Limestone and Acid Rain

Carbon Dioxide-Bicarbonate-Carbonate Equilibrium

One important buffer in surface waters is the carbon dioxide/bicarbonate/carbonate buffer. When water is in equilibrium with both CO2 form the atmosphere and carbonate containing rock, the pH of the water is buffered to a pH of 8.3, close to the pKa of the weak acid bicarbonate HCO3- (pKa = 8.4).

The buffered water resists changes to pH from additional acid or base. Why is this important?

Acid Rain

Acid rain is caused by the formation of nitric and sulfuric acids in our atmosphere. These compounds are strong acids and they are highly soluble in water and they dissolve in the water droplets within clouds.

Most of the nitrogen and sulfur oxides result from human activities. The major sources of sulfur dioxide emissions are electric utilities (60 %), industrial combustion (17 %), and industrial processes (8 %). Transportation, with internal combustion engines, produced more than 50 % of all NOx with additional emissions from electric utilities (26 %) and industrial combustion (14 %). Agricultural activities, in particular manure handling, are the largest source of ammonia emissions but some ammonia is also given off by industry and by the transportation sector.

Acid rain causes acidification of lakes and streams and contributes to the damage of trees at high elevations and sensitive forest soils.

Effect of Limestone

Calcium carbonate, [Ca][CO3] is a very common mineral. Limestone is one familiar form of calcium carbonate. Acids in acid rain promote the dissolution of calcium carbonate by reacting with the carbonate anion.

This produces a solution of bicarbonate. Because surface waters are in equilibrium with atmospheric carbon dioxide there is a constant concentration of carbonic acid, H2CO3, in the water.

The presence of limestone and other calcium carbonate rock in lakes and streams helps to maintain a constant pH because the minerals react with the excess acid. However, acid rain eventually can overcome the buffering capacity of the surface water.

Professor Patricia Shapley, University of Illinois, 2011