Box 33 Acids and bases

Acids and bases are an important class of chemical compounds, because they exert special control over reactions in water. Traditionally acids have been seen as compounds that dissociate to yield hydrogen ions in water:

HCl(a

The definition of an acid has, however, been extended to cover a wider range of substances by considering electron transfer. For example, boric acid (H3BO3), which helps control the acidity* of seawater, gains electrons from the hydroxide (OH-) ion:

For most applications the simple definition is sufficient, and we might think of bases (or alkalis) as those substances which yield OH-in aqueous solution.

Acids and bases react to neutralize each other, producing a dissolved salt plus water.

Two classes of acids and bases are recognized—strong and weak. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) (eqns 1 and 4) are treated as if they dissociate completely in solution to form ions, so they are termed 'strong'. Weak acids and bases dissociate only partly.

NH„OH(aq) - NHJ(aq) + OH-ammonium') hydroxide eqn. 5 eqn. 6

Dissociation is an equilibrium process and is conveniently described in terms of equilibrium constants for the acid (Ka) and alkaline (Kb) dissociation:

a aHCOOH M

b aNH4OH M

*The acidity of the oceans is usually defined by its pH, which is discussed in Box 3.5.

is released in a single event than for many years previously. The eruptions occur in very specific locations where there are active volcanoes. In addition to massive eruptions that push great quantities of material into the upper parts of the atmosphere, we must not neglect smaller fumarolic emissions, from volcanic cracks and fissures, which gently release gases to the lower atmosphere over very long periods of time. The balance between these two volcanic sources is not accurately known, although for SO2 it is probably about 50: 50.

Radioactive elements in rocks (see Section 2.8), most importantly potassium (K) and heavy elements such as radium (Ra), uranium (U) and thorium (Th), can release gases. Argon (Ar) arises from potassium decay and radon (Rn, a radioactive gas that has a half-life of 3.8 days) from radium decay. The uranium-thorium decay series results in the production of a particles, which are helium nuclei. Once these nuclei capture electrons, helium has effectively been added to the atmosphere.

Helium has not accumulated in the atmosphere over time because it is light enough to escape into space. The concentration of helium has been thus maintained in steady state through a balance of radioactive emanation from the crust and loss from the top of the atmosphere.

The Atmosphere 41 Table 3.2 Sources for particulate material in the atmosphere. From Brimblecombe (1986).

Source

Global flux (Tg yr1)

Forest fires

35

Dust

750

Sea salt

1500

Volcanic dust

50

Meteoritic dust

1

0 0

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