One of the more bizarre stories coming out of the 2016 Summer Olympics involves pool water. On Tuesday, the bright blue diving pool turned to swamp green in a matter of hours, while the water polo pool only a few feet away maintained its pool-blue hue:
What happened? To be clear, climate change did not suddenly turn one pool at a major sporting event green. And early reports of an algae bloom were found false. But the official reason why the pool changed colors has everything to do with climate change: water chemistry. Officials from FINA, the international swimming federation, blamed the color change on a sudden drop of water alkalinity due to a “[loss] of chemicals used in the water treatment process”.
What is this “alkalinity” thing, and why does it matter for climate change? Alkalinity is defined as the excess number of nonreactive, positively charged ions in water versus the number of nonreactive, negatively charged ions in water.
That is a mouthful, so let’s break that definition down:
- “Nonreactive ions” means ions that do not participate in chemical reactions and lose their charge. For example, the ocean has high concentrations of the positive ion sodium (Na+) and negative ion chlorine (Cl–). Combine the two together, and you get NaCl, a.k.a. table salt. But if you swim in the ocean today, you don’t find chunks of table salt everywhere. That is because the reaction of Na+ and Cl– to form NaCl is not energetically favorable in the ocean’s “conditions” (temperature, ion concentrations, etc.). In other words, Na+ and Cl– do not react because #thermodynamics.
- “Excess number” means that yes, there are more nonreactive positive ions than negative ions. Virtually all bodies of water on Earth (including pools) have a greater number of nonreactive positive ions than negative ions. This is true for the ocean, as well:
But wait, if charges in a body of water are not balanced, shouldn’t you get shocked every time you enter the water? That does not happen because in reality all waters are electroneutral: They possess exactly the same number of positive ions as negative ions.
The key for waters to obtain electroneutrality (as they must) is that alkalinity only involves nonreactive ions. The net positive charge resulting from the greater number of nonreactive positive ions is balanced by net negative charges from reactive ions. And here’s the link to climate: The largest source of reactive, negative charges that balance the excess positive charges are two compounds derived from carbon dioxide (CO2): Bicarbonate ion (HCO3–) and carbonate ion (CO32-). These compounds are linked through the following reversible acid-base reactions:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3– ↔ CO32- + H+
Here’s the climate takeaway: If alkalinity increases, the above reactions move to the right. Concentrations of HCO3– and CO32- increase, and the concentration of CO2 dissolved in the ocean decreases. This leads to two results: First, less CO2 escapes from the ocean into the atmosphere, and second, the ocean has a greater capacity to absorb CO2 from the atmosphere. As a result, increased ocean alkalinity will reduce atmospheric CO2 and cool climate. This is the crux of one theory of the ice ages for nearly 50 years: Ocean alkalinity increased prior to an ice age, drawing down CO2 and cooling climate. But the devil is in the details; how exactly ocean alkalinity might increase, both today and in the past, is an area of active debate.
Finally, back to pools: Why did the pool water turn green with less alkalinity? The short answer is, I don’t know. After a vigorous debate with fellow graduate students this morning, we came up a few ideas. Either the color change was due to ion-ion interactions changing the preferential spectral absorbance of water from red to (I’m guessing) a longer, purple-ish wavelength (remember: the color of something you see is the opposite of whatever color is being absorbed). Or the color change could have resulted from precipitation of greenish-colored minerals (such as copper compounds).
Chemist or not, what do you think happened to the Rio pool? Feel free to post ideas in the comments below.