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Using colour to determine the pH

Today in the lab I was doing an experiment involving brain cells kept in cell culture flasks, and grown in an incubator in a red solution called the "culture medium". These flasks contained millions of cells extracted from multiple brains.

A tower of cell culture flasks at the end of my experiment.
A tower of cell culture flasks at the end of my experiment.

The culture medium contains phenol red, which is a pH indicator. Phenol red changes colour according to the pH. In the above picture, the culture medium turned pink because the pH became too alkaline as the flasks were kept outside of the incubator for a long time during the experiment. This caused the pH to become too alkaline due to exposure to atmospheric gases when removed from the controlled atmosphere of the incubator. If the pH had turned too acidic, for example, because of the cells producing a lot of waste products, then the solution would instead have turned orange and eventually yellow.