[che_com_23]

Please help...
1)I was electrolysisng water using a solute of sodium bicarbonate. I immersed three + wires, three - wires coming from three different alkaline batteries. As usual, I got the violent bubbling. After 1/2 an hour, I removed the batteries. The solution wouldn't stop bubbling. What was the solution, and what was bubbling out?

2)I repeated above, using a sodium chloride electrolyte. Green-black powder went everywhere, and eventually settled to the bottom. I trated it with 10% H202, and it turned rust-color, and released gas. Was the green powder Fe(OH)3 or something else?

P.S, this is my first post, so don't be too hard on me.

[lemonoman]

That's odd...was it tap water, or water that was used in a chemical reaction...I'm guessing it wasn't deionized water...that wouldn't make too much sense...

[Donaldson Tan]

1. how hot was the solution? it could be thermal decomposition of the bicarbonate anion that was responsible for the bubbling.

2. what electrodes did you use?

[xiankai]

also note.. green powder would Fe(OH)2 and brown powder is Fe(OH)3... they have diff. oxidation numbers.

[lemonoman]

Haha, xiankai, I noticed the post you made just before this one was about the oxidation state of Fe...that worked out extremely well ! [I realize that may sound sarcastic but it was meant kindly - just an observation.  It is kinda funny!]

Anyways, Green could also be oxidized copper if he used regular wires...and by oxidized I mean CuO.

[Qazzian]

You mean green copper like the bottom of that pot yesterday?

I'm interested in what people come up with for this. I can't think of anything not stated.

[che_com_23]

I used Cu electrodes, not exacltly all that pure however. Also.. the black-green powder was absolutely insoluble in water. It would float to the bottom withing minutes. It also turned a regular black when I evaporated all the water off. The temperature was low, and I did use tap water. The water was at room temperature, so I don't think it was thermal decomposition.

[Dude]

1.  As Geodome indicated, that was almost definitely carbon dioxide coming out from decomposition of the bicarbonate.  I'd check the pH of the solution before and after electrolysis.  Since the decomposition of water requires a lot of energy, there is no way to sustain the reaction after removal of the battery link.

2.  Electrolysis of sodium chloride solutions can produce chlorine.  I would guess that the chlorine reacted with your electrodes or wires if they weren't platinum.    

[arnyk]

Argh I did a lab once we got the exact green-black copper solution you did...  Copper (II) chloride??  

[woelen]

The green stuff you get with copper wires is actually copper (II) oxychloride or copper (II) hydroxide, contaminated with some copper (I).

You sometimes also can get yellow/brown stuff, which turns dark green/brown, when allowed to stand for a long time in contact with air.

Hydrous copper (I) oxide is yellow/brown. Copper (II) compounds like copper (II) hydroxide or copper (II) oxychloride/hydroxychloride are blue or cyan or green. If a mix of these compounds is formed, then any color, ranging from yellow/brown to green/brown can be obtained, the precise color depending on the actual ratio of copper (I) and copper (II) in the precipitate. This ratio in turn depends on the applied voltage and concentration of the electrolyte.


The bubbling of the NaHCO3 solution probably is due to formation of CO2. On electrolysis of a solution of this, acid is formed at the anode, which may cause slow decomposition of the NaHCO3. The bicarbonate ion is not very stable and even the tiniest amount of acid is capable of decomposing this anion.