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culations. Among those involved in proposing reference environments, all have
arrived at the consensus of dividing the reference substances (R.S.) that compose
the R.E., into the gaseous components of the atmospheric air, solid components of
the lithosphere and molecular components of seawater. It is this conception of a
R.E. which is close to the authors' degraded Earth, seeing as it is also composed by
an atmosphere, hydrosphere and upper crust. However and whereas the theoreti-
cal degraded Earth sought in this topic, should be composed of all the substances
appearing currently on Earth, a typical R.E. includes 85 substances with only one
chemical substance assigned per chemical element
14
. Furthermore, the exact selec-
tion of the R.S. depends on the criteria of each individual author.
In this way, a group of authors derive the chemical exergy of the elements from
the hypothetical chemical equilibrium that could be attained on Earth in a very
distant future. Ahrendts (1977, 1980) and Diederichsen (1999) for example, stated
that if the amount of different elements in the reference system is known and the
temperature is fixed, the quantity of each chemical compound and the value of each
chemical potential is uniquely determined by the condition of chemical equilibrium.
Ahrendt's R.E. relied on the model of Ronov and Yaroshevsky (1969) to ascertain
15 elements, making up more than 99% of the Earth's crust: H, C, N, O, Na,
Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, Mn and Fe. Diederichsen (1999) updated
and extended Ahrendt's model with new geochemical data and obtained among
others, a R.E. including 75 elements. These elements were allowed to react until
chemical equilibrium was attained. The composition of this environment in chemical
equilibrium, had as a variable parameter the thickness of the crust layer (between
= 1 m and = 1000 m). The resulting equilibrium reference system based on a
slice of the Earth's crust ( = 1000 m), showed that the exergy of oxygen was even
smaller than that of fuels due to the formation of nitrates. He found that the exergy
of oxygen increased, when the considered thickness of the crust was smaller
15
.
However, as seen in the composition of air, atmospheric oxygen and nitrogen can
exist together during millions of years, without leading to the formation of nitrates.
Ahrendts' R.E. is thus a state very removed from current reality and would require
a very long temporal period in order to reach it. Nor is it very appropriate for the
analysis presented here, which is focused on activities produced at human temporal
scales.
In this respect, another relevant author Szargut (1957, 1989), proposed a R.E.
closer to the natural one in which among a group of reasonably abundant sub-
stances, the most stable will be selected should they also fulfill the “Earth similarity
criterion”. That is, if the stability of several potential reference substances for a spe-
cific element (measured in terms of the formation Gibbs energy) is within a certain
threshold, then the most abundant reference substance will be chosen. If however
14
For instance, the reference substance associated with aluminum in Szargut (1989) is Al
2
O
3
and
there is no other containing Al.
15
In order to overcome this paradox, Ahrendts considered a crust layer of only 1 m thick.
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