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Recently I ran out of dish washing detergent, so I decided to pick up one of the recipes online, to make my own "ecological" dish washing detergent alternative.

The recipe was:

  1. Mix ingredients:
    1. 400ml of water
    2. 200mg of kitchen salt
    3. 100ml of vinegar
    4. juice squeezed from 3 lemons
  2. boil for 10 minutes

After I stopped boiling and left the mixture to cool down, the fluid turned dark gray. Bottom of my stainless steel pot turned black. Here is a photo of it (after I did some scrubbing, but you can still see black on the side): enter image description here

What happened? (What chemical reaction?) Additionally, are any of the formed substances known to be toxic?

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    $\begingroup$ What is your pot made of? $\endgroup$ – Nilay Ghosh Sep 16 '19 at 14:54
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    $\begingroup$ In no way would the chemicals used produce a detergent. Wherever you got this information should be considered an unreliable source of information. $\endgroup$ – DrMoishe Pippik Sep 17 '19 at 0:48
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    $\begingroup$ @Alchimista why do I expect they turn something to be used for cooking in something toxic? Because they were used in pretty high dose (salt especially) and they reacted with the pot surface. So far no one was able to tell what reaction took place. $\endgroup$ – user2449761 Sep 17 '19 at 11:33
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    $\begingroup$ So, as usual, some power users just didn't want us to answer to a valid question.What is wrong with this question? $\endgroup$ – SteffX Sep 17 '19 at 14:45
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    $\begingroup$ I think there's interesting chemistry here -- mixing vinegar and/or lemon juice with salt will produce an acid environment with chloride ions, which attacks metals a lot more readily than the weak acids or the salt alone. $\endgroup$ – jeffB Sep 18 '19 at 17:24
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You used a salt/acid mixture and, upon bringing it to a boil to clean a stainless-steel pot, parts turned black.

All stainless steel is not alike, and all are not “stainless” or non-reactive towards oxidation – given the circumstances.
Getting to your specific question: The “black” you are seeing upon boiling is a Fe(II)/Fe(III) oxide, the same or similar to the black oxidative protection [the seasoning of a skillet] so valued by the fastidious owners of iron skillets.

One of many procedures for “seasoning” an iron skillet is similar to what you did. I have done the following A) cleaning the skillet down to shinny metal, particularly important if you do not know the history of the skillet. B) rub a mixture of table salt dampened with an acid [vinegar] over the pan until you get the classic yellow-rust of iron we are so familiar with C) now add water to the skillet and bring it to a boil. This step is critical. The yellow rust will turn black upon boiling. This is a protective Fe(II)/Fe(III) oxide coating.

Now repeat with the salt/vinegar rub and boil until you get a uniform blackish coating on the iron skillet. Finally, fry up some bacon and eggs to give a carbon-based coating to the skillet. The latter is more of a “paint” but with the black metal oxide coating as a base, your skillet is protected against “rust” and mild oxidation.

The “original Teflon ® coating” if you will.

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  • $\begingroup$ I wonder if some of the grey color is chromium (III) citrate given the presence of the citric acid from the lemons. I wouldn't expect very much of the Fe(II)/Fe(III) oxide to go into solution. The heat and acid must first degrade the chromium oxide layer on the stainless steel before solubilizing iron ions. $\endgroup$ – Andrew Nov 18 '20 at 22:13
  • $\begingroup$ @Andrew, Yes, the citric acid could be "the joker in the deck" here! You make a good point. However, citric acid is a 2-electron reducing agent and I suspect oxidation when I put an acid and a metal together. Even with the cast iron seasoning recipes that have been around for years the nature of the iron oxide is less than clear. For that matter the Fe(II)/Fe(III) chemistry of cyanotype process is all but settled. pubs.acs.org/doi/pdf/10.1021/ed078p311.2 $\endgroup$ – Hal Nov 19 '20 at 14:00
  • $\begingroup$ I wasn't thinking of citrate as a reducing agent, just as a chelator. for the chromium ions released by attack of chloride and acid on the chromium oxide layer of the stainless steel $\endgroup$ – Andrew Nov 19 '20 at 15:15

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