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Author: Subject: Accidentally making potassium picrate(?)
night429
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[*] posted on 5-6-2021 at 14:00
Accidentally making potassium picrate(?)


Yesterday, I tried to make some picric acid from a procedure that has been successful in the past, but I decided to use 100mL of sulfuric acid instead of 140mL in an attempt to conserve it. I used 17.5g of salicylic acid and 43.1g of potassium nitrate as well. The reaction went uneventful (I kept the temperature above 100°C), although I got it too hot at times and NO2 was produced. At the end, the temperature dropped, and a bunch of what I believed to be picric acid crashed out. I added an additional 20mL of sulfuric acid, but it didn't dissolve anything. I considered this done, even though I had a small amount of potassium nitrate left (less than a gram).

I crashed it out into around 300mL of ice and water, filtered, squeezed out the water, then recrystallized from water and let it cool. Everything was going as expected, and some nice crystals started to form, too.

Crystals growing in flask:
20210604_234643.jpg - 157kB

Then, I put the solution in the freezer, because the flasks were only slightly warm. Upon checking them 20min later, I saw a that a dust of crystals had "snowed" onto the bigger crystals. At this point, I knew something was off, but I cooled it further and filtered.

I knew right away by the color that whatever I made was not picric acid; it was a golden-yellow, and not a pale-yellow. I dried it to test its properties, and its flame sensitivity in comparison to pure picric acid was evident (it melted, then burned very quickly with a hissing noise and a lot of dark-gray smoke).

Dried product:
PotassiumPicrate.jpg - 55kB

The reason for posting this is that I don't know exactly what caused it, and this happened to me before (over a year ago), but I didn't think much of it until it happened again. I've never seen anyone report that they accidentally made it when they wanted to make just picric acid.

The only two things that I changed were the amount of sulfuric acid and the amount of ice water I crashed it into (I decreased it by about half to limit how much picric acid dissolved). Is it possible that there wasn't enough acid? Or was it the decreased amount of water that caused a higher concentration of potassium ions?

[Edited on 6-5-2021 by night429]
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