Aqueous solution chemistry of zirconium(IV). 1. Kinetic studies on hydrogen ion and general acid (HX) induced dissociations of the tetrameric ion [Zr4(OH)8(H2O)16]8+

Article abstract of DOI:10.1021/ic50217a054

The tetramer to monomer conversion [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ → Zr(H₂O)₈⁴⁺, i.e., Zr₄ → Zr, was initiated by diluting aqueous solutions of Zr(IV) (ca. 10^-2 M) in HClO₄ (ca. 0.5 M) to [Zr(IV)] ≤ 10^-4 M and increasing [H⁺] to a value in the range 2.0-4.0 M (I = 4.0 M (NaClO₄)). Reactions were carried out in the presence of 2-thenoyltrifluoroacetone (HT), which complexes rapidly with Zr⁴⁺ (but not Zr₄) and, since ZrT³⁺ is colored (λ_max 366 nm), provides the means of monitoring the reaction. Data conform to the simple rate law k_H[H⁺][Zr₄], where at 25°C k_H = 3.7 × 10^-3 M^-1 s^-1. At I = 2.0 M (NaClO₄), k_H (25°C) = 0.95 × 10^-3 s^-1, and from the temperature-dependence activation parameters ΔH? = 13.2 kcal mol^-1 and ΔS? = -28 cal K^-1 mol^-1 were obtained. Acids HX also induce the Zr₄ → Zr conversion, giving the rate law K_H[H⁺][Zr₄] + k_HX[HX][Zr₄]. Second-order rate constants k_HX (M^-1 s^-1) at 25°C, [H⁺] = 2.0 M, and I = 2.0 M are as follows: H₃PO₄ (80) > H₂C₂O₄ (44) > H₄P₂O₇ (40) > HF (30) > HSO₄^- (8.8) > CCl₃CO₂H (1.0) > CH₂ClCO₂H (0.4). Acetic acid (5 × 10^-3 M), Cl^- (2 M), and NCS^- (1.2 × 10^-3 M) have no effect on the conversion.