pH Fact Sheet

Definition: A measurement of hydrogen ion concentration (H+) in liquids and other substances. The amount of H+ can determine whether the substance is acidic or basic (alkaline).

Background:

  • Water contains both H+ (hydrogen) and OH- (hydroxyl) ions.
  • Pure distilled water has an equal number of hydrogen and hydroxyl ions.making the water neutral (pH of 7)
  • More hydrogen than hydroxyl ions results in a acidic solution, (pH <7)
  • More hydroxyl than hydrogen ions results in a basic solution, (pH >7)
  • pH is expressed on a Log10 scale from 1-14, thus a pH of 6 is 10 times more acidic than pH of 7
  • Natural rain has pH about 5.6, (CO2 + H2O forms carbonic acid, which breaks off a H+ and makes rain acidic)
  • Organic acids in upper soil layer can lower pH by producing extra H+
  • Calcium carbonate (CaCO3) rocks and soils can buffer changes in pH
  • Photosynthesis removes CO2, and eventually carbonic acid, making water more basic
  • Respiration/decomposition adds CO2, and eventually carbonic acid, making water more acidic
  • carbon dioxide + water+ solar energy = glucose + oxygen
  • 6CO2 + 6H2O + solar energy = C6H12O6 + 6O2
  • High pH prevails in summer when more photosynthesis occurs and waterways are heavily influenced by groundwater flow and buffered by limestone soils
  • Low pH prevails in late winter and spring when less photosynthesis occurs, snow melts and precipitation rapidly enter waterways, without buffering contact with calcium carbonate rocks, and buffered groundwater has less of an influence.
Environmental Impacts
  • Effects of acid rain are worse in those regions that:
    • Are downwind of industrial areas
    • Do not contain calcium carbonate in rocks and soils to reduce acidity
  • Primary cause of acid rain is from nitrogen oxides (NOx) and sulfur dioxide (SO2), from automobile and coal-fired power plant emissions, which transform into nitric & sulfuric acid
  • Resulting acidic rain precipitates to the ground, rendering waterways too acidic to support aquatic life

pH ranges in which different organisms can survive
Source:  Cuyahoga Valley environmental education Center, 1997.
Student Discovery Book 5/97 version.  Peninsula, OH
  • Most organisms are adapted to live within a specific range of pH, thus, even a slight change may be fatal
  • Acid mine drainage, from coal mining and other resource extraction, contains sulfuric acid (H2SO4) which can break off an extra H+, lowering the pH
  • Acidic water, low pH, releases metals, which can harm aquatic life
  • Average pH of natural creek water in Pennsylvania is between 6.5 and 8.5, except in acid mine drainage streams
  • Northeast USA typically receives acid rain of pH 4.5 or lower

Source:  Environmental Effects of Acid Rain
http://www.epa.gov/acidrain/effects/enveffct.html
Water Quality:
  • Natural waters should have a pH between 5-8.5
  • EPA's required pH levels for drinking water is 6.5-8.5