pH Measurements¶
What is pH?¶
Definition
pH is a measure of the acidity or alkalinity of an aqueous solution. The formal definition of pH is that it acts as an indicator of the concentration of hydrogen ions present in aqueous solutions. The concentration of hydrogen ions is calculated as:
This means that at a higher pH, there are fewer free hydrogen ions and that a change of one unit of pH reflects a tenfold change in the concentration of the hydrogen ion. For example, there are 10 times more hydrogen ions available at a pH of 7 than at a pH of 8. The pH scale ranges from 0 to 14, and a pH of 7 is considered neutral. Substances with a pH below 7 are acidic; while substances above 7 are basic or alkaline. It's an indicator of chemically changing waters.
Extra information
An interesting concept related to pH is alkalinity, which is resistance to changes in the pH of a solution.
Why is it important?¶
pH determines the solubility and biological availability of chemicals such as nutrients (for example, phosphorus, nitrogen, or carbon) and heavy metals (like lead, copper, cadmium, etc.). In other words, it indicates how much of these substances can be dissolved in the water and how much can be used by its aquatic living beings. For example, in addition to affecting how much and what type of phosphorus is most abundant in water, pH also determines whether aquatic life can use it. In the case of heavy metals, the degree of solubility determines their toxicity. Metals tend to be more toxic at a lower pH because they are more soluble.
In the case of nutrients, a change in pH can increase the solubility of phosphorus, making it more available for plant growth and resulting in a higher long-term demand for dissolved oxygen. Normally, water contamination conditions correspond to an increase in plant photosynthesis, and the resulting contamination can lead to a long-term increase in pH. The most common concern is changes in pH caused by the discharge of municipal or industrial effluents (wastewater). However, the pH of most effluents is fairly easy to control and is often regulated.
In the sea
The pH of seawater is normally moderate neutral-base values between 7.4 and 8.5.
pH level has an important role in ocean carbon cycles. There is evidence that ocean acidification has been caused by CO2 emissions. The ocean absorbs approximately 30% of CO2 emissions from the atmosphere and, when this happens, a chemical reaction occurs that generates hydrogen ions in the water. A higher concentration of hydrogen ions means a more acidic pH. As we said above, pH determines the solubility and availability of nutrients for aquatic life (nitrogen, phosphorus, etc.). In other words, it not only indicates whether aquatic living beings have nutrients at their disposal but also whether they can absorb them. An acidic pH could also limit the growth of corals, and other fixed living species, such as oysters or mussels, which in acidic waters would have more difficulty generating their shells.
The ocean and photosynthesis of its plant organisms (phytoplankton in particular) absorb CO2 and produce oxygen (O2). The ocean absorbs around 30% of the planet's CO2 and generates between 50% and 75% of the oxygen we breathe, depending on the sources and geographic areas. The warming of the ocean as well as its acidification endanger the survival of phytoplankton, seagrass meadows, mangroves, and salt marshes, which are natural CO2 traps. Nevertheless, 20% of the mangroves on the planet disappeared between 1980 and 2005. Ever more acidic, the ocean is also seeing the appearance of deoxygenated zones that are fatal for marine life and global equilibria.
Impact of acidity
The ocean's acidity has been the same for a hundred million years with a pH of 8.18 c. By absorbing the CO2 we produce, it is now at 8.08 and the trend is accelerating. Specifically, this means that the ocean is already 30% more acidic and will be 50% more acidic by 2100, with direct consequences on marine animals.
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More CO2: Ocean acidification makes it harder for small marine organisms to trap CO2 when building their shells.
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Less coral: Ocean acidification leads to bleaching and disappearance of coral, home to 25% of fish species and supporting more than a billion people.
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Less food: Ocean acidification is corrosive to the shells of small sea snails, the first level in the food chain.
Interesting fact
Waters with high pH (alkaline, basic) have a bitter taste, and cause deposits in pipes. In addition, it can cancel out the effect of chlorine at the disinfectant level.
Conversely, water with a low pH (acidic) tends to cause greater corrosion and dissolution of metals and other substances.
How is pH measured?¶
pH values can vary very quickly. This parameter should be measured in the field immediately after collecting a sample. pH can also be assessed after the fact in the laboratory, but the data will not be completely representative. Our Water Stations make use of a pH sensor produced by Atlas Scientific.
Additional resources
On ocean acidification and its impact: https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
