Icing The Hype
Apr 05, 2017
U.S. climate site for kids fails test

By James Barrante

Recently, I stumbled upon a website called “Climate Kids.” It appears to be a website under the auspices of NASA. Normally, websites like these do not get my attention, except when they are specifically designed to teach science to our children. This is because children tend to believe their teachers know what they are talking about. The topic covered in that particular article was ocean “acidification.” Unfortunately, most of it was not correct. It looked like the person(s) writing this material never took a chemistry course above what we call “kiddie chem.”

The material covered began by trying to convince the reader that adding carbon dioxide to water always acidifies it. While this is true for “fresh” water - bodies of fresh water can be slightly acidic, having a pH below 7, because of dissolved carbon dioxide - this definitely is not true of seawater. Seawater contains high concentrations of salts. And while most people believe the salt in seawater is table salt (sodium chloride), seawater also contains high concentrations of salts known as carbonates and bicarbonates, which are antacids, causing seawater to be alkaline (basic: pH above 7), not acidic. The antacid Alka-Seltzer essentially is sodium bicarbonate.

When CO2 dissolves in seawater, it forms carbonic acid, classified chemically as a weak acid. However, bicarbonates and carbonates are salts of this weak acid, and when a weak acid is combined with one of its salts, it forms what is known as a buffer system. Buffer systems hold the pH (the level of acidity or alkalinity) constant. Our oceans are giant buffer systems, buffered to a pH of about 8.2. The actual pH of any buffer system depends on the ratio of the concentrations of weak acid or weak base to their salts.

Curiously, the CO2-bicarbonate buffer system is the exact same buffer system that maintains the pH of the blood in humans to a constant value of pH 7.4. Our blood is slightly less alkaline (approaching a neutral pH 7) than is seawater because the concentration of dissolved CO2 in blood is about 130 times greater than it is in seawater. To actually acidify seawater (i.e., drop its pH to below 7) with CO2, the level of CO2 in the atmosphere would have to increase to about 70,000 parts per million by volume (ppmv), compared with the present 400 ppmv.

I should point out to those who feel compelled to nitpick, these numbers are approximate, because pH depends on temperature and other properties of solutions. For example, blood temperature is about 37 degrees C; seawater temperature varies from about 2 degrees C at the poles to about 30 degrees C at the equator. The boundary between acid-base (neutrality) is pH 7 only at 25-degrees C.

Moreover, contrary to popular belief, if a solution is less alkaline, it is not more acidic. It is not like “hot and cold,” which are relative to the surroundings. A substance can be hot and cold at the same time. But, by definition, a solution cannot be acidic and alkaline at the same time. At 25 degrees C, a solution is acidic, if its pH is below 7, and alkaline, if its pH is above 7. To suggest a solution with a pH of 8.1 is more acidic than one with a pH of 8.4 totally misrepresents the concept of acid and base.

James Barrante of Cheshire is a retired college professor of physical chemistry.

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