# Concept of thermodynamic capacity

Akila, R and Jacob, KT and Shukla, AK (1986) Concept of thermodynamic capacity. In: Bulletin of Materials Science, 8 (4). pp. 453-465.

The thermodynamic capacity of a species (C,) in a homogeneous phase is defined as $(\partial n_i/\partial\mu_i)P. T. n_j$ where $n_i$ is the total number of moles of i per unit quantity of the system irrespective of the actual system chemistry and pi is its chemical potential. Based on this definition, the thermodynamic capacity of oxygen in non-reactive and reactive gas mixtures and in binary and ternary liquid solutions has been computed. For reactive gas mixtures containing stable chemical species which do not undergo significant dissociation such as $CO + CO_2$, $H_2 + H_20$ and $H_2 + CO_2$, the capacity curves show a maximum at equimolar ratio and a minimum at higher oxygen potentials. If one of the chemical species partly dissociates as in the case of $H_2S$ in $H_2+H_2S$ mixtures or $SO_3$, in $S0_2 + S0_3$ , mixtures, capacity curves do not exhibit such maxima and minima, especially at high temperatures. It would be difticult to produce stable oxygen fugacities when the capacity has a low value, for example at compositions near the minimum. Oxygen capacities of non-ideal liquid solutions, Cu-0 and Cu-0-Sn, and heterogeneous systems formed at saturation with the rcspcctivc oxides are discussed.