What is pH?
The pH of water is a measurement of how acidic or how basic the water is. We measure pH on a scale of 0 (pure acid) to 14 (pure alkaline solution). Distilled water is neutral and has a pH of 7. Each unit in the pH scale represents a 10 fold change in acidity.
Some substances, when dissolved in water, produce charged molecules called ions. Acidic water has extra hydrogen ions (H+) and is given a pH value between 0-7, whereas basic or alkaline water has extra hydroxyl ions (OH-) and is given a pH value between 7-14.
WHY CARE ABOUT pH?
Aquatic life has adapted to the natural pH levels in the bodies of water that they live in, so even slight changes in pH can have negative impacts on the health of the aquatic community. For example, moderate changes in pH can affect fish egg production, fish and insect gills, and amphibian populations.
A change in the pH of water can also alter the behavior of chemicals in the water. For example, ammonia, which is harmless to fish in water that is acidic, can become toxic as the pH increases. Additionally, many heavy metals dissolve in acidic water.
- Allowable range for pH in Utah is 6.5-9
NATURAL FACTORS INFLUENCING pH
- Calcium Carbonate- Calcium Carbonate is the primary component of limestone. This common rock “buffers” the water against changes in pH. Calcium carbonate can exist in water in 3 different forms: ions with 2, 1, or no hydrogen atoms attached. The different molecules form in response to the concentrations of extra hydrogen or hydroxyl ions in the water. Watersheds without limestone lack this buffering ability and are vulnerable to acid rain and/or acid mine drainage.
- Pine or Fir Forests- Decomposing needles of these trees add acidity to the soil and also influence the acidity of nearby streams.
- Ground Water- Percolates through soils and if the soils are buffered, the pH may be somewhat higher (7-8).
- Precipitation- When precipitation falls through the air, it dissolves gases like carbon dioxide and forms a weak acid. Natural, unpolluted rain and snow is slightly acidic. Precipitation usually has a pH between 5 and 6.
- Season- In the fall when leaves and needles fall into the water and decompose, this may increase the acidity of the water.
- Photosynthesis and Respiration- During photosynthesis, aquatic plants remove carbon dioxide from the water. This can raise the pH in the water. Since plants photosynthesize with sunlight, the pH of the water will be highest during the middle of the afternoon, and lowest just before sunrise.
HUMAN FACTORS INFLUENCING pH
- Acid rain- Sulfuric acid (produced by coal burning industries) and nitric acid (produced by automobile engines) are major contributors to acid rain. Luckily, in Utah, the buffering soils help to decrease the effects of acid rain.
- Increased concentrations of carbon dioxide- Carbon dioxide is very soluble in water, forming weak carbonic acid. As our atmosphere’s carbon dioxide concentrations increase (Mauna Loa Observatory), the oceans are becoming more acidic. The effects are devastating to coral reefs, which are stressed or dying throughout the oceans. Animals with calcium carbonate shells are also harmed.
- Point source pollution- Dumping industrial pollutants directly into water can affect its pH.
- Mining- Mining may expose rocks to rainwater and produce acidic runoff. Mining drainage can introduce acids into a waterway, if it is poorly buffered the pH may reach toxic levels.
- Learn more about limiting human impacts: Protect Your Water.
HOW DO WE MEASURE pH?
Utah Water Watch- Learn how volunteers across the state measure pH!
Stream Side Science- Explore different lesson plans involving pH and see how they align with the core curriculums for grades K-12!