What is pCO2, and why do we measure it?

An Interview with Denis Pierrot

Interview by: Emma Pontes

I’m pleased to introduce Physical Chemist and Co-Chief Scientist of GOMECC-3, Denis Pierrot. On a normal day, he can be found in the computer lab overseeing CTD operations, but today, he’s kind enough to escort me to the Hydro Lab where the pCO2 Underway System lives. pCO2 is slightly different than regular CO2 concentration; it is the partial pressure of CO2 in a liquid or gas. On the Ron Brown, we have a pCO2 Underway System which Denis proudly tells me is state of the art, and the result of collaboration between several esteemed scientists around the world.

Image 1. Image credit: Emma Pontes.

In Image 1, you can see the conglomerate of wires, equilibrators, and pumps that make up the system. A water line is connected to the system which pumps surface water from outside our ship in a constant flow into the system. The equilibrator (red circle) sprays the incoming water in a thin sheet which allows the air inside to become equilibrated with the water. This means that the pCO2 of the water will equal the pCO2 of the air inside the equilibrator. This equilibrated air is then sent to a gas analyzer (yellow circle) that measures the pCO2. In this manner, the pCO2 of the water is measured indirectly (by measuring equilibrated air pCO2) and graphed neatly on the laptop attached.

Additionally, the pCO2 Underway System has a gas line (green circle) that draws in air from the bow of the ship, ensuring the cleanest air possible with no contamination from ship emissions. This line is connected to the gas analyzer and the incoming air is measured for its pCO2 directly. The goal of this system is to measure the pCO2 of the water AND the atmosphere. The interaction and difference between ocean and air pCO2 is the basis of Denis’ work.

Image 2. Image credit: Emma Pontes.

Image 2 shows a graph of the output from the pCO2 Underway System. The blue and brown dots represent pCO2 of the water, while the purple and green crosses represent pCO2 of the atmosphere (purple line). The goal here is to determine where and when the ocean acts as a source and sink of CO2. The ocean is a source of CO2 when the water pCO2 is higher than that of the atmosphere (above the purple line). Conversely, the ocean is a sink of CO2 when atmospheric pCO2 is higher than that of the water (below the purple line). Since gases always move from high to low concentration in an endless quest for equilibrium, Denis and his colleagues can tell if the ocean is releasing (source) or absorbing (sink) CO2 and at what rate. The rate of this CO2 exchange between ocean and atmosphere is called flux. Denis hopes to create an up to date flux map of the Gulf of Mexico complete with spatial and temporal attributes. Flux is constantly changing both seasonally and temporally, and is dependent on wind, ocean current, temperature, and other atmospheric factors.

One quarter of anthropogenic CO2 is dissolved into the ocean. This has important and unfavorable implications for calcifying organisms such as certain plankton, corals, sea urchins, and any other creature that relies on carbonate to build its shell or skeleton. CO2 is an acidic gas that lowers the pH of water which it is dissolved in. A lower pH means more acidic water, which also means less available carbonate for calcifying critters to utilize in their shell building. A more acidic ocean is one with fewer ecologically important corals (and other carbonate-reliant species) and fewer commercially important seafood items like shellfish. Denis is very passionate about his work, which has ecological and commercial implications under ocean acidification conditions.

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Co-Chief Scientist Denis Pierrot, hard at work in the Hydro Lab. Image credit: Emma Pontes.

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.

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.
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.

Breathing in Science: Oxygen Measurements At Sea

Author: Emma Pontes

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Scientist Emma Pontes performing Winkler Titration on a water sample collected from the first station of the cruise. Photo taken by Leah Chomiak.

Take a deep breath. The air you just inhaled contains about 20% oxygen, 78% nitrogen, and 2% of a few other minor gases. Some might assume that oxygen is only available to terrestrial air-breathers, however, this assumption couldn’t be further from the truth.

Oxygen (O2) generally exists in a gaseous state, but also exists in the world’s oceans as a dissolved gas. Fish and other ocean biology utilize the available dissolved oxygen just like humans do; taking up O2 and discharging carbon dioxide (CO2). Just like on land, the ocean is home to millions of photosynthetic organisms such as plankton, algae, and other underwater plants that take up CO2 and release O2 during a process called photosynthesis. Therefore, there is a constant ebb and flow of CO2 and O2 being ‘inhaled’ and released into ocean waters.

So what does this mean for ocean chemistry, and why do we care? Dissolved oxygen in the ocean is a sensitive indicator of climate-related changes. The dissolved oxygen concentration can be used to determine how much anthropogenic CO2 (carbon dioxide released by humans resulting from the burning of fossil fuels) is being taken up by the ocean. Just like oxygen, CO2 can dissolve in ocean waters, and most of human-created CO2 has been sequestered by our oceans. The uptake of anthropogenic CO2 by the world’s oceans is a leading cause of ocean acidification. Therefore, it is of high importance to determine the O2 concentration of various locations around the world’s oceans, not only to learn more about the how ocean biology is functioning, but also to examine the effects of ocean acidification.

Enter GOMECC-3, Ocean Acidification Research Cruise. In the past, research vessels have travelled our current route collecting the same data we are gathering now at the same locations. We can get an idea of how ocean chemistry is changing over time by comparing the data we get on this cruise, to the historic data sets collected on the same path we are on now.

Work days on the ship consist of lowering the CTD rosette (stands for conductivity, temperature, and depth) into the ocean at a predetermined location called a Station. The CTD is a large cylindrical ring of bottles, called Niskins, that are triggered to close and collect water samples at predetermined depths. The CTD is a useful tool for scientists onboard to get insight as to how ocean chemistry changes with depth.

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CTD being lowered into the water at the first station of the cruise. Photo taken by Leah Chomiak.

My job is to collect water samples from the Niskins and analyze each sample for its dissolved oxygen concentration using a technique called Winkler Titration. This procedure requires the addition of chemicals to the water sample that act as a fixative; the chemicals bind to the oxygen in the water and create a solid precipitate that eventually sinks to the bottom of the water sample. You can think of it as ‘pickling’ the oxygen to preserve it, so that the sample can be analyzed anywhere from 1hr to 4 weeks after being collected. To learn more about the titration procedure, check out the peer-reviewed paper entitled ‘Determination of Dissolved Oxygen in Seawater by Winkler Titration Using the Amperometric Technique’ written by Dr. Chris Langdon in 2010, which basically serves as my lab manual on the ship. I am looking forward to collecting some meaningful data that will contribute to OA research as we continue our trip around the Gulf of Mexico!

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Water sample collected by Scientist Emma Pontes to be analyzed for dissolved oxygen. The white milky-looking substance at the bottom of the sample is the bonded oxygen precipitate. Photo taken by Emma Pontes.


Langdon, Chris. “Determination of dissolved oxygen in seawater by Winkler titration using the amperometric technique.” The GOSHIP Repeat Hydrography Manual: a Collection of Expert Reports and Guidelines, edited by: Hood, EM, Sabine, CL, and Sloyen, BM (2010).