pH indicators in stains

In detail

A number of natural pigments are pH indicators: their molecules change shape, and therefore colour, depending on how acidic their surroundings are. Curcumin, the pigment in turmeric and in most prepared mustards, is yellow in acidic and neutral conditions and turns red or reddish-brown in alkaline ones. Anthocyanins, which colour berries, beetroot, red cabbage and red wine, shift across red, purple and blue-green over the same range. The practical consequence is a phenomenon people describe as the stain getting worse when they treat it. A soapy or ammonia-based cleaner is alkaline, so a pale yellow mustard mark turns orange under treatment, and a purple berry mark can turn grey-blue. Neither is the stain setting; it is the same molecule in a different form, and it reverses on acidification. That is one of the reasons an acidic rinse appears so often in the carpet institute's procedures — it lowers the alkalinity left behind by detergent or by the spill itself. It also means a colour change during treatment is diagnostic rather than alarming: it tells you the pigment is still present and still reachable.

Why

• Mustard stain is explained by pH indicators in stains
• Fruit juice is explained by pH indicators in stains