Retrieval of thermodynamic data

Calorimetric, spectroscopic and electrochemical measurements are the classical sources of thermodynamic data. In the present section it will be demonstrated that thermodynamic data of com parable quality can be extracted from the results of phase equilibria experiments at elevated pressures and temperatures. The prerequisite for data extraction are that 

Suppose P-T data are available for an equilibrium involving pure solids and pure H2O of the type 

If the thermodynamic data of H2O and all but one condensed phases are available, then those of the unfenown phase may be calculated. Depending on whether or not ACSf. g data are available as a function of temperature, the eq.(l9j on the one hand and eq.(20) or (21) on the other would apply. 

Retrieval of thermodynamic data has been commonly achieved by the so-called point--by-point method (e.g. Robie and Stout,1963 Fisher and Zen,1971 Zen,1969,1972). In this method, Sf 298 oi all the condensed phases have to be known, enabling one to solve for Gf 298 unknown phase, setting the relation (20) equals 

An alternative method of data extraction - dubbed the curve-fitting method - is based on eq.(2l). In this case the term 

---

Figure 2. Retrieval of thermodynamic data of pyrophyllite from experimental phase equilibria study of the reaction (22) hy Haas and Holdaway (1973). The data points correspond to the extremes of the reversal brackets, the arrows indicating the direction of the reaction. (a) curve-fitting technique(eq.21) ...

In summary, then, the following points should be taken into account for retrieval of thermodynamic data ... 

A large amount of experimental data has been published since the debut of the original CRC Handbook of Thermodynamic Data of Aqueous Polymer Solutions. Incorporating new and updated material, the CRC Handbook of Phase Equihbria and Thermodynamic Data of Aqueous Polymer Solutions provides a comprehensive collection of thermodynamic data of polymer solutions. It helps readers quickly retrieve necessary information from the literature, and assists researchers in planning new measurements where data are missing. 

Following extraction of thermodynamic data, it is a good practice to make sure that these are indeed consistent with the experimental phase equilibrium data from which they were obtained. This is best done by recalculating the equilibrium curve, using the retrieved thermodynamic data as input. Trivial though it may sound, a routine check would seem to be necessary in view of the fact that there are many cases of retrieved thermodynamic data in the literature not fulfilling this test of consistency. An example of such inconsistency is illustrated later (see Study Problems). 

The prefix trans tells us that the triphenylphosphine groups occupy opposite corners of a square rather than adjacent corners (cis). The use of chloro in the name (instead of chloride ) implies the absence of discrete chloride ions, as in [Ir(CO)(PPh3)2]+Cr. Names, as opposed to formulae, are easier to index or to store in a data bank for the rapid retrieval of information on compounds having specific structural features. If a chemist wished to collect the available thermodynamic data for inorganic nitrites, a computer search of formulae containing NOz or some multiple thereof would produce a vast number of organic nitro-compounds of no... 

Frequently process plants contain recycle streams and control loops, and the solution for the stream properties requires iterative calculations. Thus efficient numerical methods for convergence must be used. In addition, appropriate physical properties and thermodynamic data have to be retrieved fi om a data base. Finally, a master program must exist that links all the building blocks, physical property data, thermodynamic calculations, subroutines, and numerical subroutines, and that also supervises the information flow. You will find that optimization and economic anafy-sis are really the ultimate goal in the use of flowsheet codes. 

Onken U, Rarey-Nies J, Gmehling J (1989) The Dortmund data bank a computerized system for retrieval, correlation, and prediction of thermodynamic properties of mixtures. J Int J Thermophys 10(3) 739-747... 

Figure 3 shows the calculated equilibrium curve for the reaction (22), based on the retrieved thermodynamic data of pyrophyllite. As long as the proper combination of input data and the method of calculation is retained, identical curves result. Only when these are interchanged, do the resulting curves differ somewhat. For example, if the retrieved data obtained in Figure 2c is used as input, but AS g is assumed independent of T, the calcu-... 

Calculate the PHpO curve of the above equilibrium, using both the sets of retrieved thermodynamic data ... 

Lack of data for the formation of Si-OAA complexes makes the prediction of the behavior of quartz difficult. Dissolution experiments by Bennett et al. (1988) and Bennett (1991) as well as field studies (Bennett and Seigel 1987) show that the dissolution rate of quartz is modified in the presence of some acids but thermodynamic data cannot be retrieved from these experiments for use in chemical models. If indeed a stable Si-OAA complex forms in sedimentary environments, then the stabilities of quartz as well as all the silicate minerals will be reduced further than the destabilization shown in Figs. 8 and 9 (see also Bennett and Casey, Chap. 7, this Vol.). 

Knowledge of the temperature profile is essential for an understanding of the dynamical, thermodynamical, and chemical processes occurring in an atmosphere. In addition, the atmospheric temperature must first be obtained before information on gas or particle composition can be extracted from thermal emission measurements. More effort has been devoted to the retrieval of temperature profiles than any other atmospheric parameter and, as a consequence, this problem is better understood than other inverse radiative transfer problems. Study of this subject also provides a convenient means of illustrating some of the basic concepts of vertical profile reconstruction from remotely sensed data. 

Both derivative CV and SHAC voltammetry require specialized instrumentation. A much more simple experimental procedure has been described for electrode potential measurements which can be done with respectable precision using rudimentary instrumentation. The measurement of peak potentials during LSV is normally carried out to a precision of the order of 5 mV. This is because the peak resembled a parabola with a rather flat maximum. On the other hand, the half-peak potential where the current is half the peak value, has just as much thermodynamic significance and can be measured to about 1 mV using x-y recording with a suitable expansion on the potential axis. When used in conjunction with a digital data retrieval system the method is as precise as derivative cyclic voltammetry (Aalstad and Parker, 1980). 

Now having several empirical parameters in a fitting equation, it is no longer possible to calculate some term a directly from general or averaged data apart from this, this would be beyond and besides the ends of this work - our task is to construct a parameter set which describes the biochemical behaviour of metal ions while prediction of complex formation constants can be also applied for estimating thermodynamics of metal ion transport and retrieval in a terrestrial ecosystem. 

The Gibbs equation contains three independent variables T, a, and p (defined either via concentration or pressure, c or p, respectively), and is a typical thermodynamic relationship. Therefore, it is not possible to retrieve any particular (quantitative) data without having additional information. In order to establish a direct relationship between any two of these three variables, it is necessary to have an independent expression relating them. The latter may be in a form of an empirical relationship, based on experimental studies of the interfacial phenomena (or the experimental data themselves). In such cases the Gibbs equation allows one to establish the dependencies that are difficult to obtain from experiments by using other experimentally determined relationships. For example, the surface tension is relatively easy to measure at mobile interfaces, such as liquid - gas and liquid - liquid ones (see Chapter I). For water soluble surfactants these measurements yield the surface tension as a function of concentration (i.e., the surface tension isotherm). The Gibbs equation allows one then to convert the surface tension isotherm to the adsorption isotherm, T (c), which is difficult to obtain experimentally. 

The TRC THERMODYNAMIC TABLES Database contains the evaluated data published in the TRC Thermodynamic Tables - Hydrocarbons and Non-Hydrocarbons. The contents of this database are updated quarterly in conjunction with the publication of the hard-copy supplements. The database is accessible on-line through TRCTHERMO module on the STN international network. From 1994, a version of this database developed using B-tree index files will be available for IBM PC or compatible computers on a yearly lease basis with a subscription to the hard copy of the TRC Thermodynamic Tables. Multiple copies will be available with a single hard-copy subscription. Its features include search for compound by name, formula or CASRN, with flexible name and formula searching capabilities. Data for a selected series of compounds can be retrieved and saved to external files for later analysis. 

---