Geology Reference
In-Depth Information
and Na
1
are weakly hydrated cations, while Ca
21
and Mg
21
are
strongly hydrated). Ions with a medium ionic potential (10-20) tend
to hydrolyze and precipitate as oxides and hydroxides (e.g. Fe
III
,Al
III
,
Cr
III
). Those with a high-ionic potential (420) form soluble oxyanions
(e.g. phosphate, nitrate, sulfate).
The chemical speciation of the ions is highly dependent on pH and redox
potential. Phosphorus, for example, exists as orthophosphate anions:
H
2
PO
4
þ
H
1
H
3
PO
4
"
pK 2.15
(5.26)
H
2
PO
4
HPO
2
4
þ
H
1
"
pK 7.20
(5.27)
HPO
2
4
PO
3
4
þ
H
1
"
pK 12.35
(5.28)
Over the pH range of most soils, the dominant orthophosphate anion in
soil solution will be H
2
PO
4
(Figure 12). Only in alkaline soils will
HPO
4
2
be significant, and PO
4
3
ions are not important in soil
solution.
Interactions between ions in solution can be considered in two ways.
Long-range interactions, those 40.5 nm, are accounted for by the
Figure 12 A distribution diagram showing the percentage composition of phosphoric acid
specified as a function of pH
(Reproduced from Harrison, R.M. et al., Introductory Chemistry for the
Environmental Sciences, Cambridge University Press, Cambridge, 1991.)
