Purification of formaldehyde

[Rhodium]

An old bottle of formaldehyde of unknown concentration, with 1 cm of precipitate at the bottom, can this be distilled or something to get 37% formaldehyde solution?

[SPISSHAK]

Or polymerization, heat it and see if it dissolves.
I'm not sure exactly under what conditions formaldehyde floculates but It's a guess.

[Rhodium]

It will probably redissolve with heating, but the concentration is unknown, I'd like to make sure I have a 37% solution.

The conditions under which my bottle flocculated was by prolonged storage at 5-10°C.

[SPISSHAK]

Other than specific gravity, or by seperating the two, I think a certain percentage of methanol is added to formalin solutions to keep it from doing this so it probably is spontaneous.
If you heat it to much you'd either A: distill off the methanol
B: the methanol would condense on the formaldehyde and produce Methylal.
During this process the paraformaldehyde would depolymerize by reaction with water.
Depending on what extent you heat it to.

[lugh]

Paraformaldehyde is formed when formaldehyde solution is evaporated, it's probably what's on the bottom    When paraformaldehyde is heated with a large quantity of water, it's reconverted into formaldehyde  

[Rhodium]

How can I determine the concentration? Or perhaps I should rather heat a known weight of paraformaldehyde with water instead?

[lugh]

The specific gravity of formaldehyde is 1.075-1.1081    You could calculate the percentage of a solution prepared from heating paraformaldehyde in a large quantity of water to that to determine your concentration    The aqueous solution boils at 98°C, the boiling point of the gas is -21°C  

[b159510]

How can I determine the concentration

iodimetric titration, after 5 minutes add hydrochloric acid and back titrate with na-thiosulfate.

or like you said, mix up a new batch, which would probably be easier if you don't have a standardized triiodide solution already made.

You don't fool me, Oilman

[b159510]

Paraformaldehyde is formed when formaldehyde solution is evaporated...

I've dissolved paraformaldehyde in water, but never evaporated the solution. Isn't that how trioxymethylene is formed? (aka metaformaldehyde, 1,3,5-trioxane)You don't fool me, Oilman

[lugh]

Yes, as posted in

Post 209590 (missing)

(lugh: "Re: Acquiring Acetaldehyde OTC", Chemicals & Equipment)    Triformol is the trimer, paraformaldehyde is the hexamer, which melts at 171°C  

[b159510]

The titration I mentioned should still work for an aqueous solution of formaldehyde, even though that isn't the species that actually exists.
(What is that called, methanediol?)You don't fool me, Oilman