Project Details Recommended Reference Materials for Phase Equilibrium Studies

Project No.:
Start Date:
01 January 2012
End Date:


The objective is to provide lists of recommended reference materials (primarily organics and common gases) with critically evaluated property values for phase equilibrium studies; vapor-liquid (VLE), liquid-liquid (LLE), and solid-liquid (SLE) equilibrium.

Reference materials are long established as necessary for inter-laboratory comparisons and validation of uncertainty claims for instrumentation. Recommendations of materials for physicochemical properties (including a small amount of VLE)1 calorimetry and thermal analysis2 and have been reported. Similar work in the broader field of phase equilibrium studies is lacking. Results of this project will fill this need with results freely available on the Web in a convenient and fully traceable form, as well as in traditional Technical Report format.

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ref 1. IUPAC. Recommended Reference Materials for the Realization of Physicochemical Properties, edited by K. N. Marsh, Blackwell Scientific Publications, Oxford, UK (1987) [ISBN 0-63201-718X].

ref 2. R. Sabbah, An Xu-wu, J.S. Chickos, M.L. Planas Leitão, M.V. Roux, L.A. Torres. Reference materials for calorimetry and differential thermal analysis. Thermochim. Acta 331, 93-204 (1999).



1) Query the NIST collection of experimental data to determine systems with extensive, high-quality data for various types of phase equilibrium.

2) Select particular systems for modeling based on the query results. This part will involve significant communication between the committee members, as considerations of data availability and project scope are refined.

3) Fit mathematical models to the selected experimental data and refine the uncertainties.

4) Publish a paper in Pure and Applied Chemistry describing the development of the recommendations. The paper may also highlight areas where new reference quality measurements are needed.

5) Create a website for ready distribution of project results that includes computational tools that will allow researchers to easily calculate the recommended property values based on the developed models. Researchers will not have to program the models themselves. The website will also allow simple updating and future expansion of the project.

The NIST data collection (SOURCE) is one of the largest compilations of experimental property data in the world. This archive, which includes uncertainty estimates for all stored data, in combination with data analysis and modeling software,3 including special tools for weighting of vapor-liquid equilibrium data4 provides a strong foundation for specification of reference materials based on data and model quality. The international team of experts that form this Task Group includes journal editors from the three major journals that publish phase equilibrium data (Fluid Phase Equilibrium, The Journal of Chemical Thermodynamics, and Journal of Chemical and Engineering Data), as well as experts in data analysis and modeling.

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ref. 3 (a) M. Frenkel, R. D. Chirico, V. Diky, X. Yan, Q. Dong, C. D. Muzny. ThermoData Engine (TDE): Software Implementation of the Dynamic Data Evaluation Concept. J. Chem. Inf. Model. 45, 816-838 (2005).
(b) V. Diky, R. D. Chirico, A. F. Kazakov, C. D. Muzny, M. Frenkel, M. ThermoData Engine (TDE): Software Implementation of the Dynamic Data Evaluation Concept. 3: Binary Mixtures. J. Chem. Inf. Model. 49, 503-517 (2009).

ref. 4 J. W. Kang, V. Diky, R. D. Chirico, J. W. Magee, C. D. Muzny, I. Abdulagatov, A. F. Kazakov, M. Frenkel. A Quality Assessment Algorithm for Vapor-Liquid Equilibrium Data. J. Chem. Eng. Data 2010, 55, 3631-3640


June 2014 update: The data available in the literature for binary mixtures has been assembled and will be entered into a WEB based searchable system by October 2014. From this large collection of critically evaluated phase equilibrium data (SLE, VLE, LLE) the systems will be evaluated in terms of uncertainty, chemical system type, T, p, x, y variable range and measurement method type. Preliminary results obtained from this review will be provided to the committee members for review.


Sep 2017 update:  The task group met at the ECTP conference (21st European Conference on Thermophysical Properties) held in Graz, Austria on 4 September 2017. Significant positive input was received from members of the IUPAC Subcommittee on Solubility and Equilibrium Data (SSED). Three experts from the SSED have agreed to join the Task Group. The plan for the biennium 2018-2019 is to prepare the first Technical Report, which will focus on vapor-liquid equilibrium (VLE).

On the initial stage of the project, publications on classification of phase equilibria for mixtures were gathered. Based on the classification, it was concluded that:
a) separate reference systems should be selected for different experimental methods and conditions;
b) the first selection round should include the most common cases, which do not encompass modelling difficulties.

The first-round target mixture categories were identified as follows:
i) simple eutectic mixtures for SLE,
ii) VLE without azeotropes under near-atmospheric pressures,
iii) moderate-solubility LLE.

For screening of relevant phase equilibrium data, NIST/TRC Source database, currently containing 6.3 million data points of thermodynamic, thermophysical, and transport properties for pure compounds, binary mixture, ternary mixtures, as well as chemical reactions, has been used. The following selection criteria were established (presented sequentially as applied):
1. number of data points and data sets (systems with the large number of data);
2. falling into the target categories;
3. quality of measurements and consistency of data (systems with consistent, high-quality data);
4. suitable for modeling (systems without modeling difficulties);
5. consistency with other properties (modeling shows that target phase equilibria data are consistent with other properties);
6. additional compound-specific requirements, such as: compounds should be commonly-available (commercial), high-purity samples should be available (or effective purification is achievable), purity of such compounds can be checked by common methods; compounds should be stable at the studied conditions; compounds should be non-toxic.

Modeling of the considered phase equilibria data was preliminary conducted in the ThermoData Engine expert system (SRD 103b). The following models were currently considered: AC models (NRTL, Wilson, UNIQUAC), Peng-Robinson EOS, empirical equations (for LLE). Where available, models published in the Solubility Data Series were additionally applied for SLE and LLE selections. Another piece of software is currently under development by Prof. Kang for additional detailed phase equilibrium analysis.

During the screening, about 200, 100, and 100 binary mixtures were analyzed for VLE, SLE, and LLE categories, respectively. From all considered cases, three VLE, five SLE, and four LLE systems have been chosen and represented with models generated by ThermoData Engine. The systems and preliminary models, including comparison with the SDS equation (where available), are provided as supporting information SI1-VLESI2-SLE , SI3-LLE (references are omitted for clarity at this stage).


Nov 2018 update: Drafts of two Technical Reports on reference systems for LLE and SLE studies were presented and discussed at the task group meeting held in conjunction with the ISSP-18 conference on 16 July 2018 (Tours, France). Four system categories represented by 6 selected mixtures were considered for LLE: aqueous systems (aniline + water, phenol + water), non-aqueous systems (cyclohexane + methanol), low solubility (benzene + water, toluene + water), systems with ionic liquids (1-hexanol + 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide). Four system categories represented by 8 selected mixtures were considered for SLE: organic-aqueous systems (urea + water, sulfolane + water), inorganic-aqueous systems (potassium chloride + water, sodium chloride + water), non-aqueous systems (benzene + naphthalene, toluene + naphthalene), low solubility (benzoic acid + water, anthracene + water). It was decided to revise equations for toluene + water and 1-hexanol + 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide. An additional mixture was suggested for SLE studies – ammonium chloride + water. It was decided to exclude all systems containing benzene due to the existing restrictions. All suggestions about modeling and representation of the data made during the meeting were accepted.
A list of systems suggested at the previous task group meeting for VLE studies were discussed: methanol + water, butane + propane, and toluene + cyclohexane. It was decided to add a few additional mixtures for further consideration: benzene + cyclohexane, acetone, methanol; acetone + chloroform, water; ethanol + water.
More complex cases suggested during the meeting (e.g., high-temperature/high-pressure SLE and VLE, systems with LCST) can be considered later for an additional, forth Technical Report publication. Octanol-water partition coefficients were suggested during the meeting, but it was decided not to include them, since it is not possible to find a system with numerous reliable and consistent measurements.

Sept 2020 update: The task group has prepared the first part of the IUPAC Technical Report entitled “Reference Materials for Phase Equilibrium Studies. 1. Liquid-Liquid Equilibria (IUPAC Technical Report)” and the mss is being submitted to PAC. There will be two more parts of the TR (for Solid-Liquid Equilibria and for Vapor-Liquid equilibria).

July 2021 update: The Technical Report “Part 1. Liquid–liquid equilibria” has been published, Pure and Applied Chemistry, AOP 8 July 2021; (in print Pure and Applied Chemistry, vol. 93, no. 7, 2021, pp. 811-827)

Page last updated 5 Aug 2021