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I was going through some displacement reactions when this struck me:

What is the position of the ammonium cation in the metal reactivity series?

I can find other positive ions like carbon and hydrogen in my textbook but not ammonium. I tried to look it up in google but the search results were not up to the mark.

I could get some comparison at this site. From there, it seems that ammonium will be above zinc, lead and iron.

This post also claims its position but does not refer it Do copper and ammonium hydroxide react?.

I know 'ammonium in the metal reactivity series' sounds a bit strange but what I basically want is its position among the metals and the positive ions. Can anyone here help me with this?

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    $\begingroup$ Metal reactivity series is for metals. I don't really think you can find carbon there. (Hydrogen is a special case.) Some metals reduce others and thus displace them from their salts. But you won't get any metal, if you reduce ammonium. $\endgroup$
    – Ivan Neretin
    Apr 9, 2020 at 14:14
  • $\begingroup$ It is possible, with the right conditions, to electrolytically produce ammonium amalgam, analogous to sodium amalgam. But analogies only go so far. $\endgroup$
    – Ed V
    Apr 9, 2020 at 14:34
  • $\begingroup$ @ivan you can sort of find carbon in a reactivity series -- for high temperature oxidation. Note how the entropy effect of forming gaseous oxides from a solid element causes the carbon to become a stronger reducer at higher temperatures, cutting through the other elements instead of occupying a fixed rank. Many metallurgical processes use this property. $\endgroup$
    – Oscar Lanzi
    Apr 10, 2020 at 16:36
  • $\begingroup$ @Oscar Yeah, sure. However, when people mention "the metal reactivity series", they typically don't mean that one. $\endgroup$
    – Ivan Neretin
    Apr 10, 2020 at 16:39
  • $\begingroup$ The reference given about Ammonium Hydroxyde is not valid, because Ammonium hydroxyde does not exist. It is a solution of ammonia in water. All reactions attributed to ammonium hydroxide are reactions of ammonia $\ce{NH3}$. Ammonium hydroxyde is a molecule that has invented in the 19th century to explain why NH3 reacts so violently with water. But if both ions $\ce{NH4^+}$ and $\ce{OH-}$ are present in solution, they must react by protolysis producing ammonia $\ce{NH3}$ and water, leaving only one or two percent of the initial ions in solutions. See later on. $\endgroup$
    – Maurice
    Sep 23, 2021 at 8:20

1 Answer 1

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Let's compare ammonium cation NH4+ and the neutral molecule, NH4 with other metals.

As Ed V pointed out, with a mercury cathode and low concentration of H+, NH4+ can be reduced to NH4 electrolytically (also with sodium amalgam), which forms an amalgam that is stable for a short period of time: the mercury bubbles up, then collapses, evolving NH3 and H2. Ammonium amalgam was discovered in 1808 by Humphry Davy and Jöns Jakob Berzelius. https://en.wikipedia.org/wiki/Amalgam_(chemistry)

A recent (2016) paper http://scripts.iucr.org/cgi-bin/paper?xk5030 gives the crystal ionic radius of ammonium as falling between 140 and 154 pm, depending on the anion; this compares nicely with the atomic radius of mercury which is 150 - 155 pm, so an alloy is not inconceivable.

Metallic ammonium (NH4) has been considered for some off-world chemistry in a 1954 paper. The phase transition of mixed crystals of hydrogen and ammonia to metallic ammonium was found (by calculation) to take place at pressures almost certainly less than 250 kilobars, suggesting that Neptune and Uranus contain large deposits of metallic ammonium.

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&bibcode=1954MNRAS.114..172B&letter=0&classic=YES&defaultprint=YES&whole_paper=YES&page=172&epage=172&send=Send+PDF&filetype=.pdf

But on our world, the reaction of a metal like iron with a high concentration of NH4+ cation would produce H2, not NH4. The reaction could be examined on the basis of the pH, or concentration of H+ ions. As it turns out, the potentials are simply related to the concentration of H+ ion, which depends on the anion.

A 0.1M ammonia solution (anion = OH-) has a pH of 11.122 (http://www.chem.science.unideb.hu/Solutionsmonobasic.pdf). A solution of NH4Cl (anion = Cl-) gives a pH of 5.13 (https://socratic.org/questions/calculate-the-ph-of-the-following-aqueous-solutions).

When we displace from a solution of ammonium salts, we wind up displacing hydrogen. You could imagine that adding iron or zinc to a solution of NH4+Cl- could evolve neutral NH4 which immediately decomposes to NH3 plus a hydrogen radical H., which then dimerizes to H2, and the NH3 immediately redissolves in the water. Far better to think of the NH4+ dissociating in the aqueous solution before reacting with the iron or zinc, in other words, the reaction with the metal is a reaction of H+ (or, actually, hydrated H+, or hydronium ion) after dissociation of NH4+ in the presence of H2O.

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  • $\begingroup$ Thanks a lot! Can you give me an example (in form of a reaction) for the last paragraph? $\endgroup$
    – Shub
    Apr 12, 2020 at 12:51
  • $\begingroup$ Not this: Fe + 2 NH4Cl + nH2O --> FenH20++ + 2 NH4 + 2Cl- +nH2O, but rather: $\endgroup$
    – James Gaidis
    Apr 12, 2020 at 14:05
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    $\begingroup$ Not this: Fe + 2 NH4Cl + n H2O --> Fe++ + 2 NH4 + 2 Cl- + n H2O, but rather: Fe + 2 NH4Cl + n H2O --> Fe + 2 NH4+ + 2 Cl- + n H2O --> Fe + 2 NH3 + 2 H+ +2 Cl- + n H2O (which reflects the acidity of ammonium salts) --> Fe++ + 2 NH3 + H2 + n H2O --> Fe++ + 2NH4+ + H2 + 2 OH- + (n-2) H2O (reflecting the basicity of NH3). Or, remember that NH4+ dissociates to NH3 + H+, and simplify to Fe + 2H+ --> Fe++ + H2. It's simpler because we have no evidence of indication of NH4 ever appearing on this scene, but we can measure the presence of H+ (or H3O+) - you know what I mean. $\endgroup$
    – James Gaidis
    Apr 12, 2020 at 14:20

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