garage chemist
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Depolymerization of old Formalin
I recently acquired a Liter of very old 35% Formalin (12 Years old).
It has huge amounts of white paraformaldehyde at the bottom (the white stuff fills almost one quarter of the liquid).
The bottle says that it contains about 11% Methanol as stabilizer, but the very long storage (improperly stored as well, in an unheated garage and
without light protection) has still effected polymerization.
The solution also has only rather weak smell, very contrary to what I have heard about formaldehyde solutions.
Does anyone know how I can get the paraformaldehyde back into solution? I tried heating the entire flask in a water bath to about 40- 50°C for ca.
30min, but I'm afraid to go higher since it is made of plastic. This didn't have any visible effect on the amount of white stuff.
Are higher temperatures necessary for this, or do I just need
to maintain the 40- 50°C longer? Does heating of this aqueous suspension even work, or do I have to filter the paraformaldehyde and depolymerize it
by heating?
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chemoleo
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I believe you depolymerise it by the addition of a bit of acid, such as H2SO4, and then heat it, similar as with the depolymerisation of paraldehyde.
It should got back into solution. Then H2SO4 can be removed by adding BaCO3.
Btw depolymerisation also works with base, i.e. NaOH or Ca(OH)2. I know this because for pentaerythritol synthesis paraformaldehyde is used, and when
the alkali is present, paraformaldehyde dissolves within seconds. However, prolonged exposure lead to self-condensation reactions, which forms
polymers.
[Edited on 27-3-2006 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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garage chemist
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How long does it have to be heated, and how high? I'm afraid of the formaldehyde boiling off...
What is the optimal pH value for depolymerization?
Sorry, I don't know much about Formaldehyde and its properties...
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chemoleo
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Ok, I just checked this out a bit more, check Gattermann on page 341. It appears that paraformaldehyde is also MADE particularly in the presence of
H3O+. However, depolymerisation seems to occur at higher temperatures. Paraformaldehyde is not soluble in most common solvents, but it can be brought
back into solution through boiling with lots of water. Possibly you might want to treat your paraformaldehyde in a steam pressure cooker with lots of
water. The endpoint of the reaction is recognised easily, as no white PFA is left.
Paraldehyde, the acetaldehyde trimer, is depolymerised by H2SO4 and destillation, where the volatile acetaldehyde keeps shifting the equilibrium. So
possibly a similar approach is required for PFA, you could i.e. decompose the PFA by heat and collect the CH2O vapours in water.
As to temperature - if it's in solution it won't be above the bp of water, so that should be ok. For the polymerisation, it mentions a drop of H2SO4
in 5ml acetaldehyde, I am sure that sort of amount would be appropriate for the depolymerisation too.
Anyway... for most reactions paraformaldehyde can be used, and depolymerised in situ...
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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garage chemist
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I'll try simple heating in a closed round- bottom flask, the clear liquid in the formalin bottle will be poured off and only the white sludge
subjected to this treatment.
The presence of strong acis like H2SO4 can also catalyse the formation of trioxane, which will not depolymerise in solution again and does not give
the reactions of formaldehyde, e.g. is highly unwanted in the formaldehyde solution (it would introduce a very hard to remove impurity into products
made from the formalin, like for example pentaerythritol).
EDIT:
I looked in the new book in the library, this one:
http://www.sciencemadness.org/library/books/formaldehyde.pdf
and found that formaldehyde solution with precipitate can be clarified by heating.
The temperature required for this was not stated.
As I saw, 40°C is not enough... I'll have to do as explained above, with a glass container.
[Edited on 27-3-2006 by garage chemist]
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garage chemist
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Yesterday I heated the precipitate with about 400ml of the solution to nearly reflux for ca. 3 hours, with no visible effect on the amount of
precipitate. 
Today I vigorously refluxed the solution, with much stronger heating than yesterday.
I left the thing unattended, and as I came back into the garage, I was hit by the overpowering stench of HCHO. It was so strong, it acted like a
lachrymator and my eyes filled with tears. 
Now, the water through the dimroth condenser was still flowing, and it was still cold! The condenser was also cold on the outside, but it was entirely
filled with a white sublimate which also partially escaped through the top. That was the reason for the stench.
How can this happen?! Formaldehyde isn't supposed to escape as a gas from the solution when refluxed!
And it didn't do so yesterday!
I am pissed, this was my only formaldehyde, and I boiled it away. I can't buy it again, that's the problem!
Does anyone know a good quantitative test for determining the concentration of HCHO in aqueous solution? I want to know how concentrated it still is.
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phase_dancer
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I find a few mLs of methanol added to the polymerized form and shaken usually fixes it, although it may take a day or so to completely clear. It's
also worth adding a bit of extra methanol to any formalin intended to be kept for any length of time, as it inhibits the formation of
paraformaldehyde.
Chromatropic acid may be able to be used to determine formaldehyde levels.
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