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I have previously used drain/sink cleaner gel based on sodium hydroxide but noticed that caustic soda crystals were a lot cheaper. I decided to experiment with making my own cleaner.

I bought a 1 litre stainless steel flask and placed 500 ml of tap water in it. Then I added 500 g of caustic soda (from the hardware store) over maybe a 3 hour period. I stirred it with a new, unpainted wooden spoon. The solution got very hot! Then I poured it into a HDPE milk bottle, there was no free solid left and it had all dissolved. However the solution was pale green! When we used sodium hydroxide solution years ago in the lab, it was colourless. Also the end of the spoon which was in contact with the alkali was discoloured to a dark brown.

  1. What is the source of this green? Is it a chromium compound, having stripped this out of the stainless steel?

  2. The stainless steel flash looks normal, and has been thoroughly rinsed. Is it safe to use for holding tea/coffee?

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    $\begingroup$ I believe the sodium hydroxide attacked some organic molecule present in the wood of the spoon, and it ended up dissolved in the solution (the spoon discoloration is a sign this could be the case). About the second question, as a safety rule I'd always keep separate flasks for food and for experimentation. $\endgroup$ – IanC May 27 at 23:16
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    $\begingroup$ To second IanC: once a container is dedicated to chemical experimentation, this container either stays for chemical experimention, does not come close to food containers and must be clearly different from a food container -- or, after appropriate cleaning, goes into recycling. You won't drink from it anymore either, both because of even potential residues left as well as your concentrated lye might irreversibly dammaged the inner coating of your container, too. $\endgroup$ – Buttonwood May 28 at 21:00

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