Box 35 The pH scale

The acidity of aqueous solutions is frequently described in terms of the pH scale. Acids (Box 3.3) give rise to hydrogen ions (H+) in solution and the pH value of such a solution is defined:

We can write a similar relationship identifying pOH:

However, pH is related to pOH through the equilibrium describing the dissociation of water:

It is important to notice that this is a logarithmic scale, so it is not appropriate to average pH values of solutions (although one can average H+ concentrations).

On the pH scale, 7 is regarded as neutral. This is the point where aH+ = aOH-. There are a number of other important values on the scale (conventionally made to stretch from 0 to 14) that are relevant to the environment.

Lemon juice Coca cola Beer

Vinegar

Milk Neutral

Baking soda

Milk of magnesia

Household bleach

Acid G 1

1G 11 12 1B 14 Basic (alkali)

I I Processing

Acid rain Natural rainfall Seawater wastes with

Acid mine drainage NaOH or Ca(OH)2

Black smokers

Forest Humid Soils with soils arable soils CaCO3

Fig. 1 pH scale showing values for familiar commodities (above the scale) and various environmental fluids discussed in this book (below the scale). Soil pH is measured on pure water (pH 7) equilibrated with the soil solids. Note that naturally alkaline fluids are rare. Industrial processing that involves strong bases like NaOH (e.g. bauxite processing) or Ca(OH)2 (lime production) can contaminate river waters to around pH 10.

Photochemical smog was first noticed in Los Angeles during the Second World War. Initially it was assumed to be similar to the air pollution that had been experienced elsewhere, but conventional smoke abatement techniques failed to lead to any improvement. In the 1950s it became clear that this pollution was different, and the experts were baffled. A. Haagen-Smit, a biochemist studying vegetation damage in the Los Angeles basin, realized that the smog was caused by reactions of automobile exhaust vapours in sunlight.

Although air pollution and smoke have traditionally been closely linked, there were always those who thought there was more to air pollution than just smoke. We can now see how impurities in fuel give rise to further pollutants. In addition, the fact that we burn fuels, not in O2, but in air has important consequences. We have learnt that air is a mixture of O2 and N2. At high temperature, in a flame, molecules in air may fragment, and even the relatively inert N2 molecule can undergo reaction:

Equation 3.23 produces an oxygen atom, which can re-enter equation 3.22. Once an oxygen atom is formed in a flame, it will be regenerated and contribute to a whole chain of reactions that produce NO. If we add these two reactions we get:

The equations show how nitrogen oxides are generated in flames. They arise because we burn fuels in air rather than just in O2. In addition, some fuels contain nitrogen compounds as impurities, so the combustion products of these impurities are a further source of nitrogen oxides (i.e. NOx, the sum of NO and NO2).

Oxidation of nitric oxide in smog gives nitrogen dioxide (Box 3.6), which is a brown gas. This colour means that it absorbs light and is photochemically active and undergoes dissociation:

Equation 3.25 thus reforms the nitric oxide, but also gives an isolated and reactive oxygen atom, which can react to form O3:

Ozone is the single pollutant that most clearly characterizes photochemical smog. However, O3, which we regard as such a problem, is not emitted by automobiles (or any major polluter). It is a secondary pollutant.

The volatile organic compounds released through the use of petroleum fuels serve to aid the conversion of NO to NO2. The reactions are quite complicated, but we can simplify them by using a very simple organic molecule such as CH4, to represent the petroleum vapour from vehicles:

CH4 (g) + 2O2(g) + 2NO(g) —^ H2O(g) + HCHO(g) + 2NO2fe) eqn. 327

We can see two things taking place in this reaction. Firstly, the automobile hydrocarbon is oxidized to an aldehyde (i.e. a molecule with a CHO functional group, see Table 2.1). In the reaction above it is formaldehyde (HCHO). Aldehydes are eye irritants and, at high concentrations, also carcinogens. This equation simply shows the net reactions in photochemical smog. In Box 3.6 the process is given in more detail. In particular, it emphasizes the role of the ubiquitous OH radical in promoting chemical reactions in the atmosphere.

The smog found in the Los Angeles basin (Plate 3.1, facing p. 138) is very different from that we have previously described as typical of coal-burning cities.

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