Molecular Monday: The Imaginary Sulfide Ion

Just when I feel like this chemistry stuff is starting to make sense to me, I learn something new that makes me feel like I don’t understand it at all. Maybe I’m not the only one. Maybe even the professional chemists feel the same way sometimes. With that in mind, it’s time for another episode of Molecular Mondays.

This month (February, 2018), a paper was published by the Royal Society of Chemistry that casts doubt on a longstanding assumption made by chemists. It involves the S-2 ion in aqueous solutions.

First off, this may have been the most amusing scientific paper I’ve ever had the pleasure of reading. No, the authors didn’t make any references to unicorns, but they did mention something about fairies at the bottom of a well, and then there was this delightful quote: “[…] there has been a growing awareness amongst solution chemists that the S-2(aq) emperor may have no clothes.”

As I understand it, the existence of an aqueous S-2 ion is the kind of thing that makes sense on paper. It allows chemists to easily balance their chemical equations, and it’s been included in textbooks and chemical computer databases for so long now that everyone just takes it for granted that the thing exists.

But apparently the experimental evidence of this particular ion was lacking, and aqueous solutions incorporating several different sulfide compounds did not produce any S-2 ions. At least not according to a Raman spectroscopic analysis.

Now as I’ve written before, papers like this should NOT be interpreted as final proclamations handed down from the ivory tower of science. Rather, this kind of paper should be understood as the beginning of a conversation among scientists. Does this S-2(aq) ion exist or not? If not, how many prior scientific studies need to be reexamined?

There will be further research, and perhaps a rebuttal will be published. Then there will be a rebuttal to the rebuttal, and so forth. But I think, regardless of how this plays out, that this is a good reminder that in science—as in life—what makes sense on paper does not necessarily work in the real world.

8 thoughts on “Molecular Monday: The Imaginary Sulfide Ion

  1. Hmmmm….. I haven’t seen the paper, but I admit to a little skepticism. I understand that sulfide mainly exists as HS- in aqueous solution (depending on the pH), with only small amounts supposedly in the form of S2-, but too many well established equilibrium equations simply wouldn’t work if there wasn’t some kind of species like S2- present. Time will tell.

    Liked by 1 person

    1. I’m really glad to get your perspective on this. The article I originally read form Chemistry World made this sound like a really big deal, and the paper itself touts itself as a really big deal, but I don’t really have the knowledge or experience to judge for myself how much of a big deal this actually is.

      Even so, I’m pretty skeptical. As Sagan said, extraordinary claims require extraordinary evidence. And if this claim is as extraordinary as it sounds, then there should be a lot of follow up research, discussion, and debate to come.

      I’m guessing, since this paper was published just this month, that it’s too early for that, but I hope a debate of some kind will come, and I hope I’ll be able to follow along.


  2. Well on the face of it spectroscopic analysis has drawn a complete blank on corroborating the existence of S-2 ions which scientists have so far taken for granted. I wonder now. Who wrote the conflicting paper? because theres going to be hell to pay.

    Liked by 1 person

    1. It sounds like the idea of these S-2(aq) ions has been around for a really long time now, so maybe whoever came up with the idea is long gone by now. I don’t really know. I just know I’ll be really interested to see what happens next.


  3. I don’t know if this is a proper analogy, as I am not a chemist, but this sounds similar to the matter of the imaginary number i, the square root of -1. i is obviously not real whereas these sulfide ions may be, but still they are satisfying the needs of equations.

    Liked by 1 person

    1. I sort of thought of it the same way, which is how I came up with the title for this post. I think the problem is that if S-2(aq) doesn’t exist, it means something else must exist in its place. It would be like if the square root of -1 turned out to actually equal 0, and then we had to fix all our previous math.

      Liked by 1 person

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