Thermodynamic and electrochemical availability

Whitfield, M. and Turner, D.R. (1979). Critical assessment of the relationship between biological thermodynamic and electrochemical availability. In Chemical Modeling in Aqueous Systems, ed. Jenne, E. A., ACS Symposium Series, Vol. 93, pp. 657-680. 

Critical Assessment of the Relationship between Biological Thermodynamic and Electrochemical Availability... 

Thus, electrochemical data involving both thermodynamic and kinetic parameters of hydrocarbons are available for only olefinic and aromatic jr-systems. The reduction of aromatics, in particular, had already attracted much interest in the late fifties and early sixties. The correlation between the reduction potentials and molecular-orbital (MO) energies of a series of aromatic hydrocarbons was one of the first successful applications of the Hiickel molecular orbital (HMO) theory, and allowed the development of a coherent picture of cathodic reduction [1], The early research on this subject has been reviewed several times [2-4],... 

Hydrogen and carbonmonoxide are fuels which must be made at thermodynamic and economic cost. A principal industrial route is via the fired steam reform of natural gas, a highly irreversible process. The related thermodynamically reversible route to methane reform, and electrochemical oxidation, Figure A.3, is examined. An electrically driven electrochemical reformer at standard conditions is the model. The reformer supplies a pair of fuel cells separately utilising carbon monoxide and hydrogen. The thermodynamic data confirm that there is plenty of electricity available... 

For a particular diffusion layer thickness S the thermodynamic availability, as measured with an ISE or calculated from a speciation model, and the electrochemical availability, as measured by ASV, represent limiting cases of a continuum of trace metal availability. The nature of this continuum is most simply defined by considering the flux of the free metal ion across the diffusion layer to a surface which senses the metal availability. The ratio of the observed flux (J) to the limiting flux (J] ) is unity for ASV measurements under current limiting conditions and zero for ISE measurements. 

Several imperfections remain, both in our understanding of the chemistry of trace metals in natural waters and in the sophistication of our experimental techniques, that prevent an exact determination of the thermodynamically available fraction ( oC pb , equation 4) and the electrochemically available fraction (Ij /I j, equation 11). The stability constants used in calculating the individual o"-values (equation 2) are subject to considerable uncertainty ( 2, 21, 42) and the conventional fj -values used in their adjustment to sea water conditions are based on a multiplicity of conventions. For many complexes that may be important in natural samples the stability constants are unknown and, frequently, the ligands have not been identified. 

The recent extension of these thermodynamic models to include the kinetics and mechanisms of organo-metallic interactions has made it possible (1) to quantify the electrochemical availability of these metal complexes to voltammetric systems (Whitfield and Turner, 1980) (2) to examine diffusion and dissociation models for the tremsport of chelated iron to biological cells (Jackson and Morgan, 1978) and (3) to estimate the significance of adsorptive and convective removal processes on the equilibrium specia-tion of metals in natural waters (Lehrman and Childs, 1973). Thus both equOibrium and dynamic models have become an indispensable tool in the identification of the important chemical forms and critical reaction pathways of interactive elements in aquatic environments. 

A simplified representation of the energetic situation where a stable intermediate may form is depicted in Fig. 3.12. In both cases, the reaction A B exhibits the same overall standard free energy of reaction. However, if a stable intermediate (1) forms, the second step of the reaction 1 B is endergonic and therefore thermodynamically unfavorable. As a result, the reaction stops at I and the electrochemical data are correlated with the transition of A 1 rather than A B. Such processes can occur, if there is an element in the reaction mixture, which forms very stable compounds with one of the elements of the electrode material so that, at least gradually, A —> B becomes less and less available and the system degrades or alters its properties. The capacity may fade, if reaction A 1 is electrochemically not reversible. 

In this book we offer a coherent presentation of thermodynamics far from, and near to, equilibrium. We establish a thermodynamics of irreversible processes far from and near to equilibrium, including chemical reactions, transport properties, energy transfer processes and electrochemical systems. The focus is on processes proceeding to, and in non-equilibrium stationary states in systems with multiple stationary states and in issues of relative stability of multiple stationary states. We seek and find state functions, dependent on the irreversible processes, with simple physical interpretations and present methods for their measurements that yield the work available from these processes. The emphasis is on the development of a theory based on variables that can be measured in experiments to test the theory. The state functions of the theory become identical to the well-known state functions of equilibrium thermodynamics when the processes approach the equilibrium state. The range of interest is put in the form of a series of questions at the end of this chapter. 

---