NIST Webbook

Source

Pure component properties as used by the NIST WebBook, https://webbook.nist.gov/chemistry/ Retrieved: September 13th, 2019

Ideal Gas Molar Heat Capacity (Constant Pressure)

NIST uses the Shomate equation for the ideal gas molar heat capacity, which is shown below:

\[c_{\text{p ig}} = A + B \times t + C \times t^2 + D \times t^3 + \frac{E}{t^2}\]

where \(t = \frac{T}{1000}\).

Parameters

Symbol Parameter Name Indices Description
\(A, B, C, D, E, F, G, H\) cp_mol_ig_comp_coeff [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’, ‘H’]  

Note

The parameter cp_mol_ig_comp_coeff is also used when calculating specific enthalpy and entropy and parameters ‘F’, ‘G’ and ‘H’ are only required for these properties.

Ideal Gas Molar Enthalpy

The correlation for the ideal gas molar enthalpy is derived from the correlation for the molar heat capacity and is given below:

\[\frac{h_{\text{ig}} - h_{\text{ig ref}}}{1000} = A \times (t-t_{ref}) + \frac{B}{2} \times (t^2 - t_{ref}^2) + \frac{C}{3} \times (t^3 - t_{ref}^3) + \frac{D}{4} \times (t^4 - t_{ref}^4) + E \times (\frac{1}{t} - \frac{1}{t_{ref}}) + F - H\]
Symbol Parameter Name Indices Description
\(A, B, C, D, E, F, G, H\) cp_mol_ig_comp_coeff [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’, ‘H’]  

Note

This correlation uses the same parameters as for the ideal gas heat capacity with additional parameters F and H. These parameters account for the enthalpy at the reference state defined by NIST. Users wanting to use a different reference state will need to update H.

Ideal Gas Molar Entropy

The correlation for the ideal gas molar entropy is derived from the correlation for the molar heat capacity and is given below:

\[s_{\text{ig}} = A \times ln(t) + B \times t + \frac{C}{2} \times t^2 + \frac{D}{3} \times t^3 + \frac{E}{2 \times t^2} + G\]

Parameters

Symbol Parameter Name Indices Description
\(A, B, C, D, E, F, G, H\) cp_mol_ig_comp_coeff [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’, ‘H’]  

Note

This correlation uses the same parameters as for the ideal gas heat capacity with additional parameter G, which accounts for the standard entropy at the reference state defined by NIST. Users wanting to use a different reference state will need to update G.

Saturation (Vapor) Pressure

NIST uses the Antoine equation to calculate the vapor pressure of a component, which is given below:

\[log_{10}(P_{sat}) = A - \frac{B}{T+C}\]

Parameters

Symbol Parameter Name Indices Description
\(A, B, C\) pressure_sat_comp_coeff [‘A’, ‘B’, ‘C’]  

Note

The Antoine equation is generally written with saturation pressure expressed in bars. The units of the correlation can be converted to Pascals by adding 5 to \(A\).