Arsenazo III

Arsenazo III is an azo derivative of chromotropic acid, a metal chrome dye, a chelating agent, and a cation complexing agent. Arsenazo III forms stable 1:1 complexes with Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, K⁺, and Na⁺, and its binding affinity for Ca²⁺ depends on pH, alkali metal cation concentration, and buffer parameters. Arsenazo III serves as a colorimetric indicator for micromolar ionized Ca²⁺ in cells^[1][2]. In Vitro:Arsenazo III (0.74-193000 μM) forms stable, spectrally distinct complexes with alkaline earth metal ions (Ca²⁺ > Sr²⁺ > Ba²⁺) and K⁺ in buffered aqueous solutions, with a Ca-dye dissociation constant of 0.74 μM at pH 7.4 under low-ionic-strength conditions, and measurable changes in absorbance that enable quantitative determination of free Ca²⁺ concentrations^[1].
Arsenazo III (30-40 μM; 20°C, pH 7.0, 7.4, or 7.6) has an apparent calcium binding affinity that depends on buffer type and concentration, decreasing with increasing buffer concentration and varying across imidazole-Cl, K-Hepes, and Tris-Cl at the same concentration and pH^[2].