Gasses like carbon dioxide (CO2) or nitrogen dioxide (NO2) cause the pH to decrease when they mix with water. The natural carbon dioxide in the atmosphere causes the average pH of rain water to be 5.6. As the amount of carbon dioxide in the atmosphere increases, this number will decrease. Acid rain, with a pH of 4 or less, occurs in areas where there are a large amount nitrogen dioxide and sulfur dioxide (SO2) emissions, from power plants or car exhaust, that mix with the rain water.
Other compounds that have broken down from minerals in rocks can have the opposite impact, either increasing or helping to stabilize the pH. Two ions that are found in large amounts in ocean water are carbonate (CO3-2) and borate (H2BO3–). These ions that have a negative charge, so they can bond with positively charged hydrogen ions to form new compounds. This removes H+ from solution and raises the pH (decreasing the acidity). They also bond with any new H+ ions that are added to the water. This serves to buffer the water pH, so the concentration of H+ stays constant, and the pH doesn’t change.
Because of these compounds, the average pH of the ocean is fairly consistent throughout, ranging from 8.0 to 8.3. Ocean life is adapted to live at that pH level. Fresh water lakes usually have more variable and typically lower pH than the ocean because there are fewer mineral ions to buffer the water. Lakes and streams typically have pH values ranging from 6.5 to 8.0.
Carbonate plays another key role in the ocean. Many types of ocean life, from tiny single celled creatures to giant corals, shellfish, and some types of algae, take carbonate out of the water to make their shells. The carbonate bonds with calcium ions (Ca+2) to form calcium carbonate (CaCO3). In water with a low pH (high acidity), too much of the carbonate is already bonded to H+, so there is not enough available for animals to make shells. If the pH drops too low, delicate calcium carbonate shells can begin to dissolve. The pH and the amount of available carbonate in the water only need to drop low enough that the water does not have an over-saturation of carbonate. Recent studies have shown that this can begin to happen at a pH of 7.5 in the ocean. Decreases in pH have the potential to harm Maine’s shellfish populations.
Global and Local Changes in Ocean pH
The pH of the ocean as a whole is decreasing. This process, called ocean acidification, is happening because there are increasing amounts of CO2 in the atmosphere. This CO2 mixes with water in the ocean and in rain to form carbonic acid. Because there is such a large influx of CO2 to the atmosphere, the ocean is unable to completely buffer this added acid. As a result, the pH of the oceans has decreased globally by 0.1 pH units, a 30% increase in H+ concentration, over the past 100 years.
Although the ocean as a whole is not anywhere near the 7.5 level, there are several reasons that we may find pH values lower than the global average in the waters around Georgetown. Because fresh water typically has a lower pH than ocean water, the mixing of Sheepscot and Kennebec River waters with ocean water in the estuary around Georgetown can lead to localized areas with lower pH values. Another potential reason the pH might be low is that inputs of nutrients from the surface of the land, from fertilizers and organic waste, can lead to blooms of algae. When these blooms die and rot, the rotting releases organic acids that can locally decrease the pH of an area. Areas with large amounts of rotting organic material from dead plants or animals also have high amounts of organic acids released during the decay of the organisms that decrease the pH.
Georgetown’s Shellfish Committee has expressed an interest in finding out more about the pH of Georgetown’s waters. Clam populations seem to be decreasing in some areas of town, and no one has yet identified the reason for those decreases.
Testing pH
We will be testing pH using Baker pH paper. This pH paper has a fine resolution so that we will be able to identify relatively small changes in pH. Using three sets of pH paper, we will be able to test with a detail that ranges from 0.1 to 0.3 pH units within the range of pH 6.0 to pH 10.0.
The pH scale is not an arbitrary set of numbers; it is a precise measurement of the concentration of H+ ions in solution. The smaller the number, the higher the amount of H+ ions. The pH scale is a negative log scale, so each number smaller on the pH scale has 10 times as many H+ ions in solution. The change of even a single pH unit means a large change in the concentration of H+ ions and a large change in the acidity. A change of 0.1 on the pH scale can mean that the amount of H+ ions has doubled.
