Wikipedia:Reference desk/Archives/Science/2021 July 13

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July 13

Hydride anion in aqueous solution

In Pourbaix diagrams for hydrogen, the hydride anion H⁻ is shown as a stable species in aqueous acidic solutions at a voltage of less than −2.25. What form does the hydride anion take? For example, the hydrogen "cation" is reckoned to actually be H₄O₉⁺ rather than H⁺. Sandbh (talk) 06:24, 13 July 2021 (UTC)

doi:10.1063/1.2778423 is the first relevant-looking ref I found. Not my speciality, so I don't know how good its science is. Did you mean H₉O₄⁺ for the aqueous-H⁺ form? DMacks (talk) 06:46, 13 July 2021 (UTC)

That reference is purely theoretical. Any time I've added sodium hydride to water there was an instantaneous reaction generating hydrogen and sodium hydroxide and the sodium hydride article says (correctly in my opinion) that H⁻ ions do not exist in solution. I may be missing something but the Pourbaix diagram article doesn't appear to show H⁻ as a stable species under any condition. Mike Turnbull (talk) 11:42, 13 July 2021 (UTC)

DMacks: I was referring to the H⁻ anion rather than H₉O₄⁺. The theoretical article you found refers to the hydride anion being hydrated by up to six water molecules and represents this as H⁻(H₂O)_n. On the other hand, in the literature more generally, the hydrated proton is shown as e.g. H₉O₄⁺ rather than H⁺(H₈O₄). I don't understand the basis for the different depictions. Why not show the hydrated hydrogen anion as H₃O⁻ or (H_2n+1O_n)⁻?

Mike: Yes I understood that to be the case too, wrt to adding NaH to water. Pourbaix's Atlas of Electrochemical Equilibria in Aqueous Solutions is available from here. The potential-pH equilibrium diagram for the system hydrogen-water, at 25°C is at p. 112. At pH 0 the H⁻ anion is shown at a voltage of ca. less than −2.45.

I may be missing something too. Sandbh (talk) 01:32, 14 July 2021 (UTC)

I know exactly what you meant and am quite familiar with the difference between cations and anions. But when discussing H+, you wrote "H₄O₉⁺" (note swapped subscripts). DMacks (talk) 02:25, 14 July 2021 (UTC)

My bad, sorry. Sandbh (talk) 05:32, 14 July 2021 (UTC)

There's an example of such a Pourbaix diagram here. But other diagrams for water instead state that in those conditions, water itself is getting decomposed with liberation of hydrogen (e.g. Pourbaix_diagram#The_stability_region_of_water). Which is exactly what hydride does... Double sharp (talk) 03:43, 14 July 2021 (UTC)

That diagram is for water. Pourbaix (1974) gives separate diagrams for water; hydrogen peroxide in water; and hydrogen in water. Wulfsberg (2000, Inorganic chemistry, p. 247) similarly shows H⁻ as a stable species in aqueous solution, at 2.25 V, at pH 0. Sandbh (talk) 05:32, 14 July 2021 (UTC)

More IR and UV emission questions.

Somewhat a continued follow-up of a previous questions that I will link, I will try to make this more about emission than about absorption. I want to narrow down to 4 categories:

• absorbing then emitting IR, from thermal radiation
• absorbing then emitting IR, from electronic transitions
• absorbing then emitting UV, from thermal radiation
• absorbing then emitting UV, from electronic transitions.

Thermal radiation being changes in kinetic energy (translation, rotation, rotation). Also Jayron had said that through electronic transitions, happens only at specific discrete wavelengths, so for my purposes, let's not make them overlap. So my earlier analogy of clouds absorbing/emitting IR, falls under the 1st 1? Where clouds essentially emit 100% of the IR they absorbed? OurosboroCobra said something about #2 is not that common, and so I think if it does happen, the % emitted is not nearly as high as absorbed? And then, plants being good UV absorbers (but not emitting), is #3 or #4? Jayron also said that "Most substances (possibly all) have UV spectral series in their electron transition analogous to the Lyman series of hydrogen, and they will absorb and emit at the exact same wavelengths." I believe this is referring to #4, so for #3, so they emit at a more-different wavelength than absorbed? Here's the old discussion Wikipedia:Reference_desk/Archives/Science/2021_May_29#Chemistry_questions. Also for the newcomers feel free to answer any older questions here. I also see Jayron has been inactive for almost a month now, I hope he is doing well. 67.165.185.178 (talk) 07:21, 13 July 2021 (UTC).

I wasn't here for the last round of questions. I'm not really sure where to begin with this but I'll just through some stuff out there that doesn't seem like it was mentioned before. A lot of this depends on what kind of compounds we're dealing with and in what phase of matter because thermal equilibration happens faster in condensed phases. Absorption and emission bands are also much broader in solids and solutions than in the gas phase. I think you're right about clouds absorbing and emitting IR falling under category 1. Water absorbs IR in its vibrational absorption bands (and overtones in the near IR) and then releases the energy by emitting IR and by bumping into other molecules and transferring energy. Electronic absorption bands in the IR are rare. Ce3+ compounds are the only ones I know of.

I don't really understand what you mean by #3 and #4. Do you mean absorption of thermal energy and then emission of UV for #3? Or are you asking whether vibrational absorption bands can be found in the UV? Normally UV bands are electronic bands, but I suppose in principle high harmonics of infrared bands can also be found. The chlorophyll in plants absorbs strongly in the UV and less strongly in the visible (look up Soret band). Most of the absorbed energy is lost as thermal energy but some is fluoresced (meaning there is a low quantum yield). The emitted light is always lower in energy than the absorbed energy (see Kasha's rule). UV or visible light absorbed by chlorophyll is emitted as near-infrared light, I believe.Pelirojopajaro (talk) 09:44, 13 July 2021 (UTC)

Okay maybe you can answer this earlier question: for fluorescence and phosphorescence, both of those cases are for emitting radiation at a longer wavelength than absorbed. Is there a other-way around, and what is it called? (Where you emit radiation at a shorter wavelength than absorbed.). 67.165.185.178 (talk) 10:30, 13 July 2021 (UTC).

What you're looking for is an Anti-stokes shift or Photon upconversion Pelirojopajaro (talk) 12:02, 13 July 2021 (UTC)

As far as what do I mean for #3 and #4, Jayron had said earlier there is no "Will something absorb X and emit Y" generalization because that there were 2 factors: electronic transitions, and thermal radiation (or change in kinetic energy). Meaning, they can go in opposite directions. However, since he said electronic transitions go in specific and discrete wavelengths, then I thought to separate it out to 1 situation at a time. This is kind of like how putting compounds in water, you got solvation and dissociation, 1 can be exothermic and 1 can be endothermic, but you look at the net gain on which was bigger. I realize my 1st question for #1 and #2 was about emitting 100% of what you absorbed vs. not, whereas my 2nd question for #3 and #4 was about emitting at the same wavelength or farther, from what you absorbed. You can certainly criss-cross those questions, I'm curious to know the % emitted for UV for situations 3 and 4. 67.165.185.178 (talk) 10:40, 13 July 2021 (UTC).

I still don't really follow. Any excited system will seek to lose its excess energy and return to equilibrium. Different systems have different ways of doing this. The thing about electronic transitions being discrete wavelengths only really applies to atoms in the gas phase. They don't have vibrational degrees of freedom, so they have fewer ways to lose the energy they gain after absorption. Pelirojopajaro (talk) 12:00, 13 July 2021 (UTC)

Okay, my question was for plants and metal oxides. Metal oxides being the primary ingredient in sunscreen. Plants and metal oxides are big absorbers of UV without immediately emitting them right back out, right? Are there examples of something that absorbs and emits the UV, like clouds absorb and emits the IR? 67.165.185.178 (talk) 00:28, 14 July 2021 (UTC).

Ah okay. Laser dyes are an example of something that absorbs visible light and emits it again quickly and efficiently, maybe there are UV ones too. Rhodamine 6G for instance has a quantum yield near 100% and a very small stokes shift. Pelirojopajaro (talk) 07:04, 14 July 2021 (UTC)

There are several laser dyes that emit UV. It seems they are commonly based on the terphenyl framework. Terphenyl itself is tunable 322–360 nm as pumped by various excimer lasers, with a maximum absorbance at 275 nm and maximum fluorescence at 339 nm ("Lambdachrome Laser Dyes" catalog). DMacks (talk) 08:43, 17 July 2021 (UTC)