Ocean Acidification from a pH Perspective

Author: Jon Sharp

If you’re reading this blog, you’ve probably heard of and have some understanding of ocean acidification. Even the entirely uninitiated can deduce a bit of information from the term itself: “ocean acidification” must refer to a process by which the ocean becomes more acidic than it is already. That much is certainly true, but the details of the phenomenon are a bit more complex and—at least to ocean chemists—interesting.

Acidity is a measure of the amount of dissolved hydrogen ions (H+) in a solution (like seawater). And contrary to what the phrase “ocean acidification” may suggest, seawater is not acidic at all. Typical ocean water lies comfortably above neutral on the pH scale (within the basic range). Ahhh the pH scale; you remember it fondly from high school chemistry, right? In case not, let’s break it down.

The term “pH” can be divided into two parts. The “H” denotes what pH is actually a measure of: hydrogen ions. We chemists think about dissolved ions in terms of concentration, or the number of ions in one kilogram of seawater. The “p” in pH is simply a mathematical operation (the negative logarithm) that allows us to view hydrogen ion concentrations in numbers that are easy to digest and understand. Due to this numerical wizardry, low pH values denote high hydrogen ion concentrations, and vice versa.

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A visual representation of the pH scale. Figure courtesy of NOAA PMEL.

To learn even more about seawater pH and ocean acidification, click here.

So, “ocean acidification” implies that something is causing the quantity of hydrogen ions in seawater to increase. Conveniently, that something is easy to identify: the culprit is carbon dioxide (CO2). While CO2 is a naturally occurring gas, the burning of fossil fuels, along with other human activities, releases extra CO2 into the air. Excess CO2 that doesn’t stick around to warm our atmosphere dissolves into the ocean. Once there, CO2 undergoes a few interconnected chemical reactions with water (H2O). This process results in more dissolved hydrogen ions in seawater (lower pH). Another effect is fewer dissolved carbonate ions (CO32–), which is particularly bad news for corals and for many shellfish we like to eat.

While the mechanics driving ocean acidification are quite simple, Earth’s environment enjoys complicating things. That is why scientific expeditions like GOMECC-3 are important. Our group from the University of South Florida’s College of Marine Science is monitoring ocean acidification by examining large-scale environmental changes in pH and carbonate ion concentrations. We are also investigating localized patterns in the two parameters that may be influenced by factors other than CO2, such as ocean currents and biological activity. Multiple factors can work together to either accelerate or reduce the rate of ocean acidification in a region.

To perform measurements of pH, we use a form of analysis called spectrophotometry. We shine a beam of light through seawater that has been mixed with an indicator dye, called m-cresol purple, that forms chemical complexes with H+ ions. The dye has a basic form, which appears purple, and an acidic form, which appears yellow. Both forms absorb light at different wavelengths in the visible light spectrum. Our sensitive spectrophotometers detect how much light has been absorbed by each form of m-cresol purple, which corresponds to the pH of the sample. We perform measurements of CO32– in much the same way, only using a different indicator and examining absorbances in the ultraviolet light spectrum.

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Briefcases filled with the 10-cm cuvettes we use for spectrophotometric pH and carbonate measurements. The cases make it easy for us to transport our samples to and from the CTD rosette. Photo credit: Jon Sharp.
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Jon Sharp fills a glass cuvette from a Niskin bottle for pH measurement. The cuvettes are overflowed a number of times to ensure that our samples are uncontaminated. Photo credit: Courtney Tierney.

By measuring pH and carbonate ion concentrations with a high degree of accuracy, we are able to assess ocean acidification directly and rapidly. We can compare new measurements to those that have been made on past GOMECC cruises (2007 and 2012) to examine human-induced changes in ocean chemistry. Perhaps most importantly, we can use this information to identify locations that must be targeted and human activities that must be altered to best mitigate future ocean acidification.

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Seawater samples from greater than 3000 meters deep (left side) to the surface (right side) that have been mixed with m-cresol purple dye display an impressive spectrum of colors. Photo credit: Jon Sharp.
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