A short review and an empirical method for estimating the absorbed enthalpy of formation and the absolute enthalpy of dried microbial biomass for use in studies on the thermodynamics of microbial growth

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The equation DrX = DrH - DrQ represents a calculated free-energy change when the exchange of absor- bed thermal energy in a chemical system represented by TDrS in the Gibbs free-energy equation is replaced by DrQ. The symbol Q is used in place of H (enthalpy = HT - H0 = HT) to represent absorbed thermal energy. Acquiring the experimental data for determining both S and Q requires the use of a low-temperature calorimeter to measure Cp as a function of T/K and these are not generally available. In a previous study it was demonstrated that for one unit-carbon formula dry weight of cells, DfSbiomass =- 0.813 P Satoms and Sbiomass = 0.187 P Satoms, where P Satoms represents the sum of the entropies of the numbers and kinds of atoms in the biomass. Using similar techniques, it is shown here that DfQbiomass =- 0.648 P Qatoms and that Qbiomass = 0.352 P Qatoms, where P Qatoms represents the sum of the absorbed thermal energies of the numbers and kinds of atoms in the biomass. Because mathematically the value of TS for solid substances is twice that of Q for the same T/K (usually ref- erenced at 298.15 K), one of these values must be physically incorrect. There cannot be two different values for the quantity of thermal energy which must be absorbed to raise the temperature of the same quantity of the same substance from 0 K to a given temperature. The argument is made that the use of Q is preferable to the use of S in the calculation of free-energy changes.