Some acids are weak, e.g acetic acid (vinegar) and lemon juice. They are not harmful and are used in preparing our food. Othershowever, such as sulphuric acid (battery acid) are strong and can burn holes in our clothes.
If pure water is exposed to the air it absorbs carbon dioxide to form carbonic acid and becomes slightly acidic, dropping from pH 7 i.e. neutral, to pH 5,6. Even in remote, unpopulated areas rain can reach a pH of 4,5. However, a pH of less than 4,5 in rain is almost certainly caused by pollution.
Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NOX form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometres. Thus acid rain can fall far from the source of pollution.
When mist or fog droplets condense they will remove pollutants from the air and can become more strongly acid than acid rain. Even snow can be acid. Gases and particles, not dissolved in water, with a low pH can also be deposited directly onto soil, grass and leaves. It is possible that even more acidity is deposited in this way than by rain! Not much is known about this process, and it is particularly difficult to study.
* Acid rain can increase the acidity of soil, water and shallow groundwater.
* Acid rain has been linked with the death of trees in Europe and North America. In spite of a great deal of research, no one yet knows exactly how acid rain harms forests. Most of the forests of Europe consist of huge areas of one tree species. This encourages the spread of plant pests and diseases. It seems likely that acid rain weakens the trees, perhaps helped by other pollutants such as ozone, and then leaves the trees open to attack by disease. Acid rain also disrupts the availability of soil nutrients. The final death of a tree may result from a combination of stresses such as heat, cold, drought, nutrient disruption and disease. It seems that the slow-growing, longer lived forests of the North may be more susceptible than the faster growing, shorter lived forests of South Africa.
* Acid rain erodes buildings and monuments. Acid particles in the air are suspected of contributing to respiratory problems in people.
* Place the buckets at least 2 m above a lawn to reduce dust contamination. Soil dust will usually tend to be alkaline and neutralize acid.
* The pH of the rain can be measured with a carefully calibrated pH meter. Indicators such as litmus paper or a swimming pool testing kit can be used, but they will be less accurate than a pH meter.
* Would it be better for South Africa to provide electricity for all its people, rather than spend money controlling the power station emissions that cause acid rain?
Eskom: Communications Department. PO Box 1091, Johannesburg, 2000. Tel. 011-8002499
CSIR. P.O. Box 395, Pretoria, 0001. Ematek Tel. 012 841 4111, Forestek Tel. 012-841 2674.