Why does phenol turn pink or red on exposure to air, and does it affect product quality?

Why does phenol turn pink or red on exposure to air, and does it affect product quality?
Phenol oxidizes slowly in air, forming a pink to reddish-brown color. The color comes not from simple benzoquinone, which is yellow, but from quinhydrone—a charge-transfer complex between p-benzoquinone and hydroquinone. This complex is deep green in crystal form but appears red-brown in solution. Over time, the color deepens to brown as more oxidation products accumulate. For industrial buyers, color change indicates oxidation and potential quality degradation, especially for applications requiring high purity, such as pharmaceuticals or resins. Proper storage under inert gas or with antioxidants is essential. The oxidation also slightly lowers melting point and can affect downstream reactions, so buyers should specify color limits and freshness when sourcing.