Natural Acidity of Rainwater

Pure water has a pH of 7.0 (neutral); however, natural, unpolluted rainwater actually has a pH of about 5.6 (acidic).[Recall from Experiment 1 that pH is a measure of the hydrogen ion (H⁺) concentration.] The acidity of rainwater comes from the natural presence of three substances (CO₂, NO, and SO₂) found in the troposphere (the lowest layer of the atmosphere). As is seen in Table I, carbon dioxide (CO₂) is present in the greatest concentration and therefore contributes the most to the natural acidity of rainwater.

Gas	Natural Sources	Concentration
Carbon dioxide CO2	Decomposition	355 ppm
Nitric oxide NO	Electric discharge	0.01 ppm
Sulfur dioxide SO2	Volcanic gases	0-0.01 ppm
Table 1 Carbon dioxide, produced in the decomposition of organic material, is the primary source of acidity in unpolluted rainwater. NOTE: Parts per million (ppm) is a common concentration measure used in environmental chemistry. The formula for ppm is given by: