pH indicators in stains
Several food pigments change colour with acidity. It is why treating a yellow turmeric mark with soap turns it red — and why that reverses.
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
Sources for this page
Sources
Royal Society of Chemistry educational and reference chemistry resources
Royal Society of Chemistry · Scientific literature
Relied on for: That curcumin and anthocyanins change colour with pH, being yellow or red under acidic conditions and shifting under alkaline ones. (background and mechanism only)
Spot Solver: spot and spill removal procedures
The Carpet and Rug Institute (CRI) · Trade or professional body
Relied on for: That the vinegar solution's stated purpose is to lower alkalinity left by detergent or by an alkaline spill. (states this directly)