Wikipedia:Reference desk/Archives/Science/2020 June 7

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June 7

Mothballs

Our article on Mothball says:

"Naphthalene and 1,4-dichlorobenzene should not be used together because the mixture may cause damage to items being preserved.:

References

That page does not exist but the wayback machine has a copy at [1] which says

"Two of the most widely used fumigants are paradichlorobenzene (PDB) and naphthalene, both of which are obtainable in balls or flakes. Never mix PDB with naphthalene as they react chemically and produce a liquid that may damage the collection."

Is this an urban myth? I just happen to have some of both kinds of mothballs, so I put one of each in a small glass jar. I am not seeing any liquid. I also would like to know what the liquid that supposedly forms is. What is the chemistry of the reaction?

I am hoping to find a better citation that explains the chemistry involved so I can edit the page and add those details. --Guy Macon (talk) 15:11, 7 June 2020 (UTC)

If said adverse effect exists, then the advice given here is particularly unsound.  --Lambiam 15:29, 7 June 2020 (UTC)

Even if they don't react chemically (forming new chemicals), they would form a liquid together due to melting-point depression. The eutectic mixtures have mp well below room temperature (see doi:10.1016/j.jct.2008.10.005). These liquids could dissolve paint, and seep into objects, stain or alter other physical properties. DMacks (talk) 22:55, 7 June 2020 (UTC)

Correction...p-DCB is the one DCB isomer whose eutectic with naphthalene is solid rather than liquid at coolish RT...thanks Guy for pulling out the data values. DMacks (talk) 04:52, 8 June 2020 (UTC)

Our article defines a "eutectic system" as having a lower melting point than the constituents, but from a cursory examination it appears to me that the characteristic is not that the melting point is lower, but that the components melt (and on cooling solidify) together at the same temperature. Perhaps we should use a more recent source than one from 1884 (although this may be the coining publication).  --Lambiam 06:39, 8 June 2020 (UTC)

That explains the lack of liquid in my mothballs-in-a-jar experiment. I doubt that the mixture is just right to be eutectic.

Please tell me if I made a mistake in the following:

• Naphthalene = Freezing point at 351.3K/78.2C/172.8F) (Naptha mothballs)
• p-dichlorobenzene/1,4-Dichlorobenzene = Freezing point at 326.6K/53.45C/128.21F (Para mothballs)
• m-dichlorobenzene/1,3-Dichlorobenzene = Freezing point at 248K/-25.15C/-13.27F
• o-dichlorobenzene/1,2-Dichlorobenzene = Freezing point at 256.12K/-17.03C/1.346F
• naphthalene + p-dichlorobenzene = eutectic point at 302.85K/29.7C/85.46F (Para + Naptha mothballs)
• naphthalene + m-dichlorobenzene = eutectic point at 244.85K/-28.3C/-18.94F
• naphthalene + o-dichlorobenzene = Incongruent melting with eutectic point at 232.55K/-28.3C/-18.94F and peritectic point at 250.15K/-23C/-9.4F

Note: for substances that have different melting points and freezing points I listed freezing. That's the temperature at which you can be sure that no liquid forms to damage things.

So,

• Napthalene mothballs are solid below 78C/172F
• Para-dichlorobenzene mothballs are solid below 53C/128F
• Just the right mix of Para and Naptha gas is solid below 29C/85F

Those are all pretty low temperatures for something that might be used in a trunk stored in a hot attic.

Anyone who has used them knows that para balls fill the space with gas quicker and disapear sooner than naptha balls. So if you mix them, at first para gas dominates and later naptha gas dominates after the para balls disappear. Somewhere in between would come the moment when they are eutectic.

Final question: clearly both kinds of solid mothballs sublimate gas at far below the listed freezing point. And I noticed that a glass jar of para mothballs I stored in a hot garage has crystals on the glass wall, leading me to believe that at saturation it can go back from gas to solid without forming a liquid.

Clearly if the balls melt that could cause liquid damage. But how do I calculate the points at which para gas, naptha gas, and para plus naptha gas that came from solid balls condense into liquid? --Guy Macon (talk) 02:18, 8 June 2020 (UTC)

For a mixture's gas/liquid change, you might have to consider azeotrope effects. DMacks (talk) 04:09, 9 June 2020 (UTC)

This is a bit of a tangent but I have no trouble believing that as a generic statement, though of course better references are always good. Looking into historical preservation methods might be fruitful even if they don't focus on mothballs specifically. Preserving things like paper long-term is quite tricky as many things will slowly react with paper/ink and degrade it over time, especially if it's not archival paper to begin with. --47.146.63.87 (talk) 16:45, 8 June 2020 (UTC)

So, now I have a quandary. The claim in the article is almost certainly wrong. It would be almost certainty correct to say that para, naptha and para+naptha all have a possibility of melting or condensing into a liquid state and damaging certain kinds of item but they differ as to the temperature where this happens. Alas. the almost-certainly-wrong claim that only the para+naptha carry this risk is cited to a reliable (and now deleted but without any actual retraction) source: the United States Department of Agriculture Agricultural Research Service. My belief that the source was wrong is based upon original research. Just last month I removed some original research where an editor came to the conclusion that the holocaust was faked. Why should my original research be treated any differently? Because I am right and he was wrong? He thought he was right too. What to do, what to do? --Guy Macon (talk) 13:27, 10 June 2020 (UTC)