Why is the pH Important? - Examples
Remember: The pH scale is used to specify the acidity or basicity of a solution.
Example 1: Water pH
Aquatic life is sensitive to extremes of pH, i.e. strong acidity or basicity. The generalized range of pH in which organisms live in shown in the following diagram (fig. 1).
Figure 1: A generalized plot of the growth of an aquatic organism as a function of pH. [S.E. Manahan, Environmental Chemistry, CRC Press LLC, 8th edition, 2005.]
Some industrial discharges have the potential to contribute strong acids to water, making it more acidic. One source of acid pollution is acid mine drainage (fig. 2) which is discharged from coal mines and metal mines and contains high concentrations of sulfuric acid. The pH values encountered in acid-polluted water may fall below pH 3, a condition deadly to most forms of aquatic life.
Figure 2: Acid mine drainage in the Rio Tinto River, Spain. [This picture is public domain. https://en.wikipedia.org/wiki/File:Rio_tinto_river_CarolStoker_NASA_Ames_Research_Center.jpg]
Another example of acid pollution is acid rain which you have already encountered in the previous unit. Unpolluted rain has a pH of about 5.6, which means it is acidic. (The reason for the natural acidity of rain have been explained in the previous unit "8 Acids and Bases".) Rain with a pH below 5 is considered acidic.
Example 2: Soil pH
The pH of a soil has numerous effects on plant growth and crop yields, and it is therefore considered to be one of the most important chemical soil parameters. For example, it affects the microbial activity in soils, and thereby the speed at which organic matter is decomposed. It also affects the solubility of different substances, and thus their mobility.
Al and Fe, which are both toxic to plants, become dissolved in the soil solution at low pH values. At higher pH they are fixed in the soil’s solid matter. Al is released at pH <5, Fe is released at pH <3. Soil can also contain heavy metals that are toxic to humans, for example Pb or Cd. Pb becomes soluble at low pH (ca. pH <4.5) as well as at high pH (pH >7). When those toxic heavy metals are dissolved, they may be taken up by plants and end up in our food or they can be flushed into the groundwater, which might be used as drinking water. Nutrients in the soil, which are essential to plants, can be fixated depending on the pH. For example, phosphorous becomes less plant available above pH 6.
The acidification of soil can be caused by acid rain, which you have learned about in the previous unit "8 Acids and Bases".
H.-P. Blume et al., 2016. Scheffer/Schachtschabel – Soil Science, pp. 163, p. 469 and pp. 491. Springer-Verlag, Berlin/Heidelberg.