Metal-organic complexes in geochemical processes: Temperature dependence of the standard thermodynamic properties of aqueous complexes between metal cations and dicarboxylate ligands
P. Prapaipong et al., Metal-organic complexes in geochemical processes: Temperature dependence of the standard thermodynamic properties of aqueous complexes between metal cations and dicarboxylate ligands, GEOCH COS A, 63(17), 1999, pp. 2547-2577
By combining results from regression and correlation methods, standard stat
e thermodynamic properties for aqueous complexes between metal cations and
divalent organic acid ligands (oxalate, malonate, succinate, glutarate, and
adipate) are evaluated and applied to geochemical processes. Regression of
experimental standard-state equilibrium constants with the revised Helgeso
n-Kirkham-Flowers (HKF) equation of state yields standard partial molal ent
ropies ((S) over bar degrees) of aqueous metal-organic complexes, which all
ow determination of thermodynamic properties of the complexes at elevated t
emperatures. In cases where (S) over bar degrees is not available from eith
er regression or calorimetric measurement, the values of (S) over bar degre
es can be estimated from a linear correlation between standard partial mola
l entropies of association (Delta (S) over bar(r)degrees) and standard part
ial molal entropies of aqueous cations ((S) over bar(M)degrees). The correl
ation is independent of cation charge, which makes it possible to predict (
S) over bar degrees for complexes between divalent organic acids and numero
us metal cations. Similarly, correlations between standard Gibbs free energ
ies of association of metal-organic complexes (Delta (G) over bar(r)degrees
) and Gibbs free energies of formation (Delta (G) over bar(f)degrees) for d
ivalent metal cations allow estimates of standard-state equilibrium constan
ts where experimental data are not available. These correlations are found
to be a function of ligand structure and cation charge. Predicted equilibri
um constants for dicarboxylate complexes of numerous cations were included
with those for inorganic and other organic complexes to study the effects o
f dicarboxylate complexes on the speciation of metals and organic acids in
oil-field brines. Relatively low concentrations of oxalic and malonic acids
affect the speciation of cations more than similar concentrations of succi
nic, glutaric, and adipic acids. However, the extent to which metal-dicarbo
xylate complexes contribute to the speciation of dissolved metals depends o
n the type of dicarboxylic acid ligand; relative concentration of inorganic
, mono-, and dicarboxylate ligands; and the type of metal cation. As an exa
mple, in the same solution, dicarboxylic acids have a greater influence on
the speciation of Fe+2 and Mg+2 than on the speciation of Zn+2 and Mn+2. Co
pyright (C) 1999 Elsevier Science Ltd.