Geology Reference
In-Depth Information
Box 4.4 Colloids
Colloids consist of ultrafine particles (usually much
smaller than 1 μ m) of one phase dispersed metastably in
another. Most colloids fall into one of three categories:
Because of their enormous surface area, the chemistry of
colloids is dominated by the surface propeties of the col-
loidal particles. They possess a surface charge owing to
the adhesion of ions. When dispersed in a solution of low
ionic strength, interparticle repulsion prevents coagulation
into larger particles. In a strong aqueous solution, how-
ever, a dense ionic atmosphere forms around the parti-
cles, they repel each other less effectively, and they
therefore aggregate into larger, more stable particles. This
process, seen for example when lemon juice is added to
milk, is called flocculation . Much of the silting that occurs
in estuaries is due to flocculation of colloidal clay particles
when the river water in which they are suspended mixes
with seawater.
Mineral and organic matter colloids play an important
part in determining the cation exchange capacity of soils
(Box 8.2).
Sol: Solid particles dispersed in a liquid, like the
clay particles suspended in river water.
Certain types of sol 'set' into a turbid, semi-
solid form called a gel (e.g. gelatin).
Emulsion: One liquid dispersed in another (e.g. milk).
Aerosol:
Liquid or solid particles dispersed in a gas
(e.g. smoke and fog in the troposphere;
desert dust; sulfuric acid/sulfate aerosol in
the stratosphere 7 resulting from a major vol-
canic eruption).
7
Because such aerosols reduce the intensity of solar radiation
reaching the surface, deliberate injection of sulfate aerosol
into the stratosphere has been suggested as a 'geoengineer-
ing' strategy to counteract anthropogenic climate change.
Table 4.3 Principal ionic constituents of seawater
a realistic chemical model of seawater must distin-
guish three ionic species:
Ion
Concentration
(ppm = mg kg −1 )
Molality m i
(10 −3 mol kg −1 )
% free ion
(calc) *
γ i measured
2
+
+
Mg
HCO gHCO
3
3
Cl
19,011
535.5
100
-
as separate chemical entities. Ions can also associate by
forming coordination complexes (Chapters 7 and 9) in
which the cohesive force resembles a covalent bond
rather than an ionic one. Complex formation is particu-
larly prevalent among the transition metals.
The extent of ion pairing in seawater is shown in
Box 4.5.
Na +
10,570
459.6
99
0.70
Mg 2+
1271
53.0
87
0.26
SO 4 2−
2664
27.8
54
0.07
Ca 2+
406
10.2
91
0.20
K +
380
9.7
99
0.60
HCO 3
121
2.0
69
0.55
Br
66
0.8
-
-
CO 3 2−
18
0.3
9
0.02
*See Box 4.4. Percentage for Cl is assumed.
†See Berner (1971), Table 3.6.
pH of seawater: carbonate equilibria and buffering
If we wish to adjust the pH of an aqueous solution in
the laboratory, we may add a small amount of a strong
acid such as hydrochloric acid (HCl) or a strong alkali
such as sodium hydroxide (NaOH). 'Strong' in this
context means that the acid or base is one that is com-
pletely ionized in solution (Appendix B), and therefore
a small addition delivers a large dose of H + or OH
respectively to the solution it is added to.
Sea water, however, is devoid of strong acids and
bases, and its pH is controlled instead by the dissocia-
tion behaviour of weak acids . The most abundant
weak acid in the oceans is carbonic acid, whose partial
be of any significance. In stronger solutions like sea-
water, however, certain ions associate on a more perm-
anent and specific basis. For example, Mg 2+ and
HCO 3 (bicarbonate) ions are abundant enough for a
significant proportion of them to combine to form the
ion pair MgHCO 3 + .
About 19% of the bicarbonate in seawater is thought
to be present in this associated form (Box  4.5). Thus,
instead of considering just two ionic species:
2
+
-
Mg
HCO
3
 
Search WWH ::




Custom Search