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Author: Subject: Several interesting reactions
Havecius
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[*] posted on 25-3-2020 at 04:24
Several interesting reactions


Hello
I have encountered a few reactions in an analytical chemistry course which I can't myself explain. Maybe someone from here knows what happens.

1. mixing ammonium nitrite with thiocyanate and adding acid results in a red, bromine-smelling gas and possibly a white smoke

2. adding little base to Mn(ii) and Ag(i) solution gives a deeply black precipitate

3. Oxidizing nickel hydroxide with persulfate also gives a deeply black precipitate which may be NiOOH or nickel dioxide, not both

4. Oxidizing Ag(I) solutions with persulfate in acidic conditions results in the formation of a brown soluble cation - Ag2+ or Ag3+. The sources differ about it.

Thank you all very much
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Metacelsus
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[*] posted on 25-3-2020 at 04:54


1: The red, bromine smelling gas is almost certainly NO2

I'm not sure about the other ones; Woelen may be able to provide more information on the silver chemistry.




As below, so above.
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njl
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[*] posted on 25-3-2020 at 07:32


im guessing the dark precipitate from Mn and Ag is superfine manganese or silver oxide (or both)
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Bedlasky
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[*] posted on 25-3-2020 at 07:37


1. Along with NO2 there can be also formation of NOSCN

3. It's mixture of hydrous NiO2 and NiO(OH). Composition of this mixture is changing during time. At the beginning it's mostly NiO2. But NiO2 in aqueous solution slowly decomposes in to NiO(OH).

4. As I know it's Ag3+. But maybe I am wrong.

Woelen have some nice articles about these reactions on his website:

https://woelen.homescience.net/science/chem/riddles/thiocyan...

https://woelen.homescience.net/science/chem/exps/Ni+persulfa...

https://woelen.homescience.net/science/chem/exps/Ni+persulfa...
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chornedsnorkack
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[*] posted on 25-3-2020 at 07:46


Quote: Originally posted by Havecius  


2. adding little base to Mn(ii) and Ag(i) solution gives a deeply black precipitate

Possibly something like
Mn2++2Ag++4OH-=2Ag+MnO2+2H2O
Both MnO2 and finely dispersed metallic Ag are black.
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AJKOER
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[*] posted on 1-4-2020 at 18:35


Here is my attempt at an explanation on the likely mechanic of the cited reaction of interest: "Oxidizing nickel hydroxide with persulfate also gives a deeply black precipitate which may be NiOOH or nickel dioxide"

where persulfate is also sometimes known as peroxysulfate or peroxodisulfate.

Now apparently, one of the best ways to activate peroxydisulfate (PDS) and peroxymonosulfate (PMS) is, for example, siderite, (Fe, Mg, Ca, Mn, Zn, Co)CO3, where Fe, Mn and cobalt are the key metal ions (apparently, a trace of the toxic cobalt ions in the presence of HSO5- is capable of producing the powerful sulfate radical anion, see "COBALT/PEROXYMONOSULFATE AND RELATED OXIDIZING REAGENTS FOR WATER TREATMENT" a thesis by Georgios P. Anipsitakis, available online with a google search). The ensuing advanced oxidation process can then produce both the usual sulfate radicals (see https://www.sciencedirect.com/science/article/pii/S004313540... ) and the even more powerful hydroxyl radical (see "Activation of Persulfates Using Siderite as a Source of Ferrous Ions: Sulfate Radical Production, Stoichiometric Efficiency, and Implications", at https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.7b03948 ).

Where here Ni2+, also a transition metal ion, is similarly expected to engage in a fenton-like reaction with KHSO5 (replacing the classic H2O2) producing the sulfate (and also the hydroxyl) radical as active radicals. Proposed reactions where Ni3+ is here NiO(OH):

Ni2+ + HSO5- -> Ni3+ + •SO4- + OH- (See Thesis related Reaction 2.1)

And, I would guess, some NiO2 with excess HSO5- per the reaction:

Ni3+ + HSO5- -> Ni4+ + •SO4- + OH-

Also, of interest:

HSO5- + e- -> •SO4- + OH- (See Thesis related Reaction 2.2)

HSO5- + e- -> SO4(2-) + •OH (Thesis related Reaction 2.3)

HSO5- -> •SO5- + •H (or equivalently, H+ + e-, per related Reaction 2.4)

And, per Wikipedia (https://en.wikipedia.org/wiki/Nickel(II)_hydroxide) on Ni(OH)2:

"Nickel(II) hydroxide is frequently used in electrical car batteries.[6] Specifically, Ni(OH)2 readily oxidizes to nickel oxyhydroxide, NiOOH, in combination with a reduction reaction, often of a metal hydride (reaction 1 and 2).[10]

Reaction 1 Ni(OH)2 + OH− -> NiO(OH) + H2O + e− "

[Edited on 2-4-2020 by AJKOER]
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