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HYDROXY NAPTHOL BLUE indicator for calcium determination is make by Dissolve 0.3 g in 100 mL of water, add 10 mL of 1 N sodium hydroxide volumetric solution and 1.0 mL of calcium chloride solution (1 g in 200 mL), and dilute with water to 165 mL.
Binding of Transition Metal Ions by the Calcium Indicator Hydroxy Naphthol Blue. It has been found that the calcium indicating azo-dye, hydroxy naphthol blue (HNB), binds more strongly to transition metal ions than to the alkaline earth ions it is currently used for.
At pH 10, Ca2+(aq) ion first complexes with the indicator as CaIn+(aq) which is wine red. As the stronger ligand EDTA is added, the CaIn+(aq) complex is replaced by the CaY2-(aq) complex which is blue. The end point of titration is indicated by a sharp colour change from wine red to blue.
Use. Murexide is used in analytical chemistry as a complexometric indicator for complexometric titrations, most often of calcium ions, but also for Cu, Ni, Co, Th and rare-earth metals. It functions as a tridentate ligand. Its use has been eclipsed by calcium-ion selective electrodes.