Re: Why does a Red Cabbage indicator turn acidic ?

Any solution of base will lose pH over time if it is exposed to air.

Air contains carbon dioxide; carbon dioxide dissolves in water to give
carbonic acid; carbonic acid reacts with base to form carbonate.

Indicators which change at a relatively high pH (like
phenolphthalein, which changes around pH 9) will "go acidic" on exposure
to atmospheric CO2.

The heat from the lamp probably hastens the reaction.

                                               Dan Berger