Formaldehyde stock solution (20%)
Add 200 mg of EM-grade paraformaldehyde per milliliter of H2O. Heat at 60°C on a stir plate in a ventilated chemical fume hood to dissolve. Add a trace of NaOH to help dissolve the paraformaldehyde (no more than 1 mL of 1 N NaOH to 100 mL of H2O). Do not overheat; if the solution boils or turns brown, discard and start over. Adjust pH to ~7.0 with 1 N HCl. Formaldehyde is typically diluted to 4% in the desired buffer.
