Spectrophotometric determination of reaction stoichiometry and equilibrium constants of metallochromic indicators. II. The Ca2+-arsenazo III complexes.
The analytical method described in the preceding article was applied to spectrophotometric Ca2+-titrations of the metallochromic indicator arsenazo III (Ar). At various reactant concentrations it was determined that Ar forms 1:1,1:2 and 2 : 1 complexes with calcium. The equilibrium constants and extinction coefficients at 602 nm were determined. Corrected to zero ionic strength at 293 K and pH 7.0, the reactions Ca + Ar = CaAr, CaAr + Ar = CaAr2 and CaAr + Ca = Ca2Ar are associated with dissociation equilibrium constants k(11) = 1.6 x 10(-6)M, K12 = 3.2 x 10(-4)M and K21 = 5.8 x 10(-3)M. respectively. The extinction coefficient of unbound indicator is (602) = 9.6 (+/-0.3) x 10(3) cm(-1) M(-1). Arscnazo III complexes with monovalent ions like Na+ and K+ : at zero ionic strength, the dissociation constant of the Na+-Ar complex is about 0.1 M.
Dorogi
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Max-Planck-Institut für Biochemie, D-8033 Martinsried, Munich, FRG.
Neumann
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