Page 230 - Materials Chemistry, Second Edition

216 R.K. Rosenbaum et al.

NO 2 þ OH þ M ! HONO 2 þ M

where OH is hydroxyl radical present in the atmosphere and M is an inactive body
which can remove surplus energy.
Ammonia, which is in itself a base (absorbing hydrogen ions via the reaction
þ
NH 3 þ H þ ! NH 4 ), but upon complete mineralisation through nitrite, NO 2 ,to
þ

nitrate, NO 3 releases one net proton:

þ
NH 3 þ 2O 2 ! H þ NO 3 þ H 2 O
Strong acids like hydrochloric acid, HCl or sulphuric acid, H 2 SO 4 , which release
their content of hydrogen ions as soon as they are dissolved in water and thus also
are strongly acidifying.
Because of their high water solubility, the atmospheric residence time of these
acidifying substances is limited to a few days, and therefore acidification is a
regional effect with its extent limited to the region around the point of emission.
When acidifying compounds deposit on plant leaves or needles, they can
damage these vital plant organs and through this damage the plants. When the
acidifying compounds reach the soil, protons are released in the soil where they
may lower the pH of the soil water and cause release of metal ions bound in the soil.
Some of these metals are toxic to the plants in the soil, others are essential for plant
growth, but after their release, they wash out, and the availability of these metals to
plants may then become limiting for plant growth. The result is stress on the plants
through root and leaf damage and after prolonged exposure the plants may die as a
direct consequence of this or through diseases or parasites that benefit from the
weakened constitution of the plant. Lakes are also exposed to the acidification, in
particular through the acidified soil water leaching to the lake. When the pH of a
lake drops, the availability of carbon in the water in its dominating form around

neutral pH, which is HCO 3 , is converted to dissolved CO 2 . The solubility of toxic
metals is increased, in particular aluminium which may precipitate on the gills of
fish at pH 5. The phytoplankton and macrophyte flora gradually change and also the
fauna is affected. Humic acids that give the lakewater a brown colour are precip-
itated, and the acidified lakes appear clear and blue.
The sensitivity to acidification is strongly influenced by the geology and nature
of the soil. Calcareous soils with a high content of calcium carbonate are well
buffered meaning that they will resist the change in pH by neutralising the input of
hydrogen ions with the basic carbonate ions:

þ
H þ CaCO 3 ! Ca 2 þ þ HCO 3
þ
H þ HCO 3 ! H 2 O þ CO 2
As long as there is calcium carbonate in the soil, it will thus not be acidified.