AJKOER
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A More Correct Formulation of Aqua Regia From HNO3 and HCl ?
BACKGROUND
On this very forum Clearly_Not_Altra has noted previously here http://www.sciencemadness.org/talk/viewthread.php?tid=155922... to quote:
“The stoichiometric reaction is not the physical reaction…Aqua regia is used with a high concentration of Cl- to dissolve metals because Cl- acts
as an effective ligand for heavy metal ions.”
Also, per an educational site here https://www.scienceabc.com/pure-sciences/aqua-regia-formula-... , to quote:
“Aqua regia solution is usually a yellow, reddish-orange, fuming liquid and has a rapidly changing composition. The traditional aqua regia solution
is comprised of a 3:1 molar mixture of hydrochloric acid and nitric acid, respectively.”
to which, I completely agree with the observation “a rapidly changing composition” and the use of the phrase “traditional aqua regia
solution”, but then again cites the usual net reaction:
HNO3 (aq) + 3 HCl (aq) -> NOCl (g) + 2 H2O (l) + Cl2 (g)
And, from a gold refining forum here https://goldrefiningforum.com/threads/aqua-regia-1-4-vs-aqua... , an interesting comment:
“If you mix up 3:1 acid mixture you will have a "hotter" solution which will more aggressively attack the gold. 4:1 has less nitric available so it
is less aggressive. I prefer to do aqua regia reactions in closed condensed reactors and I mix up the acid at 5:1 because the effect of the condenser
recycles some of the nitric.”
A technical source here https://onlinelibrary.wiley.com/doi/10.1002/047084289X.rn059 notes, to quote:
“readily available as an orange-yellow gas in cylinders, easily condensed to a deep red liquid; often more convenient to generate ClNO in situ from
alkyl nitrites and HCl in ether, ethanol, or other organic solvents, or in water from HNO2 and HCl (or H+ and Cl−).”
which agrees with a path cited in Wikipedia https://en.wikipedia.org/wiki/Nitrosyl_chloride to quote:
“A more convenient laboratory method involves the (reversible) dehydration of nitrous acid by HCl [3]:
HNO2 + HCl → H2O + NOCl”
And further Wikipedia states:
“NOCl also arises from the combination of hydrochloric and nitric acids according to the following reaction [5]:
HNO3 + 3 HCl → 2[Cl] + 2 H2O + NOCl [5]
In nitric acid, NOCl is readily oxidized into nitrogen dioxide.”
where interestingly, 1st product is not Cl2 but two chlorine radicals.
========================
NEW PROPOSED PATH:
Per Wikipedia https://en.wikipedia.org/wiki/Nitric_acid on Nitric acid decomposition taking the reaction and dividing by two (for convenience in deriving the
net reaction):
2 HNO3 -> H2O + 2 •NO2 + ½ O2
Now, a simple proposed radical reaction, which would be favored in an excess chloride (HCl) presence, namely:
[ HCl + •NO2 = •Cl + HNO2 ] x 2
Support: Assumes that •NO2 here parallels the reported reaction of the hydroxyl radical with HCl discussed in this 2015 work https://pubs.acs.org/doi/10.1021/jp512021m , to quote:
“The dynamics and mode specificity of the HCl + OH → Cl + H2O reaction are investigated using a full-dimensional quantum dynamics method on an
accurate global potential energy surface. It is shown that the vibrational excitation of the HCl reactant greatly enhances the reactivity while the OH
vibrational excitation has little effect. The surprising HCl vibrational enhancement of this early barrier reaction contradicts a naive extension of
Polanyi’s rules, but can be explained by the sudden vector projection model, which attributes the promotional effect of the HCl vibration to its
strong coupling with the reaction coordinate at the transition state. In addition, it is found that the fundamental and overtone excitations of the
HCl reactant change the reaction mechanism from a direct barrier crossing process to a capture-like process.”
Next, the cited reaction producing NOCl per Wikipedia above and the technical source:
[ HNO2 + HCl → H2O + NOCl ] x 2
which all together implies a net reaction of:
2 HNO3 + 4 HCl -> 2 •Cl + 3 H2O + 2 NOCl + ½ O2
Clearly, the above reaction is promoted with an excess of chloride (or HCl) as was noted above. It also accounts for the alluded to radical formation
of the chlorine radical •Cl above per Wikipedia cited source.
Comments please.
[Edited on 30-1-2022 by AJKOER]
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j_sum1
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Comments, Sure.
What specifically is incorrect or lacking in the current understanding? You were not clear on that.
What evidence do you have that anything you have proposed is correct?
What's with the radicals? Every. Single. Post. Radicals are transient at best. And very few reactions proceed by radical propagation. Those that do
frequently require an initiator such as UV light for halogenation reactions.
What practical benefit is there in your presentation of the mechanism? (None at all if it is untrue.) Aqua regia dissolution of gold has been used for
centuries and the practical aspects are well known. What do you add here?
Speculation is one thing. Speculation that goes nowhere is quite another.
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AJKOER
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Thanks J_Sum1:
From the perspective of physical chemistry, I have suggestive evidence that my claimed radical reaction may occur and hence, may be a matter of future
investigation in that area.
Why suggestive?
Because my one radical equation accounts nicely (like fitting a key into a lock) for two events. First, the apparent ample production of NOCl (like
precisely from the recommended lab prep) and second, the formation of the Chlorine radical. Also, the cited radical interaction is not without
precedent (occurring with the hydroxyl radical in the presence of HCl) as I so documented.
Note, the updated Wikipedia sourced equation claiming chlorine radicals is still rather dated (like 1950s). However, it is not alone, note comments
here https://chemistry.stackexchange.com/questions/24726/aqua-reg... on "aqua regia and nascent chlorine" .
The importance of chlorine radicals? Well, here is an interesting 2021 commentary available at https://www.scientificupdate.com/process-chemistry-articles/... . To quote:
"A radical approach to C-H chlorination
Written By Dr John Studley on 16th March 2021. Process Chemistry Articles
My first experiences of radical chlorination where something of a baptism of fire. Generation of chlorine radicals from chlorine gas under intense UV
irradiation in a custom-made photochemical flow reactor was as scary as it sounds to a very young, yet to be trained chemistry technician starting out
in his carrier in the chemical industry. The reaction was part of the synthesis of the inhalation anaesthetic Isoflurane and this step was the
photochemical chlorination of an intermediate we called FEFME ((2-difluoromethoxy)-1,1,1-trifluoroethane, or trifluoroethyl-difluoromethyl-ether-
hence the acronym). Careful control of mixing and flow rates in a gas-phase reaction were critical to prevent over-chlorination, chlorine gas
breakthrough and contamination of the precious ethereal product (Scheme 1).1..."
where I would comment, 'Generation of chlorine radicals from chlorine gas under intense UV irradiation' appears a bit harder than HNO3/HCl to effect
an organic synthesis. In fact, there is an organic (non-aqueous) version of aqua regia per this 2010 source https://cdn.intechopen.com/pdfs/27210/InTech-Organic_aqua_re... and associated comments here https://cen.acs.org/articles/89/i14/New-Twist-Aqua-Regia.htm... .
If you prefer looking into a UV-Lamp, please remember, at least, the eye protectors.
[Edited on 30-1-2022 by AJKOER]
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AJKOER
Radically Dubious
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Now, some may wonder is there any value of my claimed new reaction, which for practitioners in the art of dissolving gold, I further simplify to:
HNO3 + 2 HCl -> •Cl + 3/2 H2O + NOCl + ¼ O2
A possible answer is yes, around the topic of so-called Poor Man's Aqua Regia (which substitutes a nitrate salt for HNO3 and adds an acid source),
with for example, mixtures of say 3 or more HCl combined with a suitable nitrate and chloride salt.
Many possible choices, but from the point of reactivity, I refer to this 2020 article "Dissolution of noble metals in highly concentrated acidic salt
solutions" here https://pubs.rsc.org/en/content/articlehtml/2020/cc/d0cc0229... , to quote:
"The work reported here was inspired by a paper by Angell and coworkers from 1973 in which it was remarked that concentrated solutions of AlCl3·6H2O
and Al(NO3)3·9H2O dissolve noble metals at a much greater rate than can be achieved with boiling aqua regia. "
as this appears to indicate effectiveness without the explicit addition of HCl (but having some water with a CO2 presence or air exposure, may assist
in my opinion, which I will expand upon if someone asks).
[Edited on 3-2-2022 by AJKOER]
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