Transcription
Today I'm going to make the chemical picric acid from this bottle of aspirin I picked up at Walmart. Picric acid is a strongly nitrated organic compound that's been used historically as a blood test for creatine, and antiseptic, and a biological stain. However, the most common use of this chemical by far has been in manufacturing explosives, particularly during the First World War.
The IUPAC name for picric acid is 2,4,6-trinitrophenol or TNP, and its ammonium salt is a far more powerful but less stable predecessor to the more common explosive TNT, which has a very similar chemical structure. Ammonium picrate, or "dunite" as it was called, was used extensively in the early 1900s for artillery shells. However, picrate has a strong tendency to react with transition metals, forming metal picrates, which are far more shock-sensitive than the parent trinitrophenol, and a major safety issue. This is is the main reason picric acid was eventually replaced by TNT in warfare, and to this day, metal picrate formation is the main reason that World War One shipwrecks are not to be disturbed.
Anyway, to get into the actual synthesis of this chemical from aspirin, the first thing I needed to do was separate the active chemical in aspirin from the pill binder material. This was done by grinding the pills into a powder and then dissolving the powder in warm isopropyl alcohol. This solution was then passed through vacuum filtration to remove the binder material, which I'm pretty certain was cornstarch. The filtrate contained the active chemical, acetylsalicylic acid, which was transferred to a crystallization dish. I went ahead and put this on a hot plate and applied the minimum possible heat under a fan to evaporate as much isopropyl as I could. This eventually crystallized the acetylsalicylic acid, and I'll stop talking for a moment so you can watch that happen.
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Now, once all the liquid isopropyl was gone, these were still slightly damp with alcohol, and to remove the rest of the moisture, I vacuum desiccated them for a few hours. This left me with perfectly clean and dry acetylsalicylic acid, which is my starting reagent. To this end, I went ahead and weighed out 24 grams of my acetylsalicylic acid before deciding I didn't really want to make quite that much picric acid, and cut it down to 20 grams. I then poured 80 milliliters of concentrated sulfuric acid into a beaker and slowly added my acetylsalicylic acid under constant stirring. This dissolves some of it, and the solution darkened a bit. Next, I moved this to my hot plate and heated the mixture to 110 degrees Celsius, which quickly dissolved the rest of the acetylsalicylic acid and turned it nearly black.
What's happening here is a multi-step reaction. In the first step, acetylsalicylic acid is hydrolyzed by the hot sulfuric acid to salicylic acid and acetic acid, and this produces a bit of a vinegar smell. In the second step, the salicylic acid is sulfonated nearly exclusively to 5-sulfo salicylic acid, which is our desired product here. This is held at 110 degrees Celsius under constant stirring for just under an hour, and then allowed to cool to around 90 degrees Celsius. I then add about 20 grams of sodium nitrate to begin the nitration process.
In the first step of the nitration process, a huge amount of foam was quickly produced as the 5-sulfo salicylic acid was decarboxylated, producing nitrophenol sulfonic acid. This particular reaction seems to be temperature-dependent in my experience and may happen later in the nitration if your starting temperature is lower than mine was here. It's also extremely exothermic and will generate a large amount of heat and toxic nitrogen dioxide gas. This is especially true if it begins to get too hot, and ideally, you'd want to use some type of temperature control here or make the sodium nitrate a distance more slowly to try and keep the temperature from going too far above about 120 degrees Celsius.
Anyway, once this cooled down a bit, the solution became a deep orange, and I went ahead and added another 20 grams of sodium nitrate. This time, yellow crystals immediately began to crash out, and I had to slowly heat the mixture up to around 125 degrees Celsius before they melted. This temperature increase also seems to have restarted the nitration, and another huge cloud of toxic nitrogen oxides formed. What's happening now, and what probably already was happening before the second addition, is that the sulfonate group, as well as the other ortho position, are concurrently nitrated, producing the target picric acid. Almost as soon as the reaction stops spewing toxic red gases, the mixture quickly turned a distinct green color, which slowly cleared up as I allowed it to cool to room temperature.
Once the mixture had cooled completely, it had become a solid mass of yellow picric acid crystals, which I now needed to clean up. To do this, I first dumped in a bunch of ice water to dissolve and break up as much of this as I could. Keep in mind here that it's important to use ice water, as there's still a lot of sulfuric acid present, which will generate enough heat to boil over if room temperature water is used. Once I break it up as much as I can, I go ahead and pass it all through vacuum filtration, which will catch the vast majority of the highly insoluble picric acid. This crude picric acid is then transferred to a one-liter beaker and dissolved in a minimal amount of boiling water for recrystallization. This ended up taking nearly a full liter of boiling water to get it to all dissolve, and the solution turned to dark orange.
Once the picric acid is totally dissolved, which again takes a while due to how crazy insoluble this stuff is, I went ahead and took the beaker off the heat and allowed it to cool slowly at room temperature. This very deliberately slow cooling is done to create large and very, very pure crystals, which actually happened really quickly at first due to the extreme saturation of the solution. I went ahead and let this crystallize overnight to form the largest crystals possible, and I'm actually going to stop talking for a moment again so that you can watch this crystallize. I've gotten a lot of requests to do that, so here you go.
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Acid crystals had formed, and the solution returned to its distinct yellow color. I went ahead and begrudgingly broke these apart and then passed this all through vacuum filtration to collect my pure recrystallized picric acid. These crystals were then vacuum desiccated, but only for about an hour, as picric acid becomes a dangerous potential explosive if it's completely dry. After this, I went ahead and weighed my product, and in the end, I got a final mass of 18.5 grams of very slightly damp picric acid, which represents about a 72.7% yield.
Now, keep in mind, in addition to storing the stuff slightly damp, you want to store it only in glass or plastic containers, and this is because, as I mentioned earlier, that picric acid is very, very prone to forming metal picrates if stored in a metal container, which are extremely shock-sensitive explosives and very dangerous. And while on this note of safety, when it comes to disposing of all the waste filtrate I collected, I actually first tried to boil it down to see if I could crystallize any more picric acid. I ended up collecting a bit this way, but the quantity was very small, only about three grams. The crystals were ugly, and the melting point was very far from the literature value. When I was done trying to isolate any more picric acid from this, I went ahead and neutralized the solution with potassium hydroxide, which formed the very, very red, dark potassium picrate. This was then reduced with any number of reducing agents. I forget which one I used, some kind of sulfite, and the point of that is to render it non-explosive. At this point, it could just be treated as ordinary toxic waste, and the big idea is that it's relatively important that all this waste does not go straight down the drain, or again, it'll react with the copper piping to produce explosive copper picrate. Anyway, so yeah, that's the entire process.
One more thing to keep in mind about this chemical is its extreme staining power. This will turn anything it touches a very, very bright yellow for a very, very long time, and here's my finger for proof. This little stain I accidentally got on my finger took over a month to go away, and it seems to particularly aggressively stain plastic. An extremely small amount of this stuff will turn a very, very large volume of water bright yellow.
Now, with that said, my biggest warning here in this project, beyond the formation of massive amounts of toxic red gas, or using strong acids that can burn through your flesh, or the potential formation of explosive compounds that are shock-sensitive, is really this staining. It's really, really annoying because, let's be clear, if you're somebody who's, you know, got the brain power to even be attempting this in the first place, you're probably smart enough to not have this detonate on you. However, these picric acid stains are insidious. They get places you didn't even know you touched, in places you didn't even know you went, and they last forever. Now, obviously, I'm joking that it's the most hazardous part of this project, but it is really annoying. Conversely, though, if you wanted to stain something yellow forever, this would be your chemical. I think it would work great for that.
Anyway, I'm starting to ramble, so I just want to say I hope you found this video interesting, and as always, I want to thank all my incredible patrons for their generous contributions. Your support is always vital and always very, very appreciated. And everyone else, if you would like to see more content like this, consider subscribing on TikTok, YouTube, Instagram, or even by becoming a patron yourself. Thank you all so much for watching, and I'll see you next time.
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