Making formalin from paraformaldehyde - help

[iranzi]

Hello,

i'm trying to make 37% formalin from paraformaldehyde but it doesn't go into solution. I used 37g paraformaldehyde, 100ml water, heated it up to about 70-80C and stirred for almost 30 minutes. Now i have a bottle of it, 2/3 sediment and 1/3 clear liquid.

Can someone point out what am i doing wrong? Perhaps pH should be adjusted?

I couldn't find the answer here, and got some confusing information from some other sites, eg:

"For all other uses of formalin made from paraformaldehyde, please refer to the formalin variant fixatives. They may be substituted directly, keeping in mind that it requires 40 grams paraformaldehyde in a litre of water to make 10% formalin (4% formaldehyde)." etc. etc...

[Photo Engineer]

Your formula and method are incorrect. 37 grams of Paraformaldehyde + 100 ml of water will not make a 37% solution. It will be more dilute than that. See here: http://en.wikipedia.org/wiki/Paraformaldehyde

Take about 50 ml of warm water and add a drop of 37% Sulfuric Acid to it. This is common battery acid. Then add 37 grams of Paraformaldehyde to that in small amounts while stirring. It should dissolve. If it does not, add another drop of acid. When all solid is added, dilute this to 100 ml and this will make a 37% (weight / volume). Be careful as Formaldehyde is a gas and can overcome you in a closed space. Wear eye and hand protection and use a breath mask or work outdoors if possible.

PE

[iranzi]

Thank you very much, PE.
I have 10% Sulfuric Acid, so I'd have to use more of it. Would that be a problem?

[michaelbsc]

If you are doing it for fun ignore me.

If you just want 37% formalin and you're in the US there is a chemical supply house on eBay that sells 500ml bottles with reasonable shipping.

[Photo Engineer]

Michael has a point. Just buy it!

If you have to make it, then for the acid, if you can, get automobile battery acid, but if not then use 3 - 6 drops of the 10% for starters. The formula that I gave is a guesstimate anyhow and you will probably have to adjust it a bit to get it right.

PE

[brianmquinn]

Formaldehyde is a gas. It will dissolve in water and when the water is saturated it is 37% formaldehyde. Formalin is a water saturated with formaldehyde (37%) with some methanol added to stabilize the formaldehyde. When this saturated solution is diluted 10 fold you get a 3.7% solution. Usually rounded to 4%.

So you see that 10% formalin is really 3.7% formaldehyde.