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Don't ask me why, but I did an experiment where I tried to remove the magnetic iron oxide layer from a floppy disk by soaking it in HCl for a week. I put pieces of floppy disk in 0 - 1 M HCl, then measured their absorbance at 700 nm. HCl concentrations at 400 mM or more were able to remove about half of the absorbance, but not more than that.enter image description here

We can even visibly see the change in color intensity. enter image description here

This suggests that something is preventing the acid from dissolving the remaining iron oxide, and that simply increasing the concentration beyond 1 M won't help. However, I can still scratch the surface and remove some of the color, suggesting that it isn't buried throughout the plastic film, but still accessible on the surface. But I don't want to spend forever scratching the iron oxide off floppy disks, so I would still like a simple chemical method to remove the metal. Does anyone have any suggestions for removing it?

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    $\begingroup$ The magnetic iron oxide is $\ce{Fe3O4}$ and it may be considered as made of $1$ Iron(II) oxide $\ce{FeO}$ plus $1$ Iron(III) oxide $\ce{Fe2O3}$. The first oxide $\ce{FeO}$ is easily soluble into diluted HCl solutions. The second oxide $\ce{Fe2O3}$ is much more difficult to dissolve into diluted HCl solutions. The dissolution is better in concentrated HCl. Why don't you try with more concentrated HCl, like HCl $10$ M ? Have you ever measured the absorption of the solution ? It would give you more useful measurements, because the absorption would be only due to the dissolved Iron ions. $\endgroup$ – Maurice Sep 21 at 13:21
  • $\begingroup$ @Maurice I have not tried measuring the absorbance of the acid solutions. I couldn't see a visible difference between the 0 M HCl and 1 M HCl, so it didn't occur to me to try it. I'm a little reluctant to increase the HCl to really high levels because I suppose that the ester bonds in the polymer (Polyethylene terephthalate) might be hydrolyzed. But there's no harm in trying. $\endgroup$ – user137 Sep 21 at 13:29
  • $\begingroup$ it is rather surprising that you see a loss of matter on the disk, and no color in the solution. It may be due to the fact that the only dissolved compound is the ferrous atom, which gives nearly colorless solutions. I would suggest you to add some Hydrogen peroxide to the HCl solutions, just to see if the solution takes a brown color. It should ! And I doubt that the polyethylene terephthalate will be so quickly hydrolyzed. Usually this plastic substance needs high temperatures to be hydrolyzed. $\endgroup$ – Maurice Sep 21 at 13:46
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    $\begingroup$ An alternative strategy might be to dissolve the polymer and leave the inorganic parts as solids. How about putting the disk in acetone? $\endgroup$ – matt_black Sep 21 at 15:40
  • $\begingroup$ @matt_black I need to keep the polymer intact. This experiment is about removing the iron oxide so I can try to label the polymer with other compounds. If I can't remove the iron oxide from real floppy disks, I'll try to cut out disks from blank PET and make my own. $\endgroup$ – user137 Sep 21 at 22:13
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I have to say thank you to Maurice for his suggestion to use more concentrated HCl. I took the same pieces of floppy disk that I used before and increased the acid concentration by a factor of 10 and let them sit for another week.

At first I was disappointed, because there was no visible change in the plastic or the solutions. But when I took the pieces out of the acid and cleaned them off with an alcohol soaked kimwipe, the remaining Iron Oxide came right off. It's not perfectly clean, most likely because I had taped the pieces into my notebook after the first treatment, and I didn't clean the tape residue off before the second treatment. This is also probably why the remaining absorbance at 700 nm is so variable. But I think this method works as long as I don't get the plastic covered in tape goo. The acid treated plastic also might feel a little less stiff than the 0 M control, but this is subjective.

enter image description here enter image description here

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