Hello Andrew Harkaway ,

Even if a clean piece of zinc foil is immersed in hydrochloric acid, it is going to look worse because the acid is converting the zinc into ZnCl2 and H2 (gas). In other words, the smooth and shiny surface of the zinc foil is being corroded (oxidized, i.e., the Zn is losing electrons, which is the defintion of oxidation, and becoming Zn2+) by the acid. I am not sure about exactly what type of Voltaic cell you are using with the MnO2/Zn counterelectrode, it sounds almost like a Leclanché dry cell, i.e., carbon-zinc cell (old style flashlight cell). In any event, if the zinc electrode is the anode (negative) electrode in your Voltaic cell, then the reaction at this electrode will be oxidation, i.e., Zn is losing electrons and becoming Zn2+, which will combine with an available anions in solution such as Cl1-, SO42-, etc. In other words, the smooth and shiny zinc surface is, again, being corroded and taking on a pitted, rough appearance. I don't believe there is any chemical way to restore the shiny and smooth surface to the zinc foil.

Regards,

Tom Cuff

Dear Andrew Harkaway many thanks for your interesting technical question. As already indicated by Thomas Cuff zinc is a non-noble metal which easily dissolves in aqeous acids such as hydrochloric acid or sulfuric acid (so does ZnO). Thus none of these acids is suitable for cleaning the surface of zinc foil. In the worst case your zinc foil will dissole completely. If at all possible, I would use fresh zinc foil for your experiments. I this is not possible, I would suggest that you try to clean your used zinc foil using just warm water and a household detergent. In this context please have a look at the following potentially useful link:

Cleaning Up Metal

https://www.metalconstructionnews.com/articles/cleaning-up-metal

Certainly the article describes cleaning of metal surfaces on a larger scale (metal panels) but I'm sure that this can be adapted to zinc foil.

Good luck and best wishes! 👍

Dear Andrew Harkaway , you may try to clean your Zn foil in an ultrasonic bath using some special detergents, Dr. Stamm, brand name "Tickopur", is a supplier of suited ready-to-use solutions ... As mentioned above by Frank & Tom, be careful since Zn may quickly dissolve in acid as well as in alkaline media.

From my own experience with Tickopur, I would recommend

TR2, TR3 or TR7 (they are suited for all metals according to the supplier) and TR14 or R36 may also work.

EMAG is another supplier of cleaning agents. They have a de-oxidation liquid, also for us with ultrasound, EM-100.

Check the internet for local suppliers or Jeff Bezos company :-)

Good luck, and keep us updated about your progress! Kind regards, Dirk