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A Reference Equation of State with an Associating Term for Thermodynamic Properties of Ammonia



Kehui Gao, Jiangtao Wu, Ian Bell, Allan H. Harvey, Eric W. Lemmon


Fluid ammonia is highly associated because of strong intermolecular hydrogen bonding. This causes different behavior of thermophysical properties from nonpolar fluids, as reflected for example in the heat capacity. In this work, a new equation of state explicit in the Helmholtz energy with independent variables of temperature and density was developed, which is supplemented with an additional associating term. Its uncertainties in densities, vapor pressures, saturated liquid and vapor densities, and caloric properties were estimated by comparisons with experimental data of these properties. The new equation of state is valid from the triple-point temperature (195.49 K) to 725 K at pressures up to 1000 MPa and densities up to 52.43 mol dm-3. Physically correct behavior within the region of validity and at extremely high temperatures and pressures, and temperatures far below the triple-point temperature, was obtained through fitting multiple constraints of various properties. The unique physical behavior of ammonia shown in some thermodynamic properties was described in detail, which will provide a preliminary template for developing equations of state for other associating fluids.
J. Phys. & Chem. Ref. Data (JPCRD) -


Ammonia, Associating Fluids, Equation of State, Helmholtz Energy, Hydrogen Bonding, Thermodynamic Properties


Gao, K. , Wu, J. , Bell, I. , Harvey, A. and Lemmon, E. (2023), A Reference Equation of State with an Associating Term for Thermodynamic Properties of Ammonia, J. Phys. & Chem. Ref. Data (JPCRD), National Institute of Standards and Technology, Gaithersburg, MD, [online],, (Accessed June 23, 2024)


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Created February 15, 2023