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The sodium bicarbonate-carbon dioxide system
How does the sodium bicarbonate-carbon dioxide system buffer the pH of cell culture medium?

Sodium bicarbonate is a buffer used to stabilize pH. Cells in culture produce CO2 but require only small amounts of the compound for growth and survival. CO2 affects the pH of medium. Increasing atmospheric CO2 decreases the pH of the medium. Decreasing the atmospheric CO2 increases the pH of the medium. As cells metabolize, they produce CO2 thus causing the media to become more acidic. If the medium contains phenol red, the color of the medium becomes more yellow. In culture media, dissolved CO2 is in equilibrium with bicarbonate ions and many cell culture media formulations take advantage of this CO2/bicarbonate reaction to buffer the pH of the media. CO2 dissolves freely into the culture media and reacts with water to form carbonic acid. As the cells metabolize and produce more CO2, the pH of the medium decreases (becomes more acidic).

Sodium bicarbonate, NaHCO3, is used as a buffer. Sodium bicarbonate dissociates into sodium and bicarbonate ions. By increasing the bicarbonate ions, the buffer drives the top equation to the left and thus increases the pH. The concentration of the sodium bicarbonate in the medium must be matched with the level of CO2 in the atmosphere above the medium. For media containing 1.5 to 2.2 g/L sodium bicarbonate, use 5% CO2. For media containing 3.7 g/L sodium bicarbonate, use 10% CO2. If the concentration of sodium bicarbonate is too high for the CO2 atmosphere in the incubator, the media becomes more alkaline (the pH increases).

Date Created07/17/2012 07:10 PM
Date Updated03/27/2014 08:39 PM

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