Environmental Engineering Reference
In-Depth Information
Fig. 7.3
Pink color
developing in solutions and in saturated sand to which alpha, alpha dipyridyl
dye has been added. Numbers represent concentrations of Fe
2+
(
μ
g/ml). In the
top figure
(a), 0.5 ml
αα
d dye was added to 6 ml of solution. In the
lower figure
(b), one drop of
αα
d dye was added to
the saturated sand
the soil solution (Childs
1981
). Except for some very unusual acid sulfate soils
where oxidation of pyrite can (at least temporarily) form Fe
2+
under oxidizing
conditions, the presence of in soils usually indicates that the soil has been saturated
and sufficiently reducing to cause Fe
3+
oxides to be reduced to Fe
2+
(Fig.
7.2
).
When alpha-alpha dipyridyl dye is applied to soils containing ferrous iron, a pink
color develops, the intensity of which is function of how much Fe
2+
is in solution.
In clear solutions, as little as 0.5
g/ml Fe
2+
can be detected using the dye
(Fig.
7.3a
). However, when applied to soils, usually a minimum of 3-5
μ
g/ml is
required for the pink color to be detected against the background soil color, and may
require even higher concentrations to be seen if the soil is dark colored (Fig.
7.3b
).
To test a soil using the dye, a core must usually be extracted and the dye is then
applied to an interior broken face of the core. Care should be taken not to apply
the dye to portions of the soil that have been in direct contact with a steel auger or
spade as reduced iron from the tools has sometimes give a false positive reaction to
the dye. Another way that the dye can be used is to extract some fresh soil solution
from a saturated soil using a piezometer or suction lysimeter and adding a few drops
of the alpha-alpha dipyridyl dye to a few ml of the extracted solution.
μ
