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by   Peter M May and Darren Rowland
Chemistry, Murdoch University, Murdoch, Australia 6150

and  Kevin Murray, Insight Modelling Services, South Africa

JESS is a powerful research tool for modelling chemical speciation in complex aqueous environments. It is designed to solve problems requiring expert knowledge of solution chemistry. It currently comprises over 400 programs, 3000 subroutines and 340,000 lines of Fortran code. The software is highly portable and has been implemented under WindowsXP, Windows 7, Unix and a number of other operating systems.

In most JESS modelling, the speciation is calculated using known thermodynamic parameters for the relevant reactions in aqueous media. All types of chemical equilibria can be modelled including protonation, complex formation, redox, solubility and adsorption interactions. The data are taken from the JESS databases, transformed into a thermodynamically consistent set of equations and solved. Automation of this procedure, achieved by a sequence of JESS programs, is the key to good, comprehensive equilibrium modelling.

Dealing with the reasons why chemical speciation models from different workers frequently exhibit substantial discrepancies is a special focus of the approach adopted by JESS.

Development of the JESS Thermodynamic database is now well advanced. The system overcomes many problems associated with existing compilations of equilibrium constants. It is fully interactive. Reactions can be expressed in any form. Any number of equilibrium constants, enthalpy, entropy and Gibbs energy values can be associated with a reaction.

Our reaction data span interactions in solution of over 100 metal ions with more than 3,000 ligands. A suite of computer programs exists to facilitate the production of sub-databases and the interchange of data between databases.

The main features of our thermodynamic database are as follows.

* Data are generally stored as closely as possible to the way they appear in the literature and are only subsequently manipulated by our system to achieve thermodynamic consistency. This has the advantage that the data in the database are not converted to conform to a particular pre-determined basis set, unlike most other speciation databases. The JESS Parent Database is by far the largest database that can be used directly by chemical speciation modelling programs.

* Since reactions are treated in an entirely general way, JESS handles many types of chemical interaction that other databases cannot. Gibbs energies and heats for formation reactions of species from their elements in their standard states therefore appear in addition to metal-ligand and ligand-proton binding reactions. Host-guest equilibria (involving anions only, say) can also be included, even though so far not very many of these constants have been entered.

* Unlike all other major compilations of equilibrium constants, ternary complex formation is naturally represented by the same conventions as our binary reactions. The data can thus be located directly by searching on the metal ion and two ligands involved. The JESS Parent Database is, overwhelmingly, the largest single source of ternary equilibrium constants.

* Chemical substances are indexed by name, by molecular formula and, often, by CAS registry number. In general, we have many more variants of chemical names than are provided in alternative sources.

* Every thermodynamic parameter value is individually associated with (i) the conditions under which it was measured, including e.g. the identity of the background electrolyte, the temperature and the ionic strength, (ii) a literature reference and (iii) our current estimation of its reliability, on a scale of 0 - 9. This approach enables the database to become both comprehensive and critical.

* JESS also has a large database for physicochemical properties of electrolytes in aqueous solution. This comprises about 405,000 property values for over 200 electrolytes. Data from the literature are available for activity coefficients, osmotic coefficients, heat capacities and densities/volumes.

Advantages of the approach taken in JESS are described in J. Chem. Eng. Data, 2001, 46, 1035, and Talanta, 2010, 81, 142.

Two facilities for predicting the properties of binary strong electrolyte solutions have been published based on the equations of Pitzer (J. Chem. Eng. Data, 2011, 56, 5066) and Hueckel (J. Chem. Eng. Data, 2014, 59, 2030).

Three papers describing some initial aspects of JESS have been published in Talanta (Talanta, 1991, 38, 1409; Talanta, 1991, 38, 1419 ; Talanta, 1993, 40, 819). Other subsequent papers are also available

Reprints can be sent on request.

Our Primer documents provide a full introduction to JESS and can be downloaded.

JESS is made available to researchers for academic (i.e. non-commercial) purposes.