Paraformaldehyde (4% in PBS)

• 1. Add 4 g of paraformaldehyde to a beaker containing 40 mL of distilled H₂O. Cover with foil and heat to 60°C in a chemical fume hood with constant stirring. Do not let the temperature rise above 65°C.

• 2. Add 1 n NaOH drop-wise, while stirring, until the solution just clears. Cool the solution on ice to room temperature.

• 3. Add 0.9 g of NaCl and stir until dissolved.

• 4. Add 40 mL of 0.2 m dibasic (Na₂HPO₄) phosphate buffer (PB) stock (pH 7.4) and 10 mL of monobasic (NaH₂PO₄) PB stock (pH 7.4).

  • Prepared PB stocks can be stored at room temperature for months.

• 5. Bring the volume to 100 mL with distilled H₂O.

• 6. Mix thoroughly and store in the refrigerator or freezer (−20°C) in small, single-use aliquots for months.

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