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How Do You Measure the Acidity (pH) of the Ocean?

Animated illustration shows a ship with a testing device, red circles with H+ inside, and says HDYMI? Ocean pH.
Credit: B. Hayes/NIST

The short answer

Different instruments, ranging from hand-held devices to large sensors mounted on ships, measure the acidity of ocean water, by either measuring the amount of positively charged hydrogen ions in the liquid or detecting the color of a pH-sensitive indicator dye added to the solution.

In a chemistry lab, you determine how acidic or basic something is by using the pH scale. The scale ranges from 0 to 14 with 7 considered neutral, as is the case with plain water. Anything below a pH of 7 is an acid, and anything above 7 is a base, or alkaline. What makes a solution more acidic is the presence of positively charged hydrogen ions (H+), and conversely the presence of negatively charged hydroxide ions (OH-) makes it more basic.

But these measurements don’t just apply to liquid solutions in beakers and flasks in a lab. The biggest water source on Earth, the ocean, has a pH too — and its pH is extremely important to our planet’s ecosystem.

The average pH of the surface water in the ocean is 8.1. Ocean water is more basic than drinking water because the presence of other weak acids and their associated bases known as buffers causes the ocean to resist changes in pH. Based on the pH scale, the ocean is considered slightly alkaline.

How do researchers measure pH in the sea? Here are two approaches:

  • A widely used instrument is the ISFET, which stands for ion-sensitive field effect transistor. This device measures the pH of a single drop from a solution or even larger volumes by using a sensor on a silicon chip that’s housed in a rigid structure, making it harder to break. The ISFET measures the concentration (activity) of hydrogen ions in the solution. The greater the concentration of hydrogen ions, the more acidic the solution or seawater. 
  • Another way to measure pH is to use pH-reactive dyes. The dye can be embedded in the surface of pH paper, and when the paper touches a solution the dye will react to the hydrogen ions and will turn a specific color, such as red or blue, indicating if the solution is more acidic or basic. Some pH paper has a wider variety of dyes and can range in color from yellow to green to red to purple. 

These methods are typically used in smaller devices measuring individual samples taken from larger bodies of water such as in coastal areas. These sample readings provide a quick sense of the pH in that area. Though these methods are quick, efficient ways to measure ocean pH, they do need to be calibrated so researchers can get consistently accurate measurements. Researchers at the National Institute of Standards and Technology (NIST) are developing standard reference materials that help calibrate these measurements so scientists can trust the results they’re getting.

To measure the pH of larger bodies of water, researchers use ships, stationary buoys and floats.  Some sensors connected to ships rely on ISFETs, which are lowered into the ocean for a certain period of time so researchers can receive ongoing data on pH levels.

Sensors can also be attached to a stationary mooring buoy that measures ocean pH in a specific area or to floats that move along with ocean currents.

Although we have many ways of measuring ocean pH, new approaches are under development. One such project is a system being developed as a way to train undergraduate and graduate students to measure ocean pH and gather data for research. The system, called pHyter, is a hand-held device that works with a cellphone app to measure the wavelengths of light that are absorbed by a water sample. Depending on how acidic or basic the water sample is, it will turn a specific color when a chemically reactive dye is added to it. The National Oceanic and Atmospheric Administration (NOAA) is funding the new system.

Now that you know how we measure ocean pH, you may also be wondering why we measure it. Knowing the ocean pH is important because changes in pH can affect the organisms living in the sea, and, as a result, humans. These properties of the ocean are classified as essential ocean variables (EOV), which are broken down into different categories, with ocean pH being an important measure for monitoring inorganic carbon. EOVs can help scientists make accurate ocean forecasts and early warnings as well as protect ocean health.

Because of this, oceanographers and other scientists find it important to not only know the ocean’s average surface pH but also what factors can cause it to change. For example, the Industrial Revolution that began in the 18th century dramatically increased the amount of carbon dioxide (CO2) released into the atmosphere. Approximately one third of the total amount of CO2 released into the atmosphere is absorbed by the ocean, a process called ocean acidification

Organisms affected by the ocean’s acidity include corals and pteropods, such as sea snails and slugs, which rely on a compound known as calcium carbonate (CaCO3) to form their shells and outer structure. Ocean acidification results in a lower ocean pH, causing CaCO3 to dissolve or break down more easily, making it more difficult for these creatures to form shells.

Changes in ocean pH can also affect how certain frequencies of sound travel through the water. Some studies show that abrupt changes in pH can affect how some animals such as whales use sound. In addition, if the animals and organisms at the base of the food chain we consume, like fish or oysters, are stressed by changes in ocean pH and are not as healthy, then that ultimately will affect us.

Scientists are developing and deploying sensors and instruments to accurately measure ocean pH. These accurate measurements mean better data, which can ultimately be beneficial for our environment by helping us figure out steps to combat changes in ocean pH and better protect the living organisms in it.

Created January 31, 2022, Updated March 2, 2022