of 30% acidification. (
pH and environmental tolerance varies by species and location. Making blanket statements about “acidification” doesn’t seem warranted. Chesapeake Bay oyster larvae are the most susceptible to pH. The best range is 6.75-8.75, a 100 fold range in hydrogen ion. The pH tolerance range for blue crabs is 6-8. Below 6 is lethal. For lobsters, the low pH tolerance seems to be 7.4-7.85 in various reports. Corals are the poster children for ocean acidification. “Modern” historical variation in coral reef pH seems to be between 7.8 and 8.2. Looking at environmental tolerances, temperature, salinity, nutrient loading and other factors seem to be more important than pH.
The 30% increase in acidity sounds good as a PR bit, but I’m not sure it really means much if the pH and the tolerance range of a specific species is not included.