Thermodynamic Stability of Zirconium(IV) Complexes with Hydroxy Ions

A protometric study on the speciation of zirconium(IV) was performed in an aqueous solution at 25 °C in KNO3 and four ionic strengths (0.2, 0.5, 1, and 2 mol·dm−3). This study indicates the formation of the soluble species Zr(OH)3+, Zr2(OH)7+, and Zr(OH)4 in the pH range 1.5–3.5. The use of an appropriate titration technique allows the precipitation of the zirconium hydroxide to be limited. The overall stability constants βmlh of these three hydroxo complexes were computed and refined, by the program PROTAF, for each ionic strength. The solubility product Ks of Zr(OH)4 was calculated by taking these βmlh constants into account. The thermodynamic values were determined by extrapolation at zero ionic strength (Specific ion Interaction Theory method): log β°10–3 = –0.52 (0.01); logβ°20–7 = –2.22(0.07); logβ°10–4 = –4.52 (0.07); logKs° = –55.26(0.08).