The IUPAC ranges for atomic masses are NOT useful

While there is considerable excitement about the 7th period of the Periodic Table being completed, I've been thinking about more mundane matters about the table.

I was recently reminded that for many of the elements on the periodic table, IUPAC has assigned ranges and not specific values to the atomic masses. I wrote about this 5 years ago and haven't done anything with it since, so I took some time over break to look into the matter further. Being an organic polymer guy, I started with carbon, since on an atomic mass fraction that element makes up the largest component of most commercial polymers. The range for this element is between 12.0096 and 12.0116 amu, which is pretty tight, about 0.017% if my math is correct. This is beyond the accuracy with which we can measure molecular weights of polymers, so even worrying about the impact on the molecular weight of ultrahigh molecular weight polyethylene (UHMWPE) is not a concern.

But I was also struck by the plot at the bottom of the IUPAC page, which is reproduced here:

This plot shows how range of atomic masses for carbons strongly depends on the source. The largest range is in marine sediments, while other sources of carbon, such as crude oil, have significantly smaller ranges. But ironically, rather than providing justification for a range, this plot completely undermines the efforts.

I completely understand the importance for knowing these variations for fields where extremely high accuracy is needed, and I would imagine that any researcher needing to know these variations would be made aware of them very early on in their research efforts. But very few if any researchers are going to be working with carbon sources that cover this vast range. Most organic chemists are going to be working with carbon supplied from crude oil and nowhere else.

So this then begs the question of whether the ranges supplied by IUPAC are of any value. If you are like most chemists, 12.011 amu, the previous standard atomic mass for carbon is likely to be adequate enough. But if it isn't, then the new range of 12.0096 - 12.0116 amu is unlikely to be of any value either. Yes, the range is large enough to cover all the various carbon sources, but is it really necessary to have a range when a single value is sufficient for most work? And for researchers that need to understand to consider a range within their samples, any of the individual ranges in the figure above would be more helpful than to use the entire range of values supplied by IUPAC which is all inclusive.

A good chemist is going to use the range of atomic masses that makes the most sense for them. The IUPAC range is not clarifying the matter, only muddying the waters.

Previous Years

January 6, 2015 - Weeding-out Engineering Students

January 6, 2014 - How Cold Is It???

January 6, 2012 - Free Access to Articles in the Polymer Literature

January 6, 2010 - Polycarbonate (and BPA and Phosgene)

Elemental Sulfur as a Monomer

It's been far tooooo long since I commented on a research article, but it's time to change that. Angewandte Chemie has an open access Early View article on a new elemental sulfur/limonene polymer. The researchers are from Flinders University (South Australia) and the research has received quite a bit of press since it is able to capture mercury ions (Hg²⁺) from water, and as a bonus, changes color it does so. A further bonus is that sulfur is a waste product of the petroleum refining industry and limonene is a by-product of the citrus industry, (although somewhat more valuable than elemental sulfur).

The reaction is straightforward:

Simply melt the sulfur, add the limonene and wait. The sulfur rings upon heating break apart and form thiyl groups which react with the unsaturated bonds in the limonene. This is remarkably similar another sulfur/organic copolymer ($) that I blogged about 2 years ago (1, 2), which took the same approach of heat and dump. In both cases, the unsaturated organic compound needs to be low enough in volatility that the hot sulfur (> 170 ^oC) doesn't evaporate too much of it off.

As is shown above, there are stretches of sulfur-sulfur (sulfide) bonds between the organic segments, and these are what capture the mercury (hence the old name for thiols of "mercaptans"). I did find it somewhat surprising that the amounts of sulfur:liminone were 50:50 on a mass basis.

"An equal mass of sulfur and limonene was chosen to maximize the content of both industrial by-products in the final material."

The authors themselves note that sulfur is produced at nearly 1000x the rate of liminone, so maybe trying to incorporate a little more sulfur would be a good idea. Additionally, the formula weight for liminone is 136 g/mol while that of sulfur (S₈) is 256 g/mol, which further skews the ratio of the comonomers towards the limonene.

I also wonder how long researchers are going to keep running these reactions in open flasks. The engineer in me is makes it really difficult to not scream at the monitor about running the reaction under pressure so that the volatility would no longer be a concern. Or maybe in a twin-screw extruder, for continuous production of the polymers. That would make for a great patent, wouldn't it. Or maybe I should say "would have"...

Previous Years

October 28, 2013 - SoBe, What Were You Thinking?

October 28, 2010 - A Foreign Body

October 28, 2010 - Is this safe to eat?

Nobel Prizes and Polymers

This year's Nobel Prizes are being announced this week. The Chemistry Prize is announced on Wednesday and I doubt that it will go to anyone researching polymers. Since the first prize was given out in 1901, only 4 times has it been given to polymer scientists: Staudinger in 1953, Ziegler/Natta in 1963, Flory in 1974 and Heeger/MacDiarmid/Shirakawa in 2000. You could argue that polymers were never in the running in 1901, as it was only in the 1920's that Staudinger began his crusade to have polymers recognized for what they are - macromolecules and not just colloidal associations as others would have argued. In that case, there were only 62 chances to win since 1953, so the odds are quite a bit better than they originally appear. (Coincidentally, only 4 women have also won the Nobel Prize in Chemistry, but that is a different discussion for another day.)

But regardless of the exact calculation, the slim number of prizes reinforces my long running argument that polymer chemistry is just a small fraction of the world of chemistry. Arguments that half chemists work in polymers have no merit. I ran some numbers earlier this year and came up with 8% or so as a more realistic number. This number in fact correlates rather well with the number of Nobel Prizes (4/62 = 6.5%).

Whoever does win, I do hope that their research can be easily explained to freshman. I have a whole class full of them and being able to include this as enrichment material on Wednesday morning would be wonderful. It's been a good year for making tie-ins with current events. The timing could not have been better for ytterbium to be reassigned a new atomic weight, since I had just been lecturing about how the fractional abundances of isotopes contribute to atomic weights, and suddenly there was this real world example. Similarly, the discover of flowing water on Mars (on the basis of sodium-/magnesium perchlorate) was perfectly timed with a lecture on electrolytes in water (thank you NASA!). So if the people in Stockholm could give me a heads up so that I have time to adequately prepare, I would appreciate it.

Previous Years

October 5, 2010 - On Units of Measure

October 5, 2010 - Plasticizers = Positive Drug Test?

October 5, 2009 - Duplicity and Siloxanes

October 5, 2009 - Public to Private and Back Again

Are Shape Memory Polymers "Plastic"?

Just a quick, semi-serious question to ponder: The word "plastic" means that something is moldable, that it can undergo a permanent change in shape. (This applies not just to polymers, but to metals and other materials as well. Blacksmiths specialized in the plastic deformation of iron). So consider shape-memory polymers, polymers that can be shaped but then when heated (or otherwise given energy) recover their original shape. Is it proper to call shape-memory polymers "plastic"? (1, 2 and 3.)

Previous Years

May 20, 2013 - That Didn't Take Long