Today’s post is by Puneet Kollipara, intern at C&EN and an aquatic acidity aficionado.
Humans pump carbon dioxide into the atmosphere by burning fossil fuels, but not all of it stays in the air. About one-fourth of the released carbon dioxide dissolves into the oceans, where it has been lowering the global average pH of seawater and thereby threatening aquatic ecosystems.
Unfortunately, the ocean is as complex as it is spacious, and ocean pH doesn’t change uniformly across its depth. To get the full picture, scientists need a lot of data, but current techniques for monitoring ocean pH are generally expensive, aren’t always reliable, and can’t go very deep underwater. Right now, the U.S. National Oceanic and Atmospheric Administration (NOAA), for instance, has 18 ocean-chemistry monitors at various locations—more than anyone else in the world—but none of these sensors takes measurements below surface waters. “As you can imagine, that does not really represent the global oceans very well,” says Christopher L. Sabine, an oceanographer at NOAA’s Pacific Marine Environmental Laboratory.
A 22-month competition launched by the XPRIZE Foundation, a nonprofit aiming to spur technological innovation for society’s betterment, seeks to change that. The newly announced $2 million Wendy Schmidt Ocean Health XPRIZE calls on innovators of all stripes, both professional and amateur, to design better pH-measurement technologies. “The idea with the XPRIZE is to develop robust, inexpensive sensors that can be deployed much more easily,” says Sabine, whose NOAA lab is partnering with XPRIZE for the competition.
Half of the $2 million prize will be awarded for the development of an affordable, reliable sensor, Sabine says. The other half will go toward a system that can accurately profile pH changes, including at great depths; such an instrument might start deep in the ocean and take real-time measurements as it’s lifted to the surface.
Two types of instruments are currently in mainstream use for measuring ocean pH, but both have significant drawbacks. The first type, potentiometric sensors, involves probing a water sample with a device containing two electrodes. One electrode is enveloped in a semipermeable membrane that lets ions pass through, and the other is exposed directly to the water as a reference. Acid hydrogen ions flow from the seawater across the membrane, and a voltmeter measures the resulting electric-potential difference compared with the reference electrode. The sensor can use that measurement to calculate the water’s pH: The more H+ ions there are, the more that flow across the membrane, and the greater the resulting voltage.
One drawback of pH electrodes, however, is that they’re very sensitive to the presence of other ions in seawater, which can also flow across the membrane. “So they work reasonably well in freshwater, but they don’t work as well in saltwater, Sabine says. “Basically seawater has a whole lot of stuff in it that kind of whacks out the electrodes.”
The other main class of instruments is spectrophotometers. These devices measure pH using a dye that changes color with pH. It’s like a souped-up version of the classic high-school chemistry experiment in which students put pH indicators into water and watch the color change before their eyes when they add H+ or hydroxide (OH-) ions.
Although spectrophotometers can be accurate and precise, they have a problem of their own: They’re very labor-intensive. For starters, highly trained scientists have to add the chemicals, Sabine says. And by adding a dye to the solution, you can throw off the pH. “It is tricky to add enough dye to get a reading, but not so much dye that it makes a noticeable difference in the pH you are trying to measure,” he says. Moreover, the dyes have to be purified to get an accurate measurement.
Given the flaws in existing methods, that’s where the XPRIZE comes in. An area of scientific research other than oceanography may already have a solution, and the XPRIZE could help link scientists with the people who developed it, Sabine suggests. “There are enough brilliant minds and people who are perhaps using pH technologies in different ways that a solution might very well be out there,” he says.
Perhaps pH tools used in other industries—chemicals manufacturing and medicine, for instance—could be adapted to the ocean arena. A home-aquarium enthusiast might have developed a durable, accurate tool to monitor the pH in a fish tank, Sabine adds. Or maybe the breakthrough will come from a youngster—think Jack Andraka, the high-school science phenom who made headlines for his novel method to diagnose pancreatic cancer far more reliably, simply, and cheaply than traditional methods have allowed.
In the end, deploying an army of better pH sensors could have massive benefits, Sabine says. Not only might they help scientists better understand how carbon dioxide will change the chemistry of our oceans, but they also might serve as invaluable tools for fishermen, marine sanctuaries, state and local water-monitoring agencies, and others.
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