Concentrated formaldehyde solution (37-40%) can be diluted in water, buffer, alcohol etc. to prepare different fixatives. One example is 10% NBF. When concentrated formaldehyde is diluted 1:10 in buffer (usually sodium phosphate) with close to neutral pH and some preservatives and stabilizers added 10% NBF is obtained. Vendors always add methanol as a stabilizer. 10% NBF contains 3.7-4% formaldehyde among other ingredients. So 10% NBF is not the same thing as 3.7-4%formaldehyde. Whether they can be used interchangeably is open to discussion and has to be validated by the end user for their purposes.
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