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Author: Subject: Liberation of polyatomic ions via electrolysis
Upsilon
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[*] posted on 13-3-2014 at 15:14
Liberation of polyatomic ions via electrolysis


I recently experimented with electrolysis in compounds containing polyatomic ions. Specifically, I tried both ammonium sulfate and calcium nitrate. With ammonium sulfate, one electrode was bubbling violently and the other seemed to be producing a small amount of a green liquid or gas. I believe ammonia and hydrogen are gathering at one electrode, and some form of sulfur oxide and oxygen are collecting at the other.

With calcium nitrate, one electrode was bubbling violently and the other seemed to be inactive (there appeared to be she green substance forming, but I am not sure that it isn't just residue from the other experiment). I think one side is producing hydrogen and the other is producing some form of nitrogen oxide and oxygen.

Any thoughts on this? Can anyone confirm my predictions? I am using copper electrodes, by the way.

[Edited on 13-3-2014 by Upsilon]
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[*] posted on 13-3-2014 at 15:59


Quote: Originally posted by Upsilon  
I recently experimented with electrolysis in compounds containing polyatomic ions. Specifically, I tried both ammonium sulfate and calcium nitrate. With ammonium sulfate, one electrode was bubbling violently and the other seemed to be producing a small amount of a green liquid or gas.... I am using copper electrodes, by the way.

[Edited on 13-3-2014 by Upsilon]


You've got hydrogen being generated at one electrode, and copper being oxidized at the other. I assume this is in aqueous solution, rather than a melt?

[Edited on 14-3-2014 by DraconicAcid]




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[*] posted on 13-3-2014 at 16:37


Quote: Originally posted by DraconicAcid  
[rqoute]

You've got hydrogen being generated at one electrode, and copper being oxidized at the other. I assume this is in aqueous solution, rather than a melt?

[Edited on 14-3-2014 by DraconicAcid]


Yes, it is a solution. I figured that the copper electrodes would ha e some effect on the reaction, but I did this in a hurry and I just stuck raw wires in the solution. I'll try some graphite electrodes later on. I get my graphite electrodes from pencils which is delicate and somewhat time-consuming.
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[*] posted on 13-3-2014 at 16:44


If you electrolyze saturated aqueous potassium sulphate at low temperatures, you can get K2S2O8.



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[*] posted on 13-3-2014 at 16:58


Quote: Originally posted by Upsilon  
I get my graphite electrodes from pencils which is delicate and somewhat time-consuming.


Is there an art supply or hobby store near you? You can buy artist's charcoals and graphite sticks in several shapes and hardness's. I haven't yet tried them as electrodes myself, but since you have had success with pencil leads, this would save you the bother of stripping them out of the pencil.


[Edited on 14-3-2014 by Artemus Gordon]
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[*] posted on 13-3-2014 at 18:00


Quote: Originally posted by Artemus Gordon  


Is there an art supply or hobby store near you? You can buy artist's charcoals and graphite sticks in several shapes and hardness's. I haven't yet tried them as electrodes myself, but since you have had success with pencil leads, this would save you the bother of stripping them out of the pencil.


[Edited on 14-3-2014 by Artemus Gordon]


I really don't mind extracting the lead from pencils, I can get around 24 electrodes out of 50 cents. The whole process of extracting the graphite from the pencil takes maybe 10 minutes in total, but I really was rushing earlier.

Anyway, would electrolysis of the calcium nitrate solution with graphite electrodes produce nitrogen dioxide? I'm curious to see if this is a practical way to make nitric acid.
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[*] posted on 16-3-2014 at 06:37


Based on my research, it looks like electrolysis of a calcium nitrate solution will just produce hydrogen and oxygen. I too have been looking for a practical way of making nitric acid, but I am looking for one that doesn't require a nitrate salt. I am constructing a Birkeland-Eyde system currently.



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[*] posted on 16-3-2014 at 07:33


Quote: Originally posted by Upsilon  
Anyway, would electrolysis of the calcium nitrate solution with graphite electrodes produce nitrogen dioxide? I'm curious to see if this is a practical way to make nitric acid.

The nitrate anion is an anion (a negatively charged ion), so it can only get oxidized at the anode during the normal electrolysis. Its oxidation does not cause its reduction (obviously), so you cannot go from NO3- to NO2 (that is, from N(V) to N(IV), which is a reduction).

However, you could reduce the nitrate at the cathode under certain conditions (for example, see DOI: 10.1016/j.elecom.2009.08.001 and 10.1007/s10800-004-8349-z and many other articles). This is not really an electrolysis since the reaction does not occur at the ion corresponding electrode - such reactions are called electrochemical reductions (where neutral or negative species get reduced at the cathode). But such reduction of the nitrate results mainly to either N2 or NH3 (only small amounts of NO or NO2 form - these cannot be targeted as end compounds).

[Edited on 16/3/2014 by Nicodem]




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[*] posted on 16-3-2014 at 10:45


Quote: Originally posted by Nicodem  
This is not really an electrolysis since the reaction does not occur at the ion corresponding electrode - such reactions are called electrochemical reductions (where neutral or negative species get reduced at the cathode).


Huh? If you're putting in electricity to force a non-spontaneous reaction to go, it's an electrolysis. It doesn't matter if you're reducing a cation, an anion or a molecule.




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[*] posted on 16-3-2014 at 13:50


Quote: Originally posted by DraconicAcid  
Quote: Originally posted by Nicodem  
This is not really an electrolysis since the reaction does not occur at the ion corresponding electrode - such reactions are called electrochemical reductions (where neutral or negative species get reduced at the cathode).


Huh? If you're putting in electricity to force a non-spontaneous reaction to go, it's an electrolysis. It doesn't matter if you're reducing a cation, an anion or a molecule.

At the risk of being accused of being semantic pedantic, I do not agree in using the term "electrolysis" for a reaction unless it is an electro-lysis (lýsis from Greek: a loosening, releasing).
Quote:
e·lec·trol·y·sis

noun
1. Physical Chemistry. the passage of an electric current through an electrolyte with subsequent migration of positively and negatively charged ions to the negative and positive electrodes.
dictionary definition from http://dictionary.reference.com

There are many types of electrochemical reactions and not all of them are electrolysis. The reduction of the nitrate anion at the cathode obviously does not fit the concept of electrolysis, but is exactly what the authors of the cited articles called it: an electrochemical reduction.




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[*] posted on 16-3-2014 at 17:32


Quote: Originally posted by Nicodem  

At the risk of being accused of being semantic pedantic, I do not agree in using the term "electrolysis" for a reaction unless it is an electro-lysis (lýsis from Greek: a loosening, releasing).


So the electrolysis of water (2 H2O -> 2 H2 + O2) isn't an electrolysis? Or do you imagine that it's only the insignificant quantity of hydronium ions and hydroxide ions that are actually getting oxidized or reduced?

Quote:
e·lec·trol·y·sis

noun
1. Physical Chemistry. the passage of an electric current through an electrolyte with subsequent migration of positively and negatively charged ions to the negative and positive electrodes.


"Migration". Not reduction or oxidation, but migration. If you reduce water molecules at the cathode, then you will generate hydrogen gas and hydroxide ions at the cathode, which will cause cations in the solution to migrate towards the cathode (particularly if they are in a salt bridge) so that the solution does not become charged.

Quote:
There are many types of electrochemical reactions and not all of them are electrolysis.

True- some of them are spontaneous, and do not require the application of an external voltage to go.

Quote:
The reduction of the nitrate anion at the cathode obviously does not fit the concept of electrolysis, but is exactly what the authors of the cited articles called it: an electrochemical reduction.
Yes, actually, it does.



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[*] posted on 19-3-2014 at 14:35


Ok, so I tried using graphite electrodes. In both solutions, the cathode seems to be getting vigorously attacked and dissolves rapidly, quite a bit more so in the ammonium sulfate solution.

[Edited on 19-3-2014 by Upsilon]
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[*] posted on 21-3-2014 at 08:47


Quote: Originally posted by Upsilon  
Ok, so I tried using graphite electrodes. In both solutions, the cathode seems to be getting vigorously attacked and dissolves rapidly, quite a bit more so in the ammonium sulfate solution.

Try with a copper cathode. It is pointless to use graphite for the cathode. Metals will do fine. Even iron can be used as a cathode with many electrolytes. It is the anode that is the troublesome issue.
Quote: Originally posted by DraconicAcid  
So the electrolysis of water (2 H2O -> 2 H2 + O2) isn't an electrolysis? Or do you imagine that it's only the insignificant quantity of hydronium ions and hydroxide ions that are actually getting oxidized or reduced?

It is, the solvent gets electrolyzed - such reactions are called electrolysis since the old times. We already went trough this issue in a few other electrolysis threads (last time here). I never said that the redox of the solvent on both electrodes is not an electrolysis. The IUPAC golden book does not define electrolysis, but if it did, it would have to account for such reactions simply because the solvent molecules are being reduced/oxidized stoichiometricaly at both electrodes.
Quote:
Quote:
The reduction of the nitrate anion at the cathode obviously does not fit the concept of electrolysis, but is exactly what the authors of the cited articles called it: an electrochemical reduction.
Yes, actually, it does.

You do realize that by your interpretation, absolutely all electrochemical reactions could be called electrolysis, including voltaic cells, conductometry, pH probes, corrosion, electrochemical oxidations and reductions, everything? This would cause a semantic non-sense, something I very much dislike. Perhaps you could try convincing the editors of the electrochemical journals and the IUPAC members to adopt your point of view, but I will rather keep on living with my own interpretation.




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[*] posted on 21-3-2014 at 10:47


Quote: Originally posted by Nicodem  
You do realize that by your interpretation, absolutely all electrochemical reactions could be called electrolysis, including voltaic cells, conductometry, pH probes, corrosion, electrochemical oxidations and reductions, everything? This would cause a semantic non-sense, something I very much dislike.


Perhaps you are completely misreading my interpretation.

Electrolysis is when a nonspontaneous reaction is forced to go by the application of an external voltage. Voltaic cells do not use nonspontaneous reactions, they use spontaneous reactions to generate a voltage. Corrosion is spontaneous.




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[*] posted on 21-3-2014 at 14:35


Quote: Originally posted by DraconicAcid  

"Migration". Not reduction or oxidation, but migration. If you reduce water molecules at the cathode, then you will generate hydrogen gas and hydroxide ions at the cathode, which will cause cations in the solution to migrate towards the cathode (particularly if they are in a salt bridge) so that the solution does not become charged.



I've always thought of 'lysis' as 'breaking' or 'destroying', see for instance in the context of virology:

"Viruses can be released from the host cell by lysis, a process that kills the cell by bursting its membrane and cell wall if present" (Wiki).

Or the electro-lysis of NaOH which 'breaks up' this compound into its constituent elements, by means of running an electrical current though it.

The migration you refer to precedes the actual reduction/oxidation the ions undergo at the respective electrodes and is necessary to sustain it, but it is not the 'lysis' itself.

[Edited on 21-3-2014 by blogfast25]




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[*] posted on 23-3-2014 at 16:06
Nice experiment!


Interesting experiment, did you consider trying this formula NH4NO3 + HCl = NH4Cl + HNO3? The result is 90% NH4Cl and 10% Nitric acid, the last layer.
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[*] posted on 23-3-2014 at 16:42
Thanks


Quite interesting, never tried before but looking forward to it, thanks for sharing
Quote: Originally posted by flashmanc  
Interesting experiment, did you consider trying this formula NH4NO3 + HCl = NH4Cl + HNO3? The result is 90% NH4Cl and 10% Nitric acid, the last layer.
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[*] posted on 23-3-2014 at 19:26


Quote: Originally posted by flashmanc  
Interesting experiment, did you consider trying this formula NH4NO3 + HCl = NH4Cl + HNO3? The result is 90% NH4Cl and 10% Nitric acid, the last layer.


It seems like it would be difficult to separate the HNO3 from the ammonium chloride. Perhaps something like this:
Ca(NO3)2 + H2SO4 = 2HNO3 + CaSO4

The insoluble CaSO4 can be easily filtered off. The HNO3 will still be contaminated with some H2SO4, in which case you can add copper metal to the solution to produce NO2, then bubble that back into some water to get pretty pure HNO3.
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[*] posted on 23-3-2014 at 22:42


Quote: Originally posted by Nicodem  
Quote: Originally posted by Upsilon  
Anyway, would electrolysis of the calcium nitrate solution with graphite electrodes produce nitrogen dioxide? I'm curious to see if this is a practical way to make nitric acid.

The nitrate anion is an anion (a negatively charged ion), so it can only get oxidized at the anode during the normal electrolysis. Its oxidation does not cause its reduction (obviously), so you cannot go from NO3- to NO2 (that is, from N(V) to N(IV), which is a reduction).

However, you could reduce the nitrate at the cathode under certain conditions (for example, see DOI: 10.1016/j.elecom.2009.08.001 and 10.1007/s10800-004-8349-z and many other articles). This is not really an electrolysis since the reaction does not occur at the ion corresponding electrode - such reactions are called electrochemical reductions (where neutral or negative species get reduced at the cathode). But such reduction of the nitrate results mainly to either N2 or NH3 (only small amounts of NO or NO2 form - these cannot be targeted as end compounds).

[Edited on 16/3/2014 by Nicodem]


But nitrate could theoretically be oxidised to the nitrogen trioxide radical, which would be very likely to decompose into various nitrogen oxides including NO2.




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[*] posted on 26-3-2014 at 03:05


Quote: Originally posted by Upsilon  
The insoluble CaSO4 can be easily filtered off. The HNO3 will still be contaminated with some H2SO4, in which case you can add copper metal to the solution to produce NO2, then bubble that back into some water to get pretty pure HNO3.


But yet, just distille the CaSO4 and HNO3 mix and collect the vapors.
Any remaining H2SO4 will be left behind since sulfuric acid has a very high boiling point.




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