Reflective equilibrium

In the seawater example (Table 6.6), the saturation indices are inflated somewhat by the choice of a rather alkaline pH, reflecting equilibrium with atmospheric CO2. If we had chosen a more acidic pH within the range observed in seawater, the indices would be smaller. The choice of large formula units for the phyllosilicate minerals, as discussed later in this section, also serves to inflate the saturation indices reported for these minerals. 

Note that simply reflects the contrast in isotopic compositions between two substances and, in terms of physical processes, could reflect equilibrium or non-equilibrium partitioning of isotopes. For an isotope exchange reaction in which one atom is exchanged, is equal to the equilibrium constant. Because a,. is very close to unity, generally on the order of 1. OOX, we may take advantage of the relation that 10 1n( 1. OQX) X, which provides the useful relation ... 

We will start our discussion by considering a special case, that is, the situation in which the molecules of a pure compound (gas, liquid, or solid) are partitioned so that its concentration reflects equilibrium between the pure material and aqueous solution. In this case, we refer to the equilibrium concentration (or the saturation concentration) in the aqueous phase as the water solubility or the aqueous solubility of the compound. This concentration will be denoted as Qf. This compound property, which has been determined experimentally for many compounds, tells us the maximum concentration of a given chemical that can be dissolved in pure water at a given temperature. In Section 5.2, we will discuss how the aqueous activity coefficient at saturation, y, , is related to aqueous solubility. We will also examine when we can use yf as the activity coefficient of a compound in diluted aqueous solution, y (which represents a more relevant situation in the environment). 

To a higher or lesser degree - depending on the experimental technique - ion initial energy distributions do not reflect equilibrium distributions and attempts to fit them to a Maxwellian distribution leads to temperatures that are too high to be realistic. 

Failure of a given water sample to reflect equilibrium with the air with which it was last in contact can be ascribed to a variety of causes. An obvious and important case is nonconservation (e.g., addition of nonatmospheric gases see Sections 4.5 and 4.6). Equally obvious is the simple lack of equilibration for kinetic reasons (e.g., rapid isolation of glacial melt), which would be difficult to treat quantitatively. This evidently is not too severe a problem, however, since equilibration times are apparently fairly short (e.g., a day or so for the top few meters of sea water cf. Broecker, 1974) in any case, we are unaware of any saturation anomalies ascribed to kinetic equilibration failure. However, a number of additional effects are likely to be significant and can be described quantitatively. These are discussed here and illustrated in Table 4.5. 

A study of kinetic acidities (which reflect equilibrium acidities) has shown that the anions from cyclic sulphones of type (22) and (23) are relatively much more stable than those derived from open-chain analogues. 

Empirical data describing the extent of chemical uptake by plants roots are generally expressed as ratios of chemical concentrations in the plant compartment of interest (e.g., shoots, roots, xylem sap) to that in the exposure medium (soil, soil pore water, hydroponic solution) measured at the time the samples are collected. These ratios are generally referred to as bioconcentration factors (BCFs) but they may or may not reflect equilibrium conditions. Plant BCF values are widely used to provide direct and approximate estimates of plant tissue concentrations from measured exposure... 

---