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Why does RPMI 1640 medium change color?
Publication date：2026-10-29
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When we routinely use RPMI 1640 medium, it’s not uncommon for the medium—whether halfway through a batch or freshly opened—to turn pink, purple, rose‑red, or other hues. What causes this? In fact, when purchasing RPMI 1640, you’ll typically find sodium bicarbonate and phenol red listed on the ingredient label; these two components are the primary culprits behind the medium’s color changes. First, consider sodium bicarbonate: once the bottle is opened, it decomposes to release CO₂, raising the medium’s pH and making it more alkaline. If you loosen the cap of the culture flask and incubate it in a CO₂ incubator for a while, the medium’s color will return to normal. Additionally, phenol red can also induce color changes in RPMI 1640. When the medium’s pH rises, phenol red shifts its color accordingly. Moreover, cells metabolize within the medium, producing byproducts that further alter the pH. Thus, the fundamental reason for RPMI 1640’s color change is a shift in the medium’s pH. When the pH exceeds 7, the color tends toward purple; conversely, if the pH falls below 7, the color shifts toward yellow.
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