pH


Definition:

pH by itself is not a pollutant, but it tells you how acid or basic the water is.

An acid is a substance that has an excess of hydrogen ions (H+).

A base has more hydroxyl ions (OH-).

How acid or basic a substance is can be measured on a scale called pH that ranges from 0 to 14. A pH measurement of 7 (exact middle of the scale) for a solution is said to be neutral because it contains equal amounts of hydrogen ion, and hydroxyl ion.

The table below shows the relationship between the pH, amount of H+ ion in the water, and their general category.

 

 

What does this table mean? It just means that pH 6 is 10 times more acidic than pH 7; whereas a pH 5 is 100 times more acidic that pH 7. This is called a logarithmic scale.

 

Why does it cause pollution?

Different organisms need different levels of acidity or alkalinity in order to survive. If the levels in the water change from what the organism needs, then it may not survive.

 

How does it get into the water?

Acids can come from a variety of different sources. Some acids are referred to as organic acids and others as inorganic acids.

Organic acids occur naturally or can be manufactured by humans. Some examples are as follows:

  • Uric Acid - Animals produce uric acid and it is excreted during urination.
  • Acetic Acid – bacteria produce a large number of organic acids. One common organic acid made by bacteria is acetic acid or vinegar.
  • Tannic Acid – plants produce a variety of organic acids. One example is tannic acid. It is found in tea, oak, and cypress trees.

Inorganic acids are formed through inorganic processes, so they come from processes that do not necessarily include living organisms. This can mean that the chemicals form naturally or they can be manmade.

  • Carbonic acid – formed from the reaction of water and CO2. This is the same acid that is found in soda drinks and is formed naturally in rain.
  • Nitric acid – formed naturally when Nitrogen gas reacts in the atmosphere when lightning strikes or as a byproduct of human activities. Nitric acid is one of the acids responsible for acid rain.
  • Sulfuric acid – formed from the bacterial oxidation of sulfides and elemental sulfur. Oxidation is a chemical reaction similar to burning. It is how some bacteria get energy. Sulfuric acid is one of the acids responsible for acid rain.
  • Alkalinity can be produced by both organic and inorganic processes and can come from a variety of sources.

 

Organic:

Both plants and animals produce alkalinity. Some animals secrete shell from alkaline ions in the water. An example of this is the calcium carbonate found in many seashells, like clams. When those animals die, the shell dissolves and the calcium carbonate is released to the water.

 

Inorganic:

The source of much of the alkalinity in natural waters is from carbonates. Baking soda and limestone are examples of carbonates that form naturally. Carbonates can form when CO2 reacts with water to form carbonic acid, or H2CO3. The carbonate, CO3, can be insoluble in surface waters and precipitates as calcium and magnesium carbonates, or limestones. If there is more Ca or Mg in the water relative to H+, then the carbonate will precipitate as Ca or Mg carbonate rocks, or limestones. As acidic waters move through limestone rich soil or rocks, the pH of the water will increase because the carbonates dissolve into the water and neutralize the acidity.

 

General Environmental Effects of Acidity and Alkalinity

Metals are more soluble in acidic solutions (pH < 7.0) and form relatively insoluble solids in basic solutions (pH >7.0). People living in a community that gets its drinking water from acidic waters are at a higher risk for metal toxicity than people who get their drinking water from basic waters.

Pesticides are chemicals used to kill plants, fungi, insects, and spiders. In general, pesticides last longer in acidic solutions than in basic solutions. People living in a community that gets its drinking water from acidic waters are at a higher risk for pesticide toxicity than people who get their drinking water from basic waters.

 

pH Controls by Aquatic Algae

One of the most important processes is the effect that aquatic algae have on pH. Algae make food for themselves through photosynthesis, just like plants. During photosynthesis carbon dioxide is combined with water within the cell of the algae to form basic sugars and oxygen. This is how algae can raise the oxygen levels, but it is also how it can raise the pH of a waterbody.

HUH? ! ?

Remember, we said above that carbon dioxide is acidic when it reacts with water. In the case of algae and photosynthesis the carbon dioxide is used to make basic sugars rather than being allowed to form carbonic acid. So, if the source of acidity is used up the pH goes up and the water becomes more basic.

The reverse of photosynthesis can happen too. It can happen in humans as well as in algae. The process is called respiration. Respiration occurs when oxygen is used to break down or burn basic sugars. This process produces carbon dioxide and water. Humans do this when they breathe. When algae respire, they put carbon dioxide directly into the water column, which results in more carbonic acid and a lower pH, thus more acidic water.

 

What land uses are the source of this type of pollution in the B-T Basin?

  • Natural Sources
  • Agricultural Runoff
  • Urban Runoff
  • Untreated Sewage
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