Potentiometric and Spectrophotometric Study of Calcium and Alizarin Red S Complexation.

Abstract

Equilibria in the system H+, Ca2+ and Alizarin Red S have been studied by means of potentiometric (glass electrode) and spectrophotometric measurements in a 0.1 M NaCl medium at 25- degrees-C. Calcium complexation is characterized by the formation of ternary species according to the reaction: pH+ + qCa2+ + rHA2- half-arrow-right-over-half-arrow-left H(p)Ca(q)(HA)r(p+2q-2r) Equilibrium constants were evaluated within the range 3.3 less-than-or-equal-to -log[H] less-than- or-equal-to 10.3 and the wavelength interval 300 less-than-or- equal-to lambda less-than-or-equal-to 700 nm with the following results: log-beta(012) = 8.04 +/- 0.04 (pot.); log-beta(012) = 8.20 +/- 0.12 (spectr.); log-beta(-111) = -3.72 +/- 0.12 (pot.); log-beta(-111) -3.74 +/- 0.15 (spectr.); log-beta(-212) = 10.11 +/- 0.20 (pot.). All errors reported are 3-sigma (log- beta(pqr)). The protonation constants for Alizarin Red S were also determined. Data were analysed using the least-squares computer programs LETAGROPVRID (ETITR and SPEFO).