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...