Database internal consistency

Analysts should not rely on databases developed by others unless citations and regression resiilts are available. Many improper conclusions have been drawn when analysts have relied upon the databases supplied with commercial simulators. While they may be accurate in the temperature, pressure, or composition range upon which they were developed, there is no guarantee that they are accurate for the unit conditions in question. Pure component and mixture correlations should be developed for the conditions experienced in the plant. The set of database parameters must be internally consistent (e.g., mixture-phase equilibria parameters based on the pure-component vapor pressures that will be used in the analysis). This ensures a consistent set of database parameters. 

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

In this chapter, we discuss double layer theory and how it can be incorporated into a geochemical model. We will consider hydrous ferric oxide (FeOOH //IFO), which is one of the most important sorbing minerals at low temperature under oxidizing conditions. Sorption by hydrous ferric oxide has been widely studied and Dzombak and Morel (1990) have compiled an internally consistent database of its complexation reactions. The model we develop, however, is general and can be applied equally well to surface complexation with other metal oxides for which a reaction database is available. 

A database of thermodynamically assessed systems, containing critically checked and internally consistent thermochemical data for the phases taking part in the equilibrium. 

Some insight into this issue may be obtained by combining the experimentally determined phase equilibria with internally consistent thermodynamic databases, which allow interpolation and extrapolation of the extant data. In a series of papers, Kerrick and Connolly (1998, 2001a,b) calculated phase equihbria and modal mineralogies... 

Runde et al. (2002a) compiled an internally consistent database to calculate solubility and speciation of plutonium in more complex low-ionic-strength waters. A specific interaction model (Grenthe et al., 1992) was used for ionic strength corrections. The reader is referred to that work for... 

Information that is put into this Database is derived from published reports of crystal structure determinations. The data extracted from the scientific literature in this way include the atomic coordinates, information on the space group, chemical connectivity, and the literature reference to each structure determination. Each compound listed in the Database is identified by a six-letter code (the REFCODE), unique to each crystal structure determination. Duplicate structures and remeasurements of the same crystal structure are identified by an additional two digits after the REFCODE. Scientific journals are scanned regularly by the Database staff for reports of crystal structure determinations, and the data are then entered into this Database. Structural data are also deposited by journals, for example. Chemical Communications, that publish articles, but do not have space for atomic coordinates. All crystallographic data reported in the literature are tested by the Database staff for internal consistency, precision, and chemical reasonableness. In... 

None of the thermodynamic databases accompanying the modeling codes are comprehensive compilations of all the aqueous and mineral species we may encounter. Neither has it been ensured that the thermodynamic data are internally consistent. Code developers and releasing agencies make no statement about what are the best thermodynamic properties of a solid or aqueous species or about the internal consistency of the data they assume this to be the responsibility of the modeler. 

Therefore, it may be that a water analysis which is error free can still have a large speciated charge imbalance if the database is of poor quality, i.e., if the equilibrium constants are not accurate or not internally consistent. 

The first stop for the modeler is to take a good look at the database that accompanies the code or codes. The modeler needs to examine what thermodynamic and kinetic parameters are included in the database. The database may or may not contain the thermodynamic and kinetic properties for the chemical species we want to include in the model, and the listed properties may not be internally consistent. See Chapter 4 for more details on the thermodynamic data and databases. 

Numerous efforts are currently underway to provide internally consistent thermodynamic data, such as the one presented in this volume by Nordstrom and others. Once attained, even an internally consistent database is still heavily dependent upon extrapolation techniques to provide the needed constants where none exist. The problem remains that the user is not provided with an indication of the magnitude of the error expected from this collection of thermodynamic data. A fundamental question still exists, however can a truly internally consistent data base ever be assembled that addresses a wide range of geochemical problems It is more likely that a database will evolve that is less than consistent, but contains known and evaluated error. 

Therefore, a prototype of a digitized version of a thermodynamic sorption database has been implemented as a relational database with MS Access RES T -Rossendorf Expert System for Surface and Sorption Thermodynamics . It is mineral-specific and can therefore also be used for additive models of more complex solid phases such as rocks or soils. An integrated user interface helps users to access selected mineral and sorption data, to extract internally consistent data sets for sorption modelling, and to export them into formats suitable for other modelling software. 

The choice of a given database as source of auxiliary values may not be straightforward, even for a thermochemist. A database should not be a mere collection of values quoted from the literature. It usually involves some critical assessment of those values and also an effort to present a consistent set of data. In a consistent database, all the standard enthalpies of formation have been recalculated from the original values reported for the reaction enthalpies, on the basis of a single set of auxiliary data which, itself, is internally consistent (and which should also be listed). 

Cha] Chatteijee, N., Kmger, R., Haller, G., Olbricht, W., The Bayesian Approach to an Internally Consistent Thermodynamic Database Theory, Database, and Generation of Phase Diagram ,... 

If several datasets have to be regressed simultaneously, it is recommended to fit them first separately to detect outliers and check their internal thermodynamic consistency. If a dataset cannot be regressed with a sufficient quality, it should be removed from the database, as it might spoil the fit of the other datasets. Afterward, it can still happen that different internally consistent datasets do not fit together. In this case, a clear judgment of the situation is not possible. To obtain at least a solution, one could try to find combinations of datasets which fit together to enhance the probability of the chosen solution. Furthermore, there should be a tendency to prefer recently evaluated data to older data due to the improvement of the measurement techniques. 

The available experimental data for binary and ternary systems can be used as primary reference data, or as the initial values for further refinement, in order to obtain recommended values, particularly, the internally consistent values that are used for thermodynamic calculations and modelling of multicomponent equilibria and reactions. However, the recommended values are derivatives and largely depend on the method of treatment based on more or less rigorous and varying models. Thus, a collection of experimental data not only incorporates original information from widely scattered scientific pubhcations, it is fundamental and provides the foundation for modem and future databases, and recommended values. 

Sublimation enthalpies. The NIST database [5] has been scanned for each compound. When several AHsubi data were present, which was usually the case for the compounds considered (Section 7.6), the choice was made on the basis of (2a) internal consistency when similar values had been obtained by different workers, the average value was assumed (2b) chemical consistency when data were available for a series of chemically similar compounds (e.g. the carboxylic acids, the chlorobenzenes) care was taken to select values which consistently fitted within the series (2c) reliability from personal judgement of the reliability of the research groups, preference being given to data coming from laboratories with a consistent and well documented experience in the business (2d)... 

Provides access to EPA information for United States and international requests, and has a range of information sen>-ices consisting of environmental and related subjects of interest to EPA staff, including online searching of commercial databases. The focus of the IRC collection is on environmental regulations, policy, planning, and administration. Hours 8 a.m. - 5p.m., Monday - Friday ESP... 

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