PowerLabs Potassium Picrate Synthesis

It is widely known that Picric Acid should not be allowed to contact metals or their salts due to the danger of formation of metal picrates. Metal picrates are salts of Picric Acid formed by the addition of the metal to the 2,4,6TriNitroPhenol ring at the first hydroxide, as seen on the example to the left (KO-C₆H₂(NO₂)₃ or KC₆H₂N₃O₇). They are all, to some degree, explosive. Their explosive strength is lower than Picric Acid’s own (between 48 and 70% depending on the particular salt), but their sensitivity is much greater, increasing with the weight of the particular metal ion used. For curiosity’s sake experiments were performed on the synthesis of a few metal picrates and they were tested as to their properties. The two most interesting ones were Potassium and Lead Picrate (with Sodium Picrate being very weak). Below, the synthesis of a small potassium Picrate batch is outlined for informational purposes only.
Metal Picrates are sensitive explosives and as such should not be manufactured at all!

  Here all the chemicals used in the synthesis are seen, from left to right, back to front: Distilled water, Potassium Hydroxide, Picric Acid Solution, 50mL glass beaker, spatula, glass rod.

C₆H₃N₃O_7(aq) + KOH_(aq) => KC₆H₂N₃O_7(s) + H₂O_(l) [Heat of formation: +117.5 Cal/mole].

40mL of hot (<80C) saturated picric acid solution are poured in a 50mL beaker.

A few Potassium Hydroxide pellets are added to the solution and stirred until dissolution occurs. They must be added one at a time and the PH must be tested with each addition until it becomes neutral. It can be seen that the solution quickly changes from fluorescent yellow to orange/brown. This is due to the tautomerism of the polynitrobenzenic ring in strong alkaline media; the same effect as on nitroalcanes that yields nitronic acids. TNP nitronates formed under these conditions are more unstable than the picrates in what they turn back after standing a while in neutral media (return to the yellow color). A safer procedure is to use potassium carbonate, which allows the PH to remains neutral throughout the process (more is added until no more CO₂ evolves). For this particular procedure the hydroxide was purposely used so as to compare the stability of its product with the carbonate-formed one. No significant differences were observed. In order to synthesize potassium picrate using potassium carbonate the procedure remains the same except K2CO3 is substituted for KOH.

As the solution is cooled gradually to 0C, the potassium picrate crystallizes out of it in the form of a framework of long needle-shaped crystals. Yield can be maximized by boiling the solution to half of its original volume and than cooling it to 0C, at which temperature the greatest number of crystals form.

The crystals are crushed somewhat by vigorously stirring the solution for several minutes (this is highly unadvisable for larger batches!). They are than filtered, whilst the solution is still cold, and washed with 30mL cold distilled water, so as to remove any traces of alkali or acid.

The final result is a mass of crystals slightly larger than what was started with in picric acid. These can be crushed carefully into a powder with the back of a plastic spoon on top of a sheet of paper. They are somewhat friction and impact sensitive, and deflagrate mildly from flame.

Deflagration is mild and accompanied by a bright purple/lilac smoky flame which is characteristic of the potassium ion color. Detonation does not ensue easily from impact or other stimuli. Click on deflagration photo to watch the (60kb) video. There is a different video for this detonation available here (358kb).

 Decomposition:
C6H2N3O7K –> combustion–> 6 CO2 + 2 H2O + 0.5 K2O + 1.5 N2 [heat of combustion by “free oxygen” Hc of 2317.67 kcal/kg].

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