Sciencemadness Discussion Board - 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 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]

Upsilon - 13-3-2014 at 16:37

[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.

DraconicAcid - 13-3-2014 at 16:44

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

Artemus Gordon - 13-3-2014 at 16:58

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]

Upsilon - 13-3-2014 at 18:00

Quote: Originally posted 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.

Gooferking Science - 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.

Nicodem - 16-3-2014 at 07:33

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]

DraconicAcid - 16-3-2014 at 10:45

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.

Nicodem - 16-3-2014 at 13:50

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.

DraconicAcid - 16-3-2014 at 17:32

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:

"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.

Upsilon - 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]

Nicodem - 21-3-2014 at 08:47

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.

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:

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.

DraconicAcid - 21-3-2014 at 10:47

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.

blogfast25 - 21-3-2014 at 14:35

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]

Nice experiment!

flashmanc - 23-3-2014 at 16:06

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

Thanks

rcfunker - 23-3-2014 at 16:42

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.

Upsilon - 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.

Pyrovus - 23-3-2014 at 22:42

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.

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

vmelkon - 26-3-2014 at 03:05