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I've got some steel wool in hydrochloric acid to attempt to make ferrous chloride and the whole solution has turned purple. From everything I've seen it's supposed to be green. I have no idea what's happened and if anyone could give any ideas that'd be great.

  • The steel wool has no soap
  • The glassware is clean of phenols/enols/anything that would give a positive result in the ferric chloride test
  • The HCl has 1% titanium dioxide for some reason (store bought in Australia)

https://imgur.com/a/q4TAhtO

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    $\begingroup$ Chromium has a rich color chemistry. Steel wool is not iron wool. $\endgroup$
    – AChem
    Aug 10, 2021 at 16:37
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    $\begingroup$ HCl with titanium dioxide? This is strange. $\endgroup$
    – AChem
    Aug 10, 2021 at 16:40

1 Answer 1

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Your observation with steel wool is somewhat perplexing because your HCl is an odd solution of titanium chloride expressed as 1% titanium dioxide. Initially, chromium complexes could be blamed for this unusual violet color after dissolving steel wool or iron wool. However it seems that something fancier is happening.

Let us assume your store HCl has dissolved titanium dioxide for whatever magical reasons (no idea why this is so). Titanium could be in $\ce{Ti^{4+}}$. When you add iron/steel wool, $\ce{Ti^{4+}}$ begins to be reduced by hydrogen evolution reaction to $\ce{Ti^{3+}}$. Fifty years ago, we could have called that nascent hydrogen is doing this magic, but iron powder in HCl can reduce the +4 oxidation state of Ti to +3 by some mechanism.

$\ce{Ti^{3+}}$ chloride solutions are beautiful violet/purplish (source: Wikipedia):

enter image description here

With some chromium, iron, and titanium, you have a dirty mix of purplish color rather than the expected green color.

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