# Why did my iron nails not dissolve in hydrochloric acid?

I was trying to dissolve iron metal in HCl to make iron chloride, but ran into some problems. I'll give a brief background of my experiment. I took iron nails and put them in a container filled with muriatic acid (chemicals for my pool). The acid contains an equivalent of concentrated HCl since it is composed of HCl by 37.5%. The other 62.5% is listed simply as "other ingredients".

The faint yellow acid began to turn a more solid green yellow. Gas immediately began to bubble on the nails' surface. I left it for two days to dissolve and when I checked back the iron nails were not dissolved. They look shiny like the rust was cleaned off. The solution is black/dark green and there is a white/green precipitate at the bottom. The nails have small green crystals growing on them

I did this experiment once previously (with the same type of nail and HCl) with just a single nail inside of a hollow plastic pen tube and it seemed to have worked very well; it formed a dark brown solution at the bottom and the rest of the nail looked very rusted to a point where I could not remove it because the rust had stuck it to the inside of the pen.

Is there any explanation for the two different results and the iron nails not dissolving in my other trial? Also is there any guess as to what my solution and precipitate contains? Let me know if any more information is needed.

• Do you have a picture of the vessel where the nails did not dissolve? And were/are you able to check the acidity of your dark green solution? We (and you) might get some clues from those observations. – Oscar Lanzi May 18 '16 at 20:01
• @OscarLanzi I just added an image of the solution – Ryan May 18 '16 at 20:40
• Btw, nobody still speculated about nature of white precipitate on the photo. – sa7 Nov 7 '16 at 16:23

It seems like there is not enough acid on the nails.

1. Perhaps a depleted layer is forming next to the nails. To avoid that issue, use a magnetic stirrer or provide a source of vibration.

2. There might not be enough $\ce{HCl}$ for the large amount of iron in the nails. Check the total amount of $\ce{HCl}$, given the volume and that ~2/3 is "other ingredients" (likely water). Weigh the nails. Calculate the number of moles of $\ce{HCl}$ needed to react with the number of moles of $\ce{Fe}$. Is the product $\ce{FeCl2}$ or $\ce{FeCl3}$?

3. Have you removed all the grease (from the manufacturing process) from the nails? A little soap or detergent and rubbing may be needed.

4. Could there be some difference in the nails you used? If some are galvanized (zinc coated, to prevent rust), then that would affect the reaction.

Let us know how you do: we learn more from our mistakes than from successes.

It could be you ran out of acid. 37.5% $\ce {HCl}$ seems like a lot, but solid iron can consume 23 times its volume in 37.5% $\ce {HCl}$. The volume occupied by two moles of $\ce {HCl}$ in the "concentrated" solution is that much greater than the volume occupied by one mole of really concentrated, solid metal.

Check this by seeing if your solution is still acidic, if you still have it.