Electrolysis

[DukerX]

I'm doing various experiments whit electrolysis, and as I don't have graphite electrodes, the metal in my electrodes goes into the reaction.

At least that's what I think.

I've tried Fe, Cu and Al electrodes.

I started using NaCl dissolved in wather, and got that eerie Cl smell during the process.

Then, I ran out of NaCl, and decided to try something else.

I have a 25lbs bag of KNO3 sitting in the basement, and I went down there and grabbed a cup of the stuff, and dissolved it in hot wather.

 

As you might know... When it was dissolved, the water wsn't very hot anymore...

Anyhow... KNO3 seems to work about the same as NaCl, only it doesn't give of any Cl smell.

 

I saw somewhere that electrolysis of KNO3 would not break up the KNO3 at all, but rather break up the H2O.

When both anion & cation resist oxidation/reduction, the reaction is decomposition of water!

When looking at it from a "pure" electrolysis perspective, the result should be H2 and O2 gas, wich bubbles from the electrodes.

 

When using metallic electrodes, the electrodes are eroded, and goes into solution as "gunk".

 

Fe electrodes = Brown gunk (Rust, I think)

Cu electrodes = Green/black gunk (CuO ??)

Al electrodes = White/gray gunk (Al2O3 ????)

 

What I'm curious about is two things:

 

Are my assumptions correct regarding the "gunk" or does the KNO3 plays trics on me, forming weird potassium compounds or nitrates or other weirdness, or does the KNO3 stay intact?

 

Are my gunk "pure" metal oxides, and if they are; How can I extract it from the KNO3 solution? The gunk is heavier then the solution, so it settles in the lower third of my container, so I guess I could decant it and put back clean wather over and over again for like... a month, but that's not very practical.

 

Please speak your mind, cause I'm almost clueless about what I'm doing here

 

-DX-

[xeluc]

I know your gunk contains no nitrated because all nitrates are water soluble.

 

With the purity of the gunk, it depends. I have gottne pure Iron Oxide from electrolyzing Iron in NaCl and I have gotten pure Cu2O from electrolyzing Cu in NaCl, but when other combinations of solution and electrode are used, different chemicals ar ecreated. For instance. Electrolysis of Cu in NaCl produces (At least for me) Cu2O. Differing voltages may have differing effects. But when you electrolyze in a solution of Copper Chloride, Cu(OH)2 is formed. So there, Hope I helped. If you have any more questions, go for it. And welcome to SFN!

 

Also, if you want only H2 and O2 given off, Try a solution of NaOH. Using copper Electrodes is fine then, the solution stays clear with no gunk forming.

[woelen]

With KNO3 you indeed can expect the metals to go into solution as Fe(3+), Cu(2+) and Al(3+). With hydroxide, formed at the cathode, these give hydroxides:

 

Fe(OH)3 --> brown

Cu(OH)2 ---> sky blue

Al(OH)3 ---> white

 

What I've noted very recently is that the anode material is not fully oxidized but some weird intermediate oxidation state is obtained. This effect is most notably with copper, magnesium and aluminium.

 

With copper this gives rise to copper (I) impurities, that's why your gunk is not bright blue, but green (brown/yellow mixed with blue).

 

With aluminium you get very finely dispersed metal, intimitely mixed with the hydroxide. This gives rise to grey gunk. When a little acid is added to the grey gunk, you will see it dissolve and a small amount of hydrogen is formed.

 

With magnesium I even get hydrogen at the anode!!! The magnesium is converted to some weird mix with oxidation state somewhere between +1 and +2. This stuff is very unstable and reacts with water, giving magnesium in the +2 oxidation state (which is normal) and hydrogen. The magnesium in lower oxidation state also disproportionates to metal and to magnesium in the +2 oxidation state.

 

I have done some literature search on this and indeed, with electrolysis, many metals are only partially oxidized. E.g. for magnesium the average reaction is:

 

[math]Mg -> Mg^{1.3+} + 1.3e[/math]

 

This Mg(1.3+) reacts with water, giving Mg(OH)2 and H2. It also disproportionates, giving Mg and Mg(2+). This explains the observations of formation of dark grey gunk instead of white gunk.

 

With Al a similar thing happens, the Al having an average oxidation state somewhere between +2 and +3. This also leads to grey gunk (disproportionation to Al(OH)3 and Al-metal).

 

With Cu you get an average oxidation state between +1 and +2. This disproportionates to copper (I) and copper (II) in an environment, where copper (I) can be stable (e.g. in table salt solution) or copper (0) and copper (II) where copper (I) cannot exist. With KNO3 you might also have the latter situation, but you can confirm that if you acidify your green gunk. If some turbidity remains then you have some copper (0), metal, in it, otherwise you have copper (I) in it. Just try it.

 

My literature study has given me a lot of new insight on electrolysis with metal anodes. My source of information is the book "Chemistry of the Elements" by Earnshaw and Greenwood. If you happen to have that book, then read the chapter on magnesium and a beautiful explanation of these intermediate oxidation state compounds is given. Very enlightening!

[vrus]

What would happen if you electrolysed weak alkalis ?

[woelen]

What would happen if you electrolysed weak alkalis ?

Depends on the electrode material and anions in solution. At the cathode you can expect formation of hydrogen, at the anode the metal forms hydroxides, or oxygen is produced if the anion is not broken down.

[vrus]

Depends on the electrode material and anions in solution. At the cathode you can expect formation of hydrogen, at the anode the metal forms hydroxides, or oxygen is produced if the anion is not broken down.

 

So the hydroxide will react with the anode ?

 

Oh and what if you used silver as the electrode ? Inert ?

[Primarygun]

So the hydroxide will react with the anode ?

 

Oh and what if you used silver as the electrode ? Inert ?

I didn't read the above threads so I generalize the case here.

Anode refers to electrodes where oxidation occurs.

Normally, reactive metals lose electrons more readily than hydroxide ion.

When in an electrolytic cell, (Check the Standard Electrode Potential), hydroxide ion loses electrons more readily than silver.

http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials