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So I made an experiment to find a good electrolyte for a water electrolysis.

I tried citric acid, which turned out to not produce any gases at the anode, I tried sodium hydroxide, which turned out to be the very best, and I tried sodium carbonate.

The $\ce{Na2CO3}$ solution was about 2 g/l.

As my electrodes I decided to try some nails, very likely made of iron or some iron alloy.

So I hooked up my power source, just two nine-volt-batteries in series, and started the electrolysis.

The kathode (negative terminal) bubbled vigorously, indicating the production of hydrogen gas.

The anode (positive terminal) however bubbled very little and started decomposing, gray-green flakes started falling off and sinking to the bottom. This decomposition started as soon as the power was supplied.

This struck me, because I was expecting oxygen to be liberated at the anode!

When I took my anode out, it was at first a dirty green or green-gray, but it quickly turned a rusty brown.

So in conclusion, the kathode produced hydrogen with no sign of wear when on the other hand the anode made little gas and decomposed into probably some iron compound, which turned into probably iron oxide (I'm judging by the color here) on prolonged contact with air.

So, what could it be? I thought it might be iron hydroxide, but that'd be brown. This however was green.

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    $\begingroup$ May you please explain your downvote so I can improve in the future? $\endgroup$ – G. Ünther Aug 31 '17 at 15:52
  • $\begingroup$ This might be iron(II) hydroxide - judged from it's green color and the capability to turn brown (Oxidation to Fe(III)) on contact with air. $\endgroup$ – lcnittl Dec 4 '17 at 19:05
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On an anode made of iron, some of the removed electrons may be used oxidizing iron metal to ferrous iron(II) ions, or oxidizing iron(II) ions to iron(III), rather than bubbling off oxygen gas. Pushing more current through the cell is likely to result in gas evolution alongside the iron oxidation, since all three of these processes will run simultaneously.

$$\ce{Fe(metal) -> Fe^{2+}(aq) + 2 e^- }$$ $$\ce{Fe^{2+}(aq) -> Fe^{3+}(aq) + e^- }$$ $$\ce{2H_2O(liquid) -> O_2 (gas) + 4H^+ (aq) + 4e^- }$$

Ferrous iron(II) ion solutions are a pale green, but rapidly oxidize in air to vivid red-brown ferric iron(III). Iron oxides and hydroxides form precipitates and slimes in a variety of colors including black (magnetite), red (iron(III) oxide), and yellow (iron(III) hydroxide).

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