An Introduction to Environmental Chemistry

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 atmos-

phere, 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 SO 2 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 radioac-

tive gas that has a half-life of 3.8 days) from radium decay. The uranium–thorium

decay series results in the production of aparticles, 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 main-

tained in steady state through a balance of radioactive emanation from the crust

and loss from the top of the atmosphere.

40 Chapter Three

Box 3.3 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:

eqn. 1

The definition of an acid has, however, been

extended to cover a wider range of

substances by considering electron transfer.

For example, boric acid (H 3 BO 3 ), which helps

control the acidity* of seawater, gains

electrons from the hydroxide (OH-) ion:

eqn. 2

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.

eqn. 3

Acids and bases react to neutralize each

other, producing a dissolved salt plus water.

HCl()aq+++NaOH()aqªCl()-aq Na()+aq H O 2 ()l eqn. 4

NaOH()aqªNa()+aq+OH()-aq

HBO 33 ()aq+OH()-aqªBOH( ) 4 - ()aq

HCl()aqªH()+aq+Cl()-aq

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.

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:

eqn. 7

eqn. 8

* The acidity of the oceans is usually defined by its pH,

which is discussed in Box 3.5.

Kb=aaNHaNH OH◊OH =¥mol l

+- (^4) --
4
180 10.^51
Ka=aaHaHCOOH◊HCOO =¥mol l
+- --
177 10.^41
NH OH NH OH
ammonium
hydroxide
4 ()aq 4 +()aq+ ()-aq
Ê
ËÁ
ˆ
̄ ̃
ª
HCOOH H HCOO
formic acid
()aq ()+aq+ ()-aq
( )
ª