10% formalin and 4% paraformaldehyde are interchangeable for most purposes (in histology at least). However there are a couple of minor differences. First, commercial formaldehyde solution contains 37% to 38% formaldehyde. Therefore diluting it 1:9 results in a solution containing 3.7% to 3.8% formaldehyde, while a 4% solution of paraformaldehyde in water contains a full 4% formaldehyde. Secondly, commercial formaldehyde solution contains 10% to 15% methanol as a preservative. Therefore diluting it 1:9 results in a solution containing 1.0% to 1.5% methanol. This is not a problem for most histological applications, but it could be a problem in a procedure where sources of methylation have to be avoided. 4% paraformaldehyde solution contains no methanol - nothing but water and formaldehyde.
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