Figure 4. Pourbaix diagram for P4 in water showing that
P4 is not thermodynamically stable in water. The region
bordered by the heavy lines is the stability field of water. No
substance outside this field is stable in the presence of water
and atmospheric oxygen. (After Pourbaix 1966.)
well below the domain of the stability of water
culate the time for a spherical particle with an
(Pourbaix 1966); that is, it can theoretically re-
initial diameter of 1 mm (surface area of 0.0314
cm2 and a mass of 953 g) to dissolve from
duce water with the evolution of hydrogen.
However, this diagram is for the state of thermo-
1.5 g
dM
=
4πr 2 .
(2)
1.25 cm ⋅ hr
2
dt
at which equilibrium is achieved. Several factors
must be taken into account to assess the persis-
Since
tence of a chunk of solid P4 introduced into water
or saturated sediments.
4
dM = ρdV = ρ πdr 3 = ρ4πr 2dr
(3)
Spanggord et al. (1983) reviewed the proper-
3
ties and processes associated with the environ-
then dM in eq 2 may be replaced:
mental fate of P4. These included aqueous solu-
bility, vapor pressure, sorption partition coeffi-
1.5 g
ρ4πr 2dr =
4πr 2dt
cient, octanolwater partition coefficient, leach-
(4)
2
1.25 cm ⋅ hr
ability, Henry's constant and rates of dissolution,
hydrolysis, oxidation, reduction and biotransfor-
mation.
t=t
r=r
1.5 g
∫ dr = 1.25 cm2 ⋅ hr (1/ρ) ∫ dt
(5)
Dissolution and aqueous solubility
t=0
r=r0
White phosphorus is nonpolar, so it does not
dissolve readily in water. Spanggord et al. (1985)
t=t
r=r
1.5 g
∫ dr = 1.25 cm2 ⋅ hr (1/ρ) ∫ dt .
measured the dissolution rate of a piece of P4 that
(6)
had a surface area of 1.25 cm2 exposed to 25C
t=0
r=r0
oxygen-saturated and nitrogen-purged water
flowing at 0.143.00 mL/min. Dissolution rates
Based on the amount of time for the radius to
ranged from 1.5 g/hr for the slowest flow rate
go to zero, a P4 particle with an initial diameter of
to 6.7 g/hr for the highest flow rate. There was
1 mm would take about 8 years to dissolve at a
constant temperature of 25C (Fig. 5) in slowly
no difference in dissolution rate for white phos-
phorus exposed to oxygen-saturated or nitrogen-
flowing water.
purged water.
Despite the slow dissolution rate, there is am-
Using these data and assuming that the rate of
ple evidence that water becomes contaminated
dissolution per unit area is constant, we can cal-
by contact with P4. In industrial settings, P4 is fre-
5
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