In cell culture experiment, why does the medium turn purple when stored in a 4°C refrigerator?

When the medium is stored in a 4℃ refrigerator, the CO2 in the medium will gradually overflow, causing the medium to become more and more alkaline, and the color of the acid-base indicator (usually phenol red) in the medium will become more purple with the increase of alkalinity. As a result of the alkaline medium, cell growth arrest or death will be caused. If the medium is alkaline, aseptic filtered CO2 can be introduced to adjust the pH value.