Picric acid is a simple enough organic acid - its nitro groups withdraw electrons, making it a pretty strong acid for a phenol. However, it's got a slightly darker side: it's to TNT what phenol is to toluene. Those nitroes come with a price.
Picric acid is funny; like raney nickel, it's much more benign under water. For this reason, it's sold that way. Dry picric acid or its metal salts can be quite nasty. From time to time you hear about an old container of the stuff being discovered and the bomb squad coming out.
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And it is great (as part of more complex mixes that have to be made fresh every time) for preserving tissues for later histological analysis. Yellow liquid, of course, as we were not crazy to buy the dry stuff.
A few years ago we had a course about pyrotechnics at the lab of a local chapter of the Swedish Young Scientist Association. It was housed in a local high school, and had inherited many chemicals from the chemistry teachers over the years. The instructor told us about interesting and risky chemicals, and when he mentioned picric acid our hosts brought out a big container of it from the storage cabinet. The instructor looked a bit surprised, but continued by pointing out that while it was a high explosive it was not *that* dangerous unless it was polluted by its metal salts. At that point he opened the container and saw the rusty metal spoon in the powder. He closed it very gently and put it away, keeping to the opposite side of the room for the rest of the evening.
Wasn't picric acid one of the chemical agents responsible for leveling half the city of Halifax, Nova Scotia during World War I?