Physical & Thermodynamic Properties

Searching for thermodynamic and physical property data of chemical substances can be time-consuming and frustrating. It truly is like searching for a very specific needle in a huge and utterly disorganized mountain of needles -- and the one you're looking for may not even exist. One of the reasons why people tend to take shortcuts and make potentially serious mistakes with data is that the recommended mechanisms for finding reliable data are imperfect, never comprehensive, and often mysterious. Defaulting to more familiar web search engines can be a waste of time as well as risky, because they are not really appropriate tools for this task.

It is usually not difficult to locate reliable data for standard gases, small organic molecules, and common inorganic substances in pure form, along with their aqueous solutions and well-known binary systems. Standard reference tools like the CRC Handbook, the NIST WebBook, and DIPPR can answer many of these basic questions. But if you are looking for a complex or proprietary material -- things like polymers, drugs, biological molecules, exotic molecules, composites and mixtures, newly synthesized compounds, or commercial products -- published data often don't exist. It can also be difficult to find data covering non-standard conditions such as extreme temperatures and pressures; extrapolation of known data to such conditions may not be reliable. Engineers often rely on property values calculated by estimation programs. These are useful within their stated limits, but are outside the scope of this guide, which focuses on finding published literature values.

High-quality data can be found in certain online databases, but these almost always require a subscription or a fee to use. Many printed secondary data compilations have been published over the years, of varying quality and scope. They are all arranged differently, sometimes incomprehensibly. They cover different types of compounds and properties, and they tend to be scattered in a library, making them hard to remember and locate. Data reported in the primary journal and technical report literature can be even more elusive.

While people often refer to scientific data points as "facts," one scientist has stated that "there are no facts - just measurements embedded within assumptions." (1)

The accuracy of data published in the primary literature (e.g., in peer-reviewed journals) should not be assumed. Reviewers rarely examine such data closely. Experimental and measurement errors can occur. Authors can be sloppy in their use of units and symbols, and errors and typos creep in during the editing process. The pressure to keep articles short and omit "unnecessary" tables and graphs means that some useful data do not appear in published articles at all. If that's not bad enough, errors that make it into the literature can be propagated elsewhere almost indefinitely, creating confusion and uncertainty about even basic properties.

Most but not all data "handbooks" are secondary sources, meaning they are compilations of data previously reported in the primary literature. The reliability of secondary sources obviously depends on both the quality of the original data and on the care taken in compiling and evaluating them. Most compilations provide literature references for the data. Those that don't include such references should be used with caution. The age of the data is also relevant. The enthalpy of a compound is the same today as it was in 1905 - what has changed is the precision of measurement and estimation methods. Older data may be perfectly valid, but they should be compared to more recent values if they can be found.

The same caveats apply to data you might find on the Internet. A value found on a college lab course web page, in an MSDS, or in Wikipedia (2) cannot be treated the same as a value contained in a NIST database. The bottom line is that all sources of data should be viewed with a critical eye. Ask these questions: Is the source cited? When was this work done, and by whom? Were the data determined experimentally or derived by calculation (estimated)? What methods, experimental parameters, or special conditions applied? If you can't answer these questions the data probably should not be trusted.

The term critically evaluated - while occasionally overused - is a useful one to look for in secondary sources. This usually implies that someone has evaluated the data and procedures for internal consistency, and, in cases where conflicting values have been reported, established a set of recommended values. It does not mean that experts have repeated and verified the measurements themselves. YS Touloukian, commenting on critical evaluation, stated that "while 'critical analysis' always sets a 'level of confidence' for the recommended values, there is no implication whatsoever of high accuracy or precision in these values." (3)  Most primary literature and secondary compilations are uncritical, however.

NOTES:

1. Bradley, J.C. Useful Chemistry [blog post since deleted] 6/22/2011.
2. Walker, M.A. "Wikipedia as a resource for chemistry." ACS Symp. Ser. 1060, 2011, 79-92.
   [Wikipedia chembox: Wikipedia pages on common chemical compounds usually contain an infobox sidebar called a chembox, which provides data values for properties that are not likely to change. Data marked with a checkmark have been "verified" by WikiProject Chemicals. However, any data lacking a specific literature citation, and any value lacking the verfication checkmark, should be verified independently before being trusted.]
3. Touloukian, Y.S. "Twenty-five years of pioneering accomplishments by CINDAS--a retrospective review." Int. J. Thermophysics 2, 1981, 205-222.  DOI:  10.1007/BF00504185

When you do a thorough literature search in SciFinder for physical and thermodynamic property data, you will probably notice that many of the earlier, more foundational studies on chemical species were published in Russian language journals.  During the early decades of the Cold War, Soviet scientists collected huge amounts of experimental data that were available nowhere else.  Starting in the 1950s some Western publishers issued English translations of a number of core Soviet technical journals (primarily in chemistry, physics and engineering), either cover to cover or selectively.  After the breakup of the Soviet Union in the early 1990s the quality of Russian science declined precipitously and these journals are no longer as important.

Chemical Abstracts indexed these journals fairly thoroughly, from the original Russian versions, with translated titles and abstracts.  Until 1995, CAS indexed only the Russian original and did not refer to the translation at all. This was a policy based on timeliness (the translations appeared months after the originals) and completeness. The translation journals were not always complete, and sometimes omitted letters, short communications and editorial matter.  After 1995 CAS began to index selected translations instead. Bibliographic citations and CA references to the original Russian titles carry Russian edition page numbers, which are different from the English edition page numbers. The table of contents of each English issue provides side-by-side page numbers for comparison. Titles that contained the words "Soviet" or "USSR" were changed around 1992.

When you locate a Russian language journal reference, the first step is to determine if there was an English translation of that journal.  The CASSI database is the best place to be sure about it - this is the master list of all sources that CAS has indexed over the decades, with vital bibliographic information about dates, languages, and translations.  You can also search the Russian journal title in our Library Catalog, as it will be included in our records for the equivalent translations.  Here are the most frequently encountered Russian chemistry journals: