HEPES is a buffer solution (1 M), also known as N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid. HEPES is a zwitterionic organic chemical buffering agent commonly used in cell culture media. It maintains the pH in a range of 6.7 – 8.6 and the addition of 10 to 25 mM HEPES provides extra buffering capacity when a cell culture requires extended periods of manipulation outside of a CO2 incubator. Our HEPES buffer solution is suitable for many cell culture systems because it is membrane impermeable, has a limited effect on biochemical reactions, and is chemically and enzymatically stable. The solution is prepared in cell culture grade water.