Geoscience Reference
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In the case considered, it is observed that Ca 2+ and Mg 2+ precipitate
very early [in the form of CaMg(CO 3 ) 2 and CaCO 3 ], while Cl - remains
totally in solution and can even be a concentration index, chlorides being
very soluble. It should be noted that the conductivity increases much
more slowly than the concentration since ions are withdrawn from the
water by precipitation of salts. It is still more complicated in the soil in
situ because it is difficult to estimate the irregularity in the distribution
of salts and irregularity in the level of water applied during furrow
irrigation. Consequently, modelling, even the best, cannot substitute for
field measurements. It supplements them.
13.4.5 Associated Concepts
In this context, we have been led to define alkalinity of a solution. The
concept was established in USA in 1950. It is the concentration of weak
bases HCO 3
Alkalinity
- , CO 3 2 - and OH - , but from this it is appropriate to deduct the
fraction already neutralized by the H + present (Bourrié 1976):
Alk = [ HCO 3 - ] + 2 [ CO 3 2 - ] + [ OH - ] - [ H + ]
The concentrations are moles/l or meq/l. When the solution is
close to neutrality, the last two terms cancel each another and can be
eliminated. But sometimes we seek to be more precise by considering
ions such as , which is a carbonate ion camouflaged by its association
with a calcium ion. Likewise, we can count undissociated that is capable,
on dissociation, of neutralizing two protons.
The complete equation for alkalinity, then, is as follows (Ribolzi et al .
1993; G. Bourrié, pers. comm.):
Alk = [ HCO 3 - ] + 2 [ CO 3 2 - ] +[ OH - ] - [ OH - ]+2[ CaCO 3 0 ]+ [ CaHCO 3 + ] + º
in which the ellipsis denotes the compounds of Na, K and Mg similar
to those given for calcium. Fortunately, it is possible to arrive at a simpler
expression by considering electroneutrality and by limiting ourselves to
the principal ions likely to be involved (Fig. 13.13).
x
NH 4 +
NO -
Mg 2+
Ca 2+
Na +
K +
Cl -
C
O 2 -
,
etc.
SO 4 2-
HCO 3 -
y
Alk
Fig. 13.13 Simplifi ed defi nition of alkalinity taking into account the principal anions and
cations.
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