The discrepant event of placing a red sponge in a red solution and having it turn blue is sure to capture your students’ attention and stimulate a lively discussion of possible explanations.
The indicator sponge is saturated with congo red solution. Congo red is a dye, a biological stain, and a pH indicator. It has been used as a direct fabric dye for cotton to produce a bright red fabric. Biologists use Congo red as a general contrast stain for cellulose. Congo red is also used as a pH indicator. The colour transition is between pH 3.0 and 5.0. Below a pH of 3.0 (very acidic solutions), the indicator is blue. Above pH 5.0, the indicator is red. When a cellulose sponge is soaked in a Congo red solution, the dye becomes permanently bonded to the cellulose fibers. The active acid/base sites on Congo red are still available and the sponge now becomes an indicator sponge for acids. It can also be used to check for acid spills on counters after students have used acids.