Skip to content

ccSTP is not an SI unit for a reason. Use moles instead.

October 31, 2019

This post is about the unit of “ccSTP,” or cubic-centimeters-at-standard-temperature-and-pressure, that is used to measure amounts of gas. This subject has been on my list to write about for a while, but I am ashamed to say that even though this posting has been sitting around in draft form for a couple of years, I couldn’t be bothered to actually finish it and push the ‘publish’ button until Marissa Tremblay brought up the subject. Really, there was no collusion:  convergent evolution actually happens.

The important difference is, I get more than 280 characters to talk about it, so I can begin at the beginning. If you are not like me and you began your career as a noble gas geochemist when you were young and impressionable in graduate school, instead of backing into it later as an already-opinionated mid-career cosmogenic-nuclide geochemist, you have absorbed the fact that these units are used in a major fraction of the scientific literature having to do with noble gas geochemistry (which is voluminous, that is, the literature is, not the gas). Your brain is hard-wired to think in ccSTP. Or, even worse, “ncc” or “pcc”, which are hopelessly uninformative shorthand in which the nano and pico are added and the STP left off. OK, I get this — as gas measurement technology initially developed, measurements of pressure, volume, and temperature were the data actually used to determine how much of a gas was present or to prepare a standard for mass-spectrometric measurements, and folks wanted to keep the units of subsequent measurements linked to the source data. So, being able to use this unit fluently indicates that you are well-grounded in the history and fundamentals of your craft. There is some analogy to the secret handshake.

If you are like me, however, you think these units are terrible, for the following reasons.

  1. I have no idea what “standard temperature and pressure” actually is. Believe it or not, Wikipedia lists eighteen different definitions of standard reference conditions used by various entities and organizations. Before sitting down to write this blog entry, I was pretty sure that most uses of STP in the noble gas literature implied something resembling 1 atmosphere at either 0° or 25° C, probably 0°. But now, having read the Wikipedia entry, I am no longer so sure. In addition, Wikipedia adds the rather disturbing information that in 1982, IUPAC changed their definition of the “P” part of STP from 101.325 kPa (1 atm) to 100 kPa (almost 1 atm). Did post-1982 geochemists know this? I probably wouldn’t have noticed for years.
  2. In cosmogenic-nuclide geochemistry, we think about pretty much everything in units of atoms. Concentrations in atoms per gram, production rates in atoms per gram per year, etc. So, in general, any measurements in the cosmogenic-noble-gas literature that are in ccSTP have to be converted to atoms or moles before one can do anything useful with them. However, the conversion factor to do this is deeply obscure. In fact, it is a testament to the profound obscurity of this unit that Google does not know how to do this. If I type “1 dram in jiggers” into a Google search box, I get the correct answer. “One furlong in cubits.” No problem. But “1 ccSTP in mol?” Total blank. It does not make me feel good about the societal relevance of noble gas geochemistry if no one at Google has even bothered to figure this out. On the other hand, perhaps they read the Wikipedia entry and they were too confused to proceed.

Hence, the rest of this entry will do two things. First, try to figure out what the ccSTP/mol/atom conversion factors are, so that even if we don’t really know what T and P were used in a particular paper, we ought to be able to get fairly close to the correct number of atoms. Second, encourage people to stop using these units. Just say no. Use moles or atoms, which at least have only one agreed-upon definition, instead.

So, conversion factors. First, to help future victims of typing ‘1 ccSTP in mol’ into Google, let’s include all the possible phrases one might want to search for. ccSTP in mol. ccSTP in atoms. nccSTP in mol. Convert ncc to moles. Convert ccSTP to moles. Convert ccSTP to atoms. That should give the Google index-bot a bit of a head start in finding this posting.

Even if Google doesn’t know about them, some references do give a conversion factor from ccSTP to mol. For example, we can consult page 6 of a fairly standard reference, the 2002 edition of Noble Gas Geochemistry by Ozima and Podosek. This page contains a table of constants that gives the conversion as 1 ccSTP = 4.465e-5 mol. OK, that is some progress, but what is STP? There are some clues in the table on this page, although it is not clear that they make things any clearer. First of all, the table reminds us that 1 atmosphere is 101.325 kPa, which makes one think that even though we are writing in 2002, we might be thinking about the pre-1982 IUPAC STP that used 1 atm instead of 100 kPa. Then the table also tabulates RT (the  ideal gas constant times the temperature) in units of cm3 atm mol-1, which also makes one think that we are using 1 atm as standard P. But the table gives RT for both 0° C and 25° C, which is rather disturbing. So, OK, if PV = nRT, P = 1 atm, V = 1 cm3, and RT = 2.24141e4 cm3 atm mol-1 (this is the value tabulated for 0° C), then n = 4.4615e-5, which is close to, but not exactly what is in the table. The difference between 4.465e-5 and 4.4615e-5 could be rounding error or a typo, but then there have to be at least two typos, because it is further stated to be equal to 2.688e19 molecules, which is, in fact, 4.465e-5 * 6.022e23. However, it does appear that these authors were, in fact, assuming 0° C and 1 atm; if we use the 25°C value in the table (2.44655e4 cm3 atm mol-1), we get n = 4.0874e-5 mol, which is much more different from the tabulated value. To the extent that this book is a good representation of the noble gas literature generally, at least we can be reasonably confident that most folks think STP is 0°C at somewhere near 1 atmosphere (even if it remains rather mysterious what the value for 25°C is doing in this table at all).

Of course, lacking an early indoctrination in the arcana of noble gas geochemistry, I may just never have been informed of a global conspiracy to stick with the pre-1982 definition, which would be almost exactly analogous to the global conspiracy among radiocarbon-dating specialists to retain the obsolete Libby half-life for data reporting purposes. However, one would think that Ozima and Podosek would have mentioned it. Regardless, let’s assume, even though we are probably wrong for a lot of papers in the existing literature, that we are talking about the current IUPAC STP, which is 100 kPa and 0° C or 273.15 ° K. We obtain the ideal gas constant from a different source (Wikipedia again) as 8.314462 m3 Pa K-1 mol-1. So, 1 cm3 is 1e-6 m3, 100 kPa is 1e5 Pa, and n = PV/RT = (1e5 * 1e-6)/(8.314462 * 273.15) = 4.4032e-5, which, again, is fairly close to but a couple of percent different from the value tabulated in Ozima and Podosek. Apparently this unit is not unlike the biblical cubit

Although the assumption that most authors were paying attention to the IUPAC in 1982 may be too optimistic, I would guess that this value is probably the most sensible thing to assume, so:

1 ccSTP = 4.4032e-5 mol = 2.6516e19 atoms

1 nccSTP = 4.4032e-14 mol = 2.6516e10 atoms

1 pccSTP = 4.4032e-17 mol = 2.6516e7 atoms = 26.516 million atoms (this is actually a useful amount for cosmogenic He-3 measurements)

These should at least get you within a few percent of the true value in most papers in the literature. If someone disagrees with this, or notices that I have made an error in the above, please let me know. And for heaven’s sake, if you are talking or writing about cosmogenic noble gases, use units of atoms or moles. You know what the pressure and temperature were when you made your gas standards, and we don’t, so please use this information to do the conversion to moles or atoms, yourself, now, before it becomes impossible later.





One Comment leave one →
  1. Brent Goehring permalink
    November 2, 2019 03:04

    Thank you. The radiocarbon community was this way for a while and I happy to say we have moved to real units. Grams of carbon and carbon ratios. Done. Molar volume as you found has many definitions.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: