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Congo Red as a pH indicator.

Congo Red is a dye once used for clothing and is also a pH indicator. It is also a Microscope Stain.

Indicator properties

Due to a color change from blue to red at pH 3.0–5.2, Congo red is used as a pH indicator. Since this color change is an approximate inverse of that of litmus, it can be used with litmus paper in a simple trick: add a drop or two of Congo red to both an acid solution and a base solution. Dipping red litmus paper in the red solution will turn it blue, while dipping blue litmus paper in the blue solution will turn it red.

Usage as Clothing Dye

Congo Red was once used as a dye in clothing, but has since been abandoned due to toxicity reasons and can change color when exposed to sweat.

References

Steensma, D. P. (2001). "Congo Red: Out of Africa?" (pdf)Archives of Pathology and Laboratory Medicine125 (2): 250–252. doi:10.1043/0003-9985(2001)125<0250:CR>2.0.CO;2PMID 11175644