A survey is made of the enthalpies of formation, third law entropies and Gibbs energies available for Krebs cycle and related compounds. These include formate, acetate, succinate, fumarate, glycine, alanine, aspartate and glutamate. The potential of the NAD+ /NADH couple is recalculated based on lhe;; e;;tbanul/acetaldehyde and· isopropanol/acetone equilibria. The reported enzyme catalyzed equilibrium constants of the Krebs cycle reactions are evaluated with estimated errors. These 28 equilibria form a network of reactions that is solved hy a lea.qt squares regression procedure giving Gibbs energies of formation for 21 Krebs cycle and related compounds. They appear to be accurate to ± 0.4 kJ .mol-1 for some compounds but ± 1 kJ·mol-1 in less favorable cases. This procedure indicates which third law 11,6 and enzyme equilibria are inaccurate, and allows very accurate ArG to be determined for compounds related to the Krebs cycle by measuring enzyme equilibrium constants.
The preparation of the different samples was made by Maj-Lis Fontell, the density measurements were made by Mary Molund, and the 12 3H spectra were recorded by Ali Khan. The starting point of this work was discussions with Heinz Hoffmann and we are grateful to him for valuable dis-cussions and for providing information prior to publication.Many important comments on this work were given by Hákan Wennerstrom and John Lang. We are grateful to Gordon Tiddy for the penetration studies reported in note 28 as well as for many useful comments and to Tommy Liljefors for the calculations reported in note 34.
We have measured specific heats of high-grade, medium-grade and low-grade samples of Athabasca oil sands over the temperature range 50–300°C. along with specific heats of components (coarse solids, fine solids and bitumen) over this same temperature range. It has been found that the specific heats of oil sands can be represented accurately as appropriate sums of the specific heats of components. Equations for convenient calculatations of all of these specific heats at temperatures to 300°C are given. Introduction Both commercial plants and all pilot plants currently in operation for the production of bitumen from the oil sands and heavy oil deposits in Alberta make use of thermal methods. Both Syncrude and Suncor use the hot water process in their commercial plants that separate bitumen from mined oil sands. Direct coking of mined oil sands has reached the pilot-plant stage. Various pilot-plant operations for in-situ production are based on steam injection or underground combustion or some combination of these methods. We cite a few useful reviews(l-4). Among the parameters needed for proper design and assessment of all thermal methods are specific heats of the material to be heated. We have therefore measured specific heats of samples of Athabasca oil sands having three different compositions. To make our results applicable to a wide range of oil sands having different compositions, we have also measured specific heats of components of oil sands and have demonstrated that the specific heats of whole oil sands can be represented as appropriate sums of specific heats of their components. We have chosen differential scanning calorimetry as our principal method for obtaining specific heats of oil sands and components. This method offers the advantages of being reasonably fast and makes use of instrumentation that is commercially available so that others may conveniently extend our work. On the other hand, it must be noted that there are two problems to consider in connection with measurements made by this method. Most researchers interested in accurate thermodynamic data have regarded differential scanning calorimetry as an unsatisfactory method, largely on the basis of inaccurate data published a decade and more ago. More recently, as a result of improvements in the instruments and greater care in operation, it has been demonstrated that differential scanning calorimetry can yield thermodynamically accurate results, as summarized in several papers(5–11). Because oil sands are obviously non-homogeneous, the small samples ordinarily used in differential scanning calorimetry can lead to misleading results. We have minimized this problem by using larger-than-usual samples (20–55 mg) and by averaging the results obtained on several different calorimetric samples taken from the same bulk sample. In order to obtain a check on the validity of our minimization of this problem of small sample size and non-homogeneity, we have also made a few measurements with larger calorimeters. Experimental Most of our calorimetric measurements of specific heats have been made with a Perkin-Elmer DSC-2 differential scanning calorimeter, with output recorded on a Perkin-Elmer single-channel multi-range thermal analysis recorder.
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