Hydroxy naphthol blue disodium, an azo dye, is a metal indicator for calcium and a colorimetric reagent for alkaline earth metal ions ( Λmax = 650 nm). In the pH range between 12 and 13, the solution of the indicator is reddish pink in the presence of calcium ion and to deep blue in the presence of disodium EDTA[1].
[1]. Motoki Goto, et al. Colorimetric detection of loop-mediated isothermal amplification reaction by using hydroxy naphthol blue. Biotechniques. 2009 Mar;46(3):167-72.