PowerLabs Potassium Picrate Synthesis

A POWERLABS exclusive!

Potassium Picrate (2,4,6-Trinitrophenol, potassium
salt) Laboratory Synthesis:

INTRODUCTION:

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!

MATERIALS:

Reactants:

Glassware:

Potassium Hydroxide (KOH(s))

250mL Glass beaker.

Picric Acid (saturated solution)
(C6H3N3O7(aq))

Pipette

Distilled Water (H2O(l))

Stirring Rod

Funnel/filter
paper

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.

PROCEDURE:

C6H3N3O7(aq)
+ KOH(aq) =>
KC6H2N3O7(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.

RESULTS:

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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Last updated 30/01/04

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