The average pH of seawater is approximately 8.1. This is slightly alkaline (basic), not acidic. However, it's crucial to understand that this is an average, and the actual pH can vary depending on location and other factors.
Factors Affecting Seawater pH
Several factors influence the pH of seawater:
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Carbon Dioxide Absorption: The ocean absorbs a significant amount of atmospheric carbon dioxide (CO2). This leads to the formation of carbonic acid, which lowers the pH, making the ocean more acidic. As noted in multiple sources (NOAA, EPA), the ongoing absorption of CO2 is causing a decrease in ocean pH.
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Local Conditions: Local factors such as temperature, salinity, and biological activity can also affect the pH. As stated in a University of Florida publication (https://sfyl.ifas.ufl.edu/media/sfylifasufledu/miami-dade/documents/sea-grant/Temperature,-Salinity-and-pH.pdf), the pH can range from 7.5 to 8.5 depending on these local conditions.
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Long-Term Trends: Studies show a long-term decline in seawater pH. For example, data from the European Environment Agency (https://www.eea.europa.eu/en/analysis/indicators/ocean-acidification) indicates a decrease from 8.11 in 1985 to 8.05 in 2021. This is further supported by research on long-term pH trends in Japanese coastal waters (https://bg.copernicus.org/articles/16/4747/2019/).
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Measurement Methods: The precise measurement of seawater pH is complex. Different methods and instruments exist, ranging from handheld devices to sophisticated sensors, as highlighted by the National Institute of Standards and Technology (https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-acidity-ph-ocean). There isn't a universally agreed-upon pH scale for seawater, adding to the complexity (https://en.wikipedia.org/wiki/Seawater).
