Science History Punctilio: The Free Solvated Electron Was Discovered Before its Mass Was Determined.

[NNadir]

I've been thinking a lot about approaches to defluorinate the very troublesome perfluoro acids and sulfonates, which are turning into some very serious environmental issues owing to their wide technological use and their chemical inertness - the latter accounting for their wide use. Being who I am, this interest is driven by a possible application for radiation. Maybe I'll write about all of this later this weekend.

It turns out that the active species for defluorination of fluoroalkanes in solution - specifically the troublesome case where they contaminate waster supplies, which they do, and which is becoming a real health concern - is the solvated free electron. In a bout of intellectual laziness, I have long assumed that the active species was the hydride or hydroxide radicals, but on checking my assumptions I was wrong.

Anyway, this all inspired me to read more about solvated electrons, of which I've been aware for most of my adult life, but about which I didn't think all that much. So to read about solvated electrons, I pulled up a nice little review paper, this one: The Hydrated Electron (Herbert and Koons, Annual Review of Physical Chemistry Vol. 68:447-472 (2017)).

It's open sourced, anyone can read it.

Anyway, a fun thing to do in chemistry is to dissolve an alkali metal like potassium or sodium in liquid ammonia which gives a beautiful chemistry. When I was a lazy kid, more interested in very strong bases for organic synthesis, I was more interested in the salts one can obtain by evaporating the ammonia, sodium amide, etc, etc, but the solution that forms is a solution of free electrons, not sodium or potassium amide salts.

The paper has this very interesting note that this fact discovered only a few years after J.J. Thompson proved the (postulated) existence of the electron, and several years before Millikan showed what the mass of the electron was in the famous oil drop experiment.

Here's the text from the cited paper:

...Following the liquefaction of ammonia, the blue color of sodium/ammonia mixtures was noted by Weyl in 1864 (2), who attributed the blue color to formation of a chemical compound, NaNH3. This idea held sway for some time until convincing evidence against it was finally presented by Kraus in 1908 (3), in experiments originally intended to test the idea that electrons are the charge carriers in metallic conduction (4). The existence of dissolved ions as the charge carriers in electrolyte solutions had been established much earlier by Kohlrausch (5), but in Kraus's view, “knowledge of the solid state of matter… is far too limited to enable us to determine the nature of the processes which are specifically involved when electricity passes through a metal” (4, p. 1558), and he supposed that solutions of metals in nonconducting solvents might provide simpler systems on which to test theories of electrical conduction in metals.

To this end, Kraus measured the electrical conductivity of solutions of alkali metals dissolved in liquid ammonia, and already in the first of these papers (in 1908), he proposed that the charge carriers were “electrons surrounded by an envelope of ammonia” (6, p. 1332), i.e., solvated electrons, formed via the dissociation equilibrium Na⇇Na++e−. This inference seems all the more profound when one considers that the nature of the electron as the charge carrier in cathode ray tubes had been established only about ten years earlier (7), and Kraus's 1908 paper predates the publication of Millikan's oil drop experiment (8)...

OK, I'm a dork, but I find this very cool.

If one must be locked down, thinking about this kind of science history is better than watching reruns of the movie, The Stand based on Steven King's book.

I think so anyway.

I hope you'll have some fun this weekend.

[Bernardo de La Paz]

1. Rarely is mass of something determined before it is discovered. . . . nt

[NNadir]

2. Thank you. I seem to have had a typo in the title, which I fixed.

The electron was postulated before it was discovered, but while the discovery did not fully characterize its mass, only its charge to mass ratio.

[Bernardo de La Paz]

3. You fixed the double paste, but not the logical error. . . . nt

[NNadir]

4. Ok, whatever. I don't see a logical error, but if you do, you're free to enlighten me.

Or, you can avoid bothering. I really am not interested.

This post was designed around a throw away "punctilio," for fun, not a picayune quiz game. I was simply noting a historical point that the nature of solvated electrons was discovered very early in the history of understanding electrons.

If there's some deeper meaning, I don't get it.

Jeezus.

[Bernardo de La Paz]

5. As I previously stated: "Rarely is mass of something determined before it is discovered."

It is hard to know how much an oompha loompha weighs until one is discovered.

How much does a unicorn weigh?

[eppur_se_muova]

6. The *solvated* electron was discovered, before the electron was weighed.

A little confusion re. the antecedent of "its".

[Bernardo de La Paz]

7. Thanks. It seemed it would have been easy to make that kind of clarification, but .... . . . nt