50 mM Sodium phosphate, containing 150 mM NaCl (pH 7.0)

Combine 100 ml of 0.5 M Na₂HPO₄ with 150 ml of 1 M NaCl in a 1-liter volumetric flask. Adjust to 1 liter with H₂O.

Combine 50 ml of 0.5 M NaH₂PO₄ with 75 ml of 1 M NaCl in a 500-ml volumetric flask. Adjust to 500 ml with H₂O.

While monitoring the pH, combine the two solutions (from Steps 1 and 2) until the pH reaches 7.0.