TRC (formerly the Thermodynamics Research Center) was founded in 1942 by Dr. Fredrick D. Rossini [Chief of the Section on Thermochemistry and Hydrocarbons at the National Bureau of Standards, now the National Institute of Standards and Technology (NIST)] to undertake American Petroleum Institute Research Project 44. The purpose of that project was to obtain information on thermodynamic and thermophysical properties of selected hydrocarbons and their sulphur-containing derivatives. Such information was critically important to the development of new refinery technologies that were vital during World War II. Data tables were first circulated in loose-leaf sheets, and then published by the Government Printing Office in bound book form around 1948. "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds", comprising the tables of API-RP-44 extant as of December 31, 1952, were published for API by Carnegie Press. The outstanding accomplishments of the staff of API Research Project 44 were readily apparent by the uniform acceptance of the work by industry and educational institutions worldwide. API Research Project 44 operated at NBS from its beginning in 1942 until 1950, when it moved to the Carnegie Institute of Technology (now Carnegie Mellon University), where Dr. Rossini was the Silliman Professor and Head of the Department of Chemistry.
In 1955 TRC started another national project — Manufacturing Chemists' Association (MCA), (subsequently the Chemical Manufacturers Association, and now the American Chemistry Council) Research Project — at the Carnegie Institute of Technology, Pittsburgh, PA. The purpose was to expand coverage to all organic compounds, using the same kind of loose-leaf tables as API-RP-44. Dr. Bruno Zwolinski joined API-RP-44 and the MCA Project as Associate Director in 1958. In 1961, Dr. Rossini left TRC to become Dean of the College of Science and Acting Head of the Department of Chemistry at the University of Notre Dame. TRC was relocated to Texas A&M University as a part of the Chemistry Department. Dr. Bruno Zwolinski became the second Director of TRC.
Later, the project name was changed to the "Chemical Thermodynamic Properties Data Project" and the name of the tables was changed to "TRC Thermodynamic Tables - Hydrocarbons" and "TRC Thermodynamic Tables - Non-Hydrocarbons". The NIST Standard Reference Data program supported special data evaluation projects at TRC, but not for the TRC Thermodynamic Tables. The Thermodynamic Tables became self-supporting, and included spectral data sheets, which were part of the API-RP-44 and MCA projects from the beginning.
In 1986, Dr. Randolph C. Wilhoit designed and created an electronic database, "TRC SOURCE", for managing numerical values of thermodynamic, thermochemical and transport properties of pure compounds and mixtures extracted from the world's scientific literature. It marked the entrance for TRC into a new phase of computerized data management and processing. This led to an electronic version of the TRC Thermodynamic Tables, called the "TRC Table Database", for PCs.
In September, 2000 after TRC rejoining the National Institute of Standards and Technology, where it is currently operated within the Physical and Chemical Properties Division, the publication of both series of the TRC Thermodynamic Tables resumed at NIST.
The critically evaluated physical and thermodynamic data are provided in tabular form according to property groups. There are eight property groups included in the TRC Web Thermo Tables (WTT). They are: (1) critical properties, (2) vapor pressure and boiling temperatures, (3) phase transition properties, (4) volumetric properties, (5) heat capacities and derived properties, (6) transport properties, (7) refraction, surface tension, and speed of sound, and (8) reaction state-change properties. Detailed lists of the properties available within each group are provided with the Help Documentation for that group. Not all properties have been evaluated for all compounds.
The tabulated values were published originally as hardcopy tables, as described in the History section of Help. For a number of properties, the hardcopy tables included identical properties in several different tables. For example, one table included condensed-phase density values at p = 101.3 kPa for T = 293.15 K and T = 298.15 K only, while a second table included values over broad temperature ranges, including 293.15 K and 298.15 K. Generally, redundant values were checked and adjusted for self-consistency in the sense that the same property of the same compound reported in different tables should be the same, and that thermodynamic and other relationships among physical properties should be satisfied. Each data set is dated to indicate when the critical evaluation was done. Data sets having the same evaluation date should be self-consistent. However, data from one set may not always be consistent with data on the same property published earlier in another set. In such cases, the data with the most recent evaluation date are considered to be the best values.
In the critical evaluation process, the data compiler tries to select the most accurate values available at the time of evaluation. Whenever possible, the numbers reported in the tables are based on experimental measurements, the results of which have been published in the scientific literature or have been obtained through personal communication with the investigator. When more than one source exists, the selected value may be taken from the source judged to be the most reliable. More often, however, the selected value is obtained by some averaging, smoothing, or extrapolation of data from several sources. Older data may be corrected or recalculated using updated values of auxiliary data, fundamental constants, and/or conversion factors, when deemed appropriate. In making the final choice, consideration is given not only to the directly measured property values, but also to other data related by thermodynamic principles to the one in question. Where experimental data are missing or unreliable, the data in the tables are obtained by a correlation or estimation procedure. The tables do not show whether a particular number is based on experimental measurements or on an estimation procedure.
Thermophysical property values critically evaluated for the Web Thermo Tables were validated, where possible, with the NIST ThermoData Engine software (NIST Standard Reference Database 103: http://www.nist.gov/srd/nist103.htm).
The basis for the selection or derivation of a number is important for someone who uses the tabulated values to develop or test new correlation methods. The distinction between experimental and estimated data may not always be sharp, because some values are the result of combining different kinds of data. For example, an enthalpy of vaporization may be obtained from a vapor pressure equation derived from measured values combined with an equation of state based on estimated parameters. In some instances, an estimation may influence the final choice among conflicting experimental values.
References to experimental data sources and information and methodology related to the evaluations can be found in:
TRC Thermodynamic Tables — Hydrocarbons. Ed. M. Frenkel, National Institute of Standards and Technology, Boulder, CO, Standard Reference Data Program, Publication Series NSRDS-NIST- 75 (1942-2007), Gaithersburg, MD.
TRC Thermodynamic Tables — Non-Hydrocarbons. Ed. M. Frenkel, National Institute of Standards and Technology, Boulder, CO, Standard Reference Data Program, Publication Series NSRDS-NIST- 74 (1955-2007), Gaithersburg, MD.
uncertainties
The estimated uncertainties in the tabulated values may be inferred from the number of significant figures used to display them. The uncertainty reflects, in the view of the compiler, the combined standard uncertainty, as defined in the references given below. For values listed as a function of temperature or pressure, one extra figure beyond the one which is uncertain is generally listed. For single-valued properties, the last figure is uncertain. The exception is ideal-gas property data, which are listed generally with four or five digits of precision, as traditionally reported in the literature. In these cases only, no uncertainty estimate can be deduced from the values provided.
References for uncertainty definition:
Guide to the Expression of Uncertainty in Measurement (International Organization for Standardization, Geneva, Switzerland, 1993). This Guide was prepared by ISO Technical Advisory Group 4 (TAG 4), Working Group 3 (WG 3). ISO/TAG 4 has as its sponsors the BIPM, IEC, IFCC (International Federation of Clinical Chemistry), ISO, IUPAC (International Union of Pure and Applied Chemistry), IUPAP (International Union of Pure and Applied Physics), and OIML. Although the individual members of WG 3 were nominated by the BIPM, IEC, ISO, or OIML, the Guide is published by ISO in the name of all seven organizations.
U. S. Guide to the Expression of Uncertainty in Measurement, ANSI/NCSL Z540-2-1997, ISBN 1-58464-005-7. NCSL International: Boulder, Colorado. 1997.
Chirico, R. D.; Frenkel, M.; Diky, V. V.; Marsh, K. N.; Wilhoit, R. C. ThermoML — an XML-based Approach for Storage and Exchange of Experimental and Critically Evaluated Thermophysical and Thermochemical Property Data. 2.Uncertainties. J. Chem. Eng. Data 2003, 48, 1344-1359.
DESCRIPTION
OF THE NUMERICAL DATA TABLES
Every data table includes the following information:
property name
property value(s)
units
phase(s) present
evaluation date
For some properties, multiple phases are listed to represent saturation conditions or to fully specify phase transitions, such as vaporization or melting. If saturation conditions are represented, Phase 1 is the phase associated with the property and Phase 2 is the second phase present. In the case of phase transitions, Phase 1 represents the initial phase and Phase 2 the final phase for the physical process. See the Help associated with each particular property group for additional details.
PHASE
SPECIFICATIONS IN THE TABLES
C: crystal
C1: the stable crystal form immediately below the melting temperature
C2, C3, etc.: the sequential stable crystal forms at temperatures below C1
CM: metastable crystal form
CM1, CM2: metastable crystal forms occurring at increasing lower temperatures
FL: fluid (i.e., T > Tc)
L: liquid
Critical Properties
Properties within this group:
Critical temperature, Tc
Critical pressure, pc
Critical density, ρc
Critical molar volume, Vc
Critical compressibility factor, Zc
Zc = pcVc/RTc where R is the gas constant.
Vapor Pressure and Boiling Temperatures
Properties within this group:
Normal boiling temperature, Tb
Vapor pressure, psat
Sublimation pressures are included within this property. The identity of the condensed phase is always specified in the data table.
PHASE
TRANSITION PROPERTIES
Properties within this group:
Triple point temperature, Ttp
This property includes transition temperatures between crystal forms at saturation pressure, as well as crystal-liquid-gas triple-point temperatures. The condensed phases are always specified.
Normal melting temperature, Tm
Enthalpy of vaporization, ΔvH
This property also includes enthalpies of sublimation. The condensed phase is always specified.
Enthalpy of phase transition, ΔtrH
This property includes enthalpies for phase transitions between crystal forms, as well as crystal-liquid phase transitions. The initial (Phase 1) and final (Phase 2) phases are always specified.
Entropy of vaporization, ΔvS
This property also includes entropies of sublimation. The condensed phase is always specified.
Entropy of phase transition, ΔtrS
This property includes entropies for phase transitions between crystal forms, as well as crystal-liquid phase transitions. The initial (Phase 1) and final (Phase 2) phases are always specified.
VOLUMETRIC
PROPERTIES
Properties within this group:
Specific density, ρ
Properties are available for single phases (crystal, liquid, gas, fluid) at specified pressures and at liquid-gas saturation conditions. For properties under saturation conditions, Phase 1 is the phase of the property and Phase 2 is the second phase present.
Adiabatic compressibility, -(1/V)(∂V/∂p)S
See note for specific density, ρ.
Second virial coefficient, B
The second virial coefficient is defined as
B = RT {(Z - 1)/p} in the limit of p approaching 0, where Z - 1 = Bp/RT.
HEAT CAPACITIES AND DERIVED PROPERTIES
Properties within this group:
Heat capacity at constant pressure, Cp
This property includes values for the ideal-gas state and condensed phases at 1 bar.
Heat capacity at saturation, Csat
Heat capacities for condensed phases at vapor saturation pressure are listed in the data tables.
Derived condensed-phase thermodynamic properties
The basic experimental data needed for evaluation of condensed phase thermodynamic properties are heat capacities from near 0 K to the temperature of interest, and temperatures and enthalpies of phase transitions. By use of the following formulas, other thermodynamic properties may be derived:
where Tx is the lowest temperature of measurement. It should be noted that for crystal heat capacity measurements, the experimental values are usually Csat rather than Cp, although their differences are insignificant at vapor pressures below about 100 kPa. H(0) is the enthalpy of the crystal at 0 K and S(0) is the entropy at 0 K for perfect crystalline substances. The following derived properties for the condensed phases are included in the database.
Entropy, S (T) - S (0 K)
Entropies for condensed phases (i.e., crystal and liquid) are relative to the stable crystalline form at 0 K.
Derived ideal-gas properties
The tabulated thermodynamic properties for chemical substances in the ideal gas state were calculated with the methods of statistical mechanics or group contributions. The statistical methods are most effective for small molecules (i.e., fewer than 5 carbons), while properties for larger molecules are typically estimated with group contributions. Where possible, such calculations are checked against experimental heat capacity or entropy data, and in some cases, adjustments are made in the molecular parameters to achieve better agreement. The standard state pressure for all ideal gas properties is 1 bar (105 Pa). Calculation of ideal-gas properties is also described in the following reference.
Entropy, S (T) - S (0 K)
Entropies for the ideal-gas phase are relative to the ideal gas at 0 K.
Enthalpy function, {H°(T) - H°(0 K)}/T
Gibbs energy function, -{G°(T) - G°(0 K)}/T
TRANSPORT PROPERTIES
Properties within this group:
Viscosity, η
This property is available for single phases (liquid, gas, fluid) at specified pressures and at liquid-gas saturation conditions. For properties under saturation conditions, Phase 1 is the phase of the property and Phase 2 is the second phase present.
Kinematic viscosity, ν
This property is only available for selected hydrocarbons and has not been updated since 1955. Generally, values are for the liquid phase at 101.3 kPa, with some single values for the liquid at saturation pressure. For properties under saturation conditions, Phase 1 is the phase of the property (liquid) and Phase 2 is the second phase present (gas).
Thermal conductivity, λ
Thermal conductivities are available for single phases (liquid, gas, fluid) at specified pressures and at liquid-gas saturation conditions. For properties under saturation conditions, Phase 1 is the phase of the property and Phase 2 is the second phase present.
Refraction, Surface Tension, and Speed of Sound
Properties within this group:
Surface tension, σ
Values for this property are available for the liquid phase at 101.3 kPa, with some single values for the liquid at saturation pressure. For properties under saturation conditions, Phase 1 is the phase of the property (liquid) and Phase 2 is the second phase present (gas).
Speed of sound, u
This property is available for single phases (liquid, gas, fluid) at specified pressures and at liquid-gas saturation conditions. For properties at saturation conditions, Phase 1 is the phase of the property and Phase 2 is the second phase present.
Refractive index, n
Generally, values are given for the liquid phase, with only a few values listed for the gas phase. All values are at pressure p = 101.3 kPa. For some compounds, the refractive index is provided as a function of wavelength.
Reaction State-Change Properties
Properties within this group:
Enthalpy of formation, ΔfH°
Gibbs energy of formation, ΔfG°
The enthalpy and Gibbs energy of formation of a compound refer to its formation from the pure elements in their reference states. For a compound in phase φ whose formula is CnHmOp, the reaction is:
nC(graphite) + 0.5m H2(g) + 0.5p O2(g) = CnHmOp(φ).
The reference states for the elements between 0 and 5000 K are:
Carbon: Graphite [0 to 5000 K]
Hydrogen: H2(g), equilibrium ortho/para mixture [0 to 5000 K]
Oxygen: O2(g) [0 to 5000 K]
Nitrogen: N2(g) [0 to 5000 K]
Sulfur: S(crystal II) [0 to 368.3 K]; S(crystal I) [368.3 to 388.36 K]; S(l) [388.36 to 881.21 K]; and S2(g) [881.21 to 5000 K].
Fluorine: F2(g) [0 to 5000 K]
Chlorine: Cl2(g) [0 to 5000 K]
Bromine: Br2(cr) [0 to 265.90 K; Br2(l) [265-332.503 K]; and Br2(g) [332.503 to 5000 K]
Tables of heat capacities, absolute entropies, and enthalpy increments for the elements used in the calculation of the formation properties are given here. Values were obtained from the NIST-JANAF Thermochemical Tables.
Enthalpy of combustion (gross), -ΔcH°
The gross enthalpy of combustion at T = 298.15 K, which is tabulated for some hydrocarbon and sulfur-containing compounds only, corresponds to the following reaction.
CnHmSk + (0.25m + n)O2(g) + 115k H2O(l) = 0.5m H2O(l) + n CO2(g) + k H2SO4(aq, 115 H2O)
The negative of the gross enthalpy of combustion is also known as the higher heating value or gross heating value.
Enthalpy of combustion (net), -ΔcH°
The net enthalpy of combustion at T = 298.15 K, which is given for some hydrocarbon and sulfur-containing compounds only, corresponds to the following reaction.
The negative of the net enthalpy of combustion is also known as the lower heating value or net heating value.
Auxiliary Data for enthalpies of formation and combustion
Auxiliary data (enthalpies of formation at T = 298.15 K) used in calculation of values of enthalpy of formation and combustion are given here.
Δf
Δf
Δf
Δf
Δf
Data sources for the auxiliary data are:
CODATA Recommended Key Values of Thermodynamics 1977, Codata Bulletin No. 28, April 1978.
Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds, Academic Press: London, 1970, p. 132.
ACKNOWLEDGMENT
Interactive plotting in WTT is performed with Ptplot Java Applet written by E. A. Lee and C. Brooks.