physical chemistry

Separating the lanthanides: physical versus chemical methods?

How do you separate dirt?

Photo credit: Reuters**

There has been much talk about rare earth metals recently. In short, the People’s Republic of China has become the dominant source of rare earth* elements in the world; the PRC government has used that fact to their strategic advantage. I don’t really wish to get into the political debate; suffice it to say that I think there’s more smoke than fire here and that predictions of war are probably overblown.

There are quite a number of articles on the subject, but only one talked about the chemistry. I was struck by a quote in an article on ForeignPolicy.com by Tim Worstall, a trader in scandium and other rare earths (now there’s a job I didn’t know about):

Another possibility is that we find a new and different way to separate rare earths, as we find new and different sources for the ores. The main difficulty is that chemistry is all about the electrons in the outer ring around an atom, and the lanthanides all have the same number of electrons in that outer ring. Thus we can’t use chemistry to separate them. It’s very like the uranium business: Separating the stuff that explodes from the stuff that doesn’t is the difficult and expensive part of building an atomic bomb precisely because we cannot use chemistry to do it — we have to use physics.

It’s quite apparent that Mr. Worstall is referring to the unusual electronic configuration of the lanthanides, where the 4f orbitals are ‘hidden’ behind the 4d and 5d orbitals. This electronic configuration is also responsible for the lanthanide contraction, in which the atomic radii of the lanthanides are smaller than predictable by periodic trends.

However, I’m not quite sure what Mr. Worstall means when he draws a distinction between chemical and physical separation of the elements. Both this article (from Oxford) and the Wikipedia article on the lanthanides suggest that countercurrent exchange methods are used on industrial scale; it appears that separation is performed by means of ionic radii and size. While this certainly doesn’t rely on the reaction chemistry of the lanthanides (because it appears they all act similar), I have a difficult time calling these techniques physics-based.

Readers, can you shed any more light on the issue? Do you agree with Mr. Worstall’s distinction between chemical and physical means of purifying elements?

*It should be noted that the rare earths are, as they say, neither rare or nor earths.
**Photo from this International Business Times article.

Puzzling polymorphs

Polymorphism is a common and sorta crazy issue in pharmaceutical process chemistry. Basically put, a drug molecule in the solid state can have multiple crystal forms. Different impurity profiles and different crystallization techniques (solvents, heating/cooling rates) can produce different polymorphs, which can have wildly different physical properties and bioavailabilities. A famous story of troublesome polymorphism is Abbott’s ritonavir, where in the middle of manufacturing for sale (not during the R&D phase!), a new, much less soluble polymorph started showing up in batches. Moreover, once the new polymorph showed up, it was very difficult to generate the previous polymorph. Even crazier, a team of scientists went to another plant in Italy where the process was still working as desired, and soon after the team left, the new polymorph appeared. It took a crash program to understand which conditions were generating the new crystal form to get it under control.

A recent article by Pradash et al. in Organic Process Research and Development illustrates the problems of polymorphism similarly: once the authors determined that there was another crystal form (‘Form A’) than the original (‘Form B’), they undertook a screening process (looking at varieties of solvent and crystallization techniques) to find other polymorphs. Interestingly, once they discovered a new polymorph (‘Form C’), they found that it was impossible to generate Form B in their laboratories. They selected Form C for its physical properties and moved it into the pilot plant; lo, they then found Form D. This new crystal form began predominating and “those seeded crystallization processes that consistently produced Forms A and C started to produce predominately Form D in the laboratory.” (Click on image to see pictures of the polymorphs and the structure itself.)

When I read these accounts, I am filled with admiration for pharmaceutical process chemists, the interesting science that they get to do and the vast reserves of patience and sangfroid they must have.  Chemistry (and manufacturing chemistry, especially!) is based on reproducibility and consistency; when issues arise, I suspect that there is a great deal of checking and double-checking to make sure that “this is really happening to us.” Also, I can’t help but wonder if those process chemists, when these issues are discovered, wonder if the laws of the physical universe are being temporarily suspended and some Loki-like diety is having its way with them.

Electroneutrality is dead?

pollack
Gerald Pollack

That is the highly controversial claim made by Kate Ovchinnikova and Gerald Pollack in Langmuir earlier this year.[Langmuir] Electroneutrality is a guiding principal in electrochemistry and is a method to understanding electrolytic cells (Pt electrodes in dilute aqueous NaCl solutions). It stipulates that any charge imbalance across an electrochemical system is quickly (~ns) balanced by the salt present in the water being driven by the electric field in such a way to neutralize that charge imbalance. Thus the need for salt bridges and all that wonderful G-chem stuff we have learned. There is even a cool little applet you can play with electroneutrality by the Harvey Project. When I tried to sit down with electrochemists to discuss the claims by O&P they quickly dismissed them out of hand after reading the beginning of their paper. So the big question is, did O&P stumble across something amazing or did they spectacularly overstate the results of their experiment.

I can summarize their paper succinctly:

electrochem setup
  1. Insert electrodes into electrolytic cell
  2. Turn on power supply
  3. Disconnect the electrodes from the circuit
  4. Remove the bridge between beakers
  5. Reconnect electrodes to measure residual charge in the two beakers.

The design seems thoughtful enough, but before I get into the merits of their results I need to take time to mention a few gems in their paper. Here is a quote from them.

Bubble formation occurred in all experiments (n > 20), although position and growth rate were inconsistent. In most cases, formation began during the charging phase and continued through discharge. Characteristics of bubble formation were not pursued in any detail, but may warrant future study.

But it doesn’t warrant further study,  all chemists know where their bubbles came from.

$$ \text{Cathode: } \text{H}_2\text{O} + 2\text{e}^- \rightarrow 2\text{HO}^- + \text{H}_2$$

$$ \text{Anode: } \text{H}_2\text{O} \rightarrow 2\text{H}^+ + \frac{1}{2} \text{O}_2 + 2\text{e}^-$$

usb-6009

An other eye catcher is that they didn’t use a standard electrochemical setup. They used my trusty NI USB-6009, I know that product well as a chunk of my thesis was acquired with it. It doesn’t make the experiment invalid, but why use crap when you are trying to disprove such a time honored concept as electroneutrality. Maz and I know from experience that the USB-6009 floats if their isn’t a sufficient load on it or if their isn’t an appreciable external voltage.

Here is a quote from them contemplating that HCl solutions have an overall positive charge.

One might speculate, for example, whether ordinary acidic solutions, which have low pH, might contain net positive charge, while ordinary basic solutions might contain net negative charge.

So far everything has been “quirky”, it isn’t until the end when you perceive something really odd.

Water appears able to adopt two structural networks that have mirror symmetry to one another. The fact that these networks are macro phenomena deserves further study.

A second and related issue is the potential for disturbance of these structural networks. It is now established that when water is left standing for long periods, it develops thixotropic properties, implying macrostructure.7 Such macrostructure is expected to be fragile. The fact that removing and inserting electrodes did not apparently ruin the charge-containing structure implies that, once formed, the structural network can re-form rather readily. This is an additional subject requiring further study.

7:Vybiral, B. Water and the Cell; Pollack, G. H., Cameron, I., Wheatley, D., Eds.; Springer: New York, 2006; pp 299-314.

It is with that last statement you say to yourself, “Oh, I get it. This is a homeopathy paper.” Water being able to adopt structures of the solutes that were dissolved in it is a hallmark of the quackery that is homeopathy. O&P’s claim isn’t that bold, but it has hints of the same idea. Claiming macrostructures (~mm) of water that extend past the picosecond domain is absurd.

Although I haven’t discussed the results of their paper, would you really trust it anyways?

Horacio Corti and Agustin Colussi have done an excellent job dissecting the technical irregularities of the paper and I encourage you to read their comments on the article (link below). If you come to a different conclusion or find me in error, please leave a comment and join the discussion.

Links

Mitch

By September 3, 2009 4 comments opinion, physical chemistry

Survivor: Mechanisms (now accepting logo submissions)

gibbsfreepassI read an interesting article in May’s issue of J. Chem. Ed. titled “Can Reaction Mechanisms Be Proven?” by Allen Buskirk and Hediyeh Baradaran of BYU.  Intriguing.  So I pop open the pdf and a Note from the Editor is boxed at the top of the page before the article starts.  It says:

“Can Reaction Mechanisms Be Proven?” generated spirited responses from its reviewers. The reviews were approximately evenly divided, and all were of very high quality. The authors agreed with the editor’s proposal that the reviewers convert their reviews into rebuttals or affirmations of the authors’ position for publication along with the article, which has been revised based on the reviews. Most agreed to such a process and their comments appear here. We hope that publication of this paper and well-reasoned rebuttals such as those provided here will initiate a wide-ranging discussion. JCE will provide an online forum for further discussion of the issue. Our hope is that both faculty and students will contribute their opinions and ideas to this discussion. -JWM

Huh.  You don’t usually hear about that happening too often.  So now I had to read the article.  It’s pretty fascinating, and I encourage you to read it all.  I’ll summarize and give my thoughts below the jump

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