RogueRose
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Electrolysis of Copper in ammonium sulfate
I'm curious what may be the result of using 2 copper electrodes in a solution of ammonium sulfate. Electrolysis of copper in sulfuric acid results in
copper sulfate and adding ammonia to a solution of copper sulfate results in Tetraamminecopper(II) sulfate.
Would Tetraamminecopper(II) sulfate result from the electrolysis of copper in ammonium Sulfate?
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j_sum1
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I think it is safe to say that the reaction at the anode would be Cu --> Cu2+ + 2e-
I don't know what the reduction reaction at the cathode would be.
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AJKOER
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Actually a very dangerous idea. Here is an extract from a prior thread (link: http://www.sciencemadness.org/talk/viewthread.php?tid=63121#... ):
"I agree with Papaya comment. One should not perform electrolysis of an increasing acidic ammonium solution. The summary explanation is the potential
formation of aqueous NH4NO2 which is prone to detonation in lower pH solutions. Here is a relevant extract from one of my prior threads (link: http://www.sciencemadness.org/talk/viewthread.php?tid=18912 ):
Quote: Originally posted by AJKOER  | OK back on topic, here is an interesting account on the electrolysis of aqueous ammonia in the presence of NaOH and, separately, Cu(OH)2 from an old
(1905) report (see page 242 at http://books.google.com/books?pg=PA242&lpg=PA242&dq=... from Journal Chemical Society, London, Volume 88, Part 2), to quote:
"Electrolytic Oxidation of Ammonia to Nitrites. Erich Muller and Fritz Spitzer (Ber., 1905, 38, 778—782. Compare Traube and Biltz, Abstr., 1904, ii,
727).—In the presence of a small amount of sodium hydroxide, ammonia may be oxidised electrolytically to nitrite even in the absence of copper
compounds.
In the presence of copper hydroxide and sufficient alkali, the oxidation of ammonia to nitrite does not cease suddenly when the nitrite concentration
has reached a certain value, but appears to proceed quite independently of the nitrite concentration. In these experiments, the oxidation was allowed
to proceed for a comparatively short time only, so that the amount of alkali present was not greatly reduced. The formation of nitrite is intimately
connected with the amount of alkali present, and when no sodium 'hydroxide is present, but only ammonia, nitrite, and copper hydroxide, it is found
that the nitrite is transformed into nitrate more rapidly than the ammonia into nitrite, and thus the concentration of the nitrite tends to decrease.
Nitrogen is also formed during the oxidation. J. J. S."
The source also notes, to quote:
"In continuation of the previous experiments, the influence of changing the concentration of the free alkali or ammonia on the rate of the
electrolytic oxidation of ammonia has been investigated. In presence of much ammonia, the amount of nitrite can be increased to about 11 per cent,
before oxidation to nitrate begins, whilst from an 11 per cent, nitrite solution to which ammonia, sodium hydroxide, and copper hydroxide had been
added a solution containing as much as 17 per cent, nitrite was obtained on hydrolysis."
Apparently replacing NaOH with Cu(OH)2 favors the formation of nitrate over nitrites, and increasing the ammonia concentration raises the yield.
Caution: product could include some copper ammonium nitrate, see discussion at http://www.pyrosociety.org.uk/forum/topic/3303-electrolysis-... . This experiment may be inherently dangerous as the author states "In these
experiments, the oxidation was allowed to proceed for a comparatively short time only, so that the amount of alkali present was not greatly reduced"
together with the observed formation of N2. From this I suspect the presence of NH4NO2 (decomposing to form nitrogen), which is inherently unstable
(explosive) as the pH is lowered, which could be particular problematic in the presence also of any copper ammonium nitrate.
On the surface IMHO, this appears to be a simple, educational and safe experiment, but upon adding NaOH and/or Cu(OH)2 to the aqueous ammonia, things
apparently could go very wrong, especially if one attempts to recover the dry salts.
[Edited on 30-3-2013 by AJKOER] |
Now as to how such products can be formed, I provide this link http://chemistry.stackexchange.com/questions/12315/electroly... to the underlying processes involved in electrolysis of water itself which
involves the formation of several radical species including the active hydroxyl radical. The latter acts on NH3 in the presence of O2 to form NH4NO2
and NH4NO3. Here is an abstract outlining the process to quote from "Removal of ammonia by OH radical in aqueous phase" by Huang L1, Li L, Dong W, Liu
Y, Hou H.:
"Many advanced oxidation technologies have been developed to remove ammonia in wastewater. All these technologies have one common characteristic, that
is, the removal processes involve OH radical (*OH). In this research work, H2O2 was selected as *OH precursor. The removal of ammonia under 253.7 nm
irradiation from low-pressure mercury lamp in the presence of H2O2 was studied to investigate the ammonia removal efficiency by *OH. Results show that
the *OH, generated by H2O2 photolysis, could oxidize NH3 to NO2- and further to NO3-. Removal efficiencies of ammonia were low and were affected by
initial pH value and ammonia concentration. Laser flash photolysis technique with transient absorption spectra of nanosecond was used to investigate
the oxidation pathway and kinetics of ammonia oxidation by *OH. Results illustrate that *OH could oxidize NH3 to form *NH2 with a second-order rate
constant of (1.0 +/- 0.1) x 10(8) M(-1) s(-1) (20 degrees C). *NH2, the main product of *OH with NH3, would further react with H2O2 to yield *NHOH.
Since *NHOH could not stay stable in solution, it would rapidly convert to NH2O2- and consequently NO2- and NO3-. The rate constants for these
elementary reactions were also given. The low removal efficiency of ammonia by *OH was mainly due to the slow reaction rate constant"
Link: http://www.ncbi.nlm.nih.gov/pubmed/19031904
[Edited on 28-8-2016 by AJKOER]
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moominjuice
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I have performed many copper/ammonia electrolysis experiments and my experience is as follows;
the electrolysis of ammonium sulphate with 2 copper electrodes produces only copper hydroxide (and a slight smell of ammonia when conducted at higher
temperatures than 0c).
this is easily explained by the equilibrium between copper and ammonia where complexes only form when ammonia is in excess.
a mixture of salts cannot contain a large excess of NH3 under these conditions, because there is by default an excess of anion for every NH3 which
might leave as a gas (albeit a very soluble one).
Le chateliers principle is at play here.
the continued electrolysis at STP will slowly loose NH3 to the atmosphere.
the solution will in fact become acidic fairly rapidly.
it would require an excess of NH3 in solution to drive the reaction toward complex formation. i have tried this, but this does (without an additional
base) also drive the reaction toward the formation of the copper hydroxide BECAUSE: the complex absorbs most of the base (NH3).
thus formation of complex, whilst plausible, produces more copper hydroxide, but reducing the concentration of FREE BASE.
the result is that an acidic solution would be formed but the complex is unstable in acid, and if say there is only a very LOW concentration of a base
such as NaOH, then we form copper hydroxide, ammonium sulphate remains largely catalytic except for losses as gas at higher temperature.
it is worth noting that oxidation of NH3 at electrodes, while a source for concern, it is under only strongly basic conditions where this occurs at
any noticeable level. most conditions arrived at using just ammonium sulphate and copper are unlikely to produce basic conditions. oxidation of
ammonia in electrolytic situations tend only to happen in basic conditions and poorly without catalysis (eg platinum electrodes) see the work of
Gerardine Botte Ohio university and work with ammonia electrolysis fuel cell.
large scale electrooxidation of ammonia in neutral solutions is hard to achieve, trust me ive tried. this would only become a problem in strongly
basic solutions.
it is worth being careful though because many metals are catalytic to ammonia elecrtooxidation including copper and nickel, but the issue here is
control of equilibrium and pH.
edit; electrooxidation can occur with high concentrations of NH3/NH4OH but for the same reasons as above, in that it increases pH, but in this latter
situation the concentration of NH3 in solution (is higher and) makes it more likely that electrooxidation of NH3 can occur at the electrode simply due
to its higher concentration of NH3 at the electrode interaction between them being more probable.
(edit typo)
[Edited on 7-9-2016 by moominjuice]
(edit; regarding Cu(OH)2 in electrolysis)
copper hydroxide is only soluble in any significant way under the conditions of an excess of NH3, giving the soluble complex, this will not occur in
near neutral conditions thus Cu(OH)2 wont be able to contribute OH- ions to the reaction without excess NH3.
[Edited on 7-9-2016 by moominjuice]
[Edited on 7-9-2016 by moominjuice]
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moominjuice
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edit; i have removed this comment because it was just rude and i apologize for getting into a crappy mood and taking it out on someone else, my bad,
im very sorry 
[Edited on 8-9-2016 by moominjuice]
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