While experimenting with primary cells using aluminium, copper, and caustic soda both electrodes begin to gas under certain load conditions.

It is common knowledge that the gas from the aluminium electrode must be hydrogen, but bubbles from the copper are larger, non-fizzy, and rise slowly through the elecrolyte. I assume it is oxygen, but researching the reaction between al and naoh gives some confusing answers, but nowhere is the production of oxygen mentioned.

The copper does not appear to be consumed, and the electrolyte does not turn bluish. It is as if the "mystery gas" becomes dissolved in the electrolyte then adheres to the copper rather like dissolved air in fresh tapwater adheres to the side of the glass. I think I have also observed slow-rising gas bubbles evolving from areas of electrolyte away from electrodes and reaction vessel sides.

 

Possible clue: The double-gassing effect only occurs when cell output has been loaded down to about 0.4v and less. When the load is reduced and the voltage rises towards 1v, the bubbles do not appear to re-dissolve. Could there be a secondary, voltage-dependent reaction going on here?

 

I have found no literature relating to this.....electron shell energy threshold effect?

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Well, scratch the voltage threshold observation, further messing about has cast serious doubt on it.

I replaced the resistive load with a diode, holding cell voltage to about .63, and gassing at the cathode still occurs. Removing the anode (aluminium) and shaking the cell causes a surprising amount what I presume is oxygen(?) to come out of solution, like shaking a bottle of pop. Could it be that when plenty of hydrogen is produced, this is combining with oxygen to form water, but insufficient hydrogen causes th oxygen to simply dissolve?

The thought of possibly producing hydrogen and oxygen simultaneously in an enclosed (but ventilated) reaction vessel is making me a bit nervous!

 

No replies yet...I suppose it must be just boring!!

Edited July 29, 200916 yr by gcol

In a working cell electrons travel from the Al electrode, through the external circuit to the copper electrode where they reduce water to hydrogen.

Aluminium is attacked by NaOH (whether there's a cell there or not) generating hydrogen

Thank you, but I think that much is already clear. Perhaps I am looking for a reaction that produces excess unattached oxygen, where this may be the result of unbalanced amounts of NaOH and Al?

 

I have come across several attempts to balance this reaction, with conflicting results depending on initial proportions, but none seem to indicate the creation of excess oxygen, but experimental observations seem clear.