α-Naphtholphthalein (C₂₈H₁₈O₄) is a

phthalein dye Phthalein dyes are a class of dyes mainly used as pH indicators, due to their ability to change colors depending on pH. They are formed by the reaction of phthalic anhydride with various phenols. They are a subclass of triarylmethane dyes. Common ...

used as a

pH indicator A pH indicator is a halochromism, halochromic chemical compound added in small amounts to a Solution (chemistry), solution so the pH (acidity or Base (chemistry), basicity) of the solution can be determined visually or spectroscopically by chang ...

with a visual transition from colorless/reddish to

greenish blue ''Icaricia saepiolus'', the greenish blue, is a butterfly of the family Lycaenidae. It is found from the northwestern United States to southern Saskatchewan and Alberta. The wingspan The wingspan (or just span) of a bird or an airplane i ...

at pH 7.3–8.7.

References

PH indicators 1-Naphthols Phthalides Triarylmethane dyes {{Ether-stub