o-Cresolphthalein, Indicator grade 25G

o-Cresolphthalein is a reversible pH indicator. The kinetics of the reverse pathway (red-purple to colorless) is slow, thereby making it an appropriate molecule to check flow patterns in ducts with corrugated walls. It is also used for the colorimetric determination of calcium content via the o-cresolphthalein complexone method. Binding of calcium to o-cresolphthalein, in the presence of alkaline conditions (pH 10-12), results in the formation of a red complex with an absorbance at 570-575nm.